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id	title	slug	type	status	content	archieml	archieml_update_statistics	published_at	updated_at	gdocSuccessorId	authors	excerpt	created_at_in_wordpress	updated_at_in_wordpress	featured_image	formattingOptions	markdown	wpApiSnapshot
56832	Influenza	influenza	page	publish	<!-- wp:html --> <!-- formatting-options toc:false bodyClassName:topic-page --> <!-- /wp:html --> <!-- wp:owid/sticky-nav --> <!-- wp:navigation-link {"label":"Introduction","url":"#introduction","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Key Insights","url":"#key-insights","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Data Explorer","rel":"","url":"#explore-our-data-on-influenza","title":"","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Research \u0026amp; Writing","url":"#research-writing","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Charts","url":"#interactive-charts-on-influenza","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Endnotes","url":"#endnotes","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Cite This Work","url":"#citation","kind":"custom","isTopLevelLink":false} /--> <!-- wp:navigation-link {"label":"Reuse This Work","url":"#licence","kind":"custom","isTopLevelLink":false} /--> <!-- /wp:owid/sticky-nav --> <!-- wp:owid/front-matter --> <!-- wp:columns {"className":"front-matter"} --> <div class="wp-block-columns front-matter"><!-- wp:column --> <div class="wp-block-column" id="introduction"><!-- wp:paragraph --> <p>Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.</p> <!-- /wp:paragraph --></div> <!-- /wp:column --> <!-- wp:column --> <div class="wp-block-column"><!-- wp:group {"className":"related-topics"} --> <div class="wp-block-group related-topics"><!-- wp:paragraph --> <p>Related topics</p> <!-- /wp:paragraph --> <!-- wp:list --> <ul><li><a href="https://ourworldindata.org/causes-of-death">Causes of Death</a></li><li><a href="https://ourworldindata.org/vaccination">Vaccination</a></li><li><a href="https://ourworldindata.org/coronavirus">Covid-19</a></li></ul> <!-- /wp:list --></div> <!-- /wp:group --></div> <!-- /wp:column --></div> <!-- /wp:columns --> <!-- /wp:owid/front-matter --> <!-- wp:owid/key-insights-slider {"title":"Key insights on Influenza","slug":"key-insights"} --> <!-- wp:owid/key-insight {"title":"Seasonal flu kills hundreds of thousands of people worldwide each year","slug":"seasonal-flu-kills-hundreds-of-thousands-of-people-worldwide-each-year"} --> <!-- wp:paragraph --> <p>The flu is estimated to cause <a href="https://ourworldindata.org/influenza-deaths">400,000 respiratory deaths each year</a> on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This shows the mean estimate of annual influenza mortality between 2002–2011, excluding the 2009 “Swine flu” pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: <a href="https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf">https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf</a> {/ref} </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>People also die from other complications of the flu – such as a stroke or heart attack – but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate “does not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.”</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361–1369.<a href="https://doi.org/10.1016/j.vaccine.2021.11.080"> https://doi.org/10.1016/j.vaccine.2021.11.080</a></p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.<a href="https://doi.org/10.1001/jamanetworkopen.2022.8873"> https://doi.org/10.1001/jamanetworkopen.2022.8873</a>{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>The flu is <a href="https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu">most severe</a> in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O’Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. <a href="https://doi.org/10.1098/rsos.211498">https://doi.org/10.1098/rsos.211498</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>But it’s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In this article, we provide more detail:</p> <!-- /wp:paragraph --> <!-- wp:owid/prominent-link {"linkUrl":"https://ourworldindata.org/influenza-deaths","className":"is-style-thin"} --> <!-- wp:paragraph --> <p></p> <!-- /wp:paragraph --> <!-- /wp:owid/prominent-link --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.<br><br></li><li>These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu – such as a stroke or heart attack – which are not included here.<br><br></li><li>Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:html --> <iframe src="https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe> <!-- /wp:html --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"Social distancing during COVID-19 had a large impact on the flu","slug":"social-distancing-during-covid-19-had-a-large-impact-on-the-flu"} --> <!-- wp:paragraph --> <p>In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142–151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. <a href="https://doi.org/10.1016/j.jet.2021.105293">https://doi.org/10.1016/j.jet.2021.105293</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., … Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. <a href="https://doi.org/10.1371/journal.pone.0253071">https://doi.org/10.1371/journal.pone.0253071</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This decline was very large because of the mathematics of epidemics. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases <em>falls</em> exponentially.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}Biggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. <em>BMC Infectious Diseases</em>, <em>14</em>(1), 480.<a href="https://doi.org/10.1186/1471-2334-14-480"> https://doi.org/10.1186/1471-2334-14-480</a>{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. <a href="https://doi.org/10.1038/s41467-022-29402-5">https://doi.org/10.1038/s41467-022-29402-5</a><br>Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). <a href="https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753">https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} We therefore show the share of tests that were positive for the influenza virus.</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:html --> <iframe src="https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe> <!-- /wp:html --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"Seasonal flu used to be far more deadly","slug":"seasonal-flu-used-to-be-far-more-deatly"} --> <!-- wp:paragraph --> <p>Over time, <a href="https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past">the severity of the flu has declined</a> among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in <a href="https://ourworldindata.org/sanitation">sanitation</a>, neonatal healthcare, and <a href="https://ourworldindata.org/vaccination">childhood vaccination</a> for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115–E120. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. <a href="https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm">https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm</a> {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref} In lower-income countries, the flu could become a larger burden as <a href="https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age">their populations continue to age</a>.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In this article, we provide more detail:</p> <!-- /wp:paragraph --> <!-- wp:owid/prominent-link {"linkUrl":"https://ourworldindata.org/influenza-deaths","className":"is-style-thin"} --> <!-- wp:paragraph --> <p></p> <!-- /wp:paragraph --> <!-- /wp:owid/prominent-link --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}<br><br></li><li>The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.<br><br></li><li>The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:image {"id":56833,"sizeSlug":"large","linkDestination":"none"} --> <figure class="wp-block-image size-large"><img src="https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png" alt="People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980." class="wp-image-56833"/></figure> <!-- /wp:image --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"Flu seasons vary between countries","slug":"flu-seasons-vary-between-countries"} --> <!-- wp:paragraph --> <p>Several respiratory infections, including the flu, are more common in the winter. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. <a href="https://doi.org/10.1038/nrmicro.2017.118">https://doi.org/10.1038/nrmicro.2017.118</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In the chart, you can see the share of flu tests that were positive in different countries.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118–124. <a href="https://pubmed.ncbi.nlm.nih.gov/36773717/">https://pubmed.ncbi.nlm.nih.gov/36773717/</a> <br><br>Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. <a href="https://doi.org/10.1038/nrmicro.2017.118">https://doi.org/10.1038/nrmicro.2017.118</a><br><br>Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. <em>Epidemiology & Infection</em>, <em>143</em>(6), 1110–1118.<a href="https://doi.org/10.1017/S0950268814002702"> https://doi.org/10.1017/S0950268814002702<br><br></a>Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010–19: A retrospective analysis. The Lancet Global Health, 11(5), e729–e739. <a href="https://doi.org/10.1016/S2214-109X(23)00109-2">https://doi.org/10.1016/S2214-109X(23)00109-2</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Newman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. <a href="https://doi.org/10.1371/journal.pone.0193263">https://doi.org/10.1371/journal.pone.0193263</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>You can see this in the chart for Singapore and Thailand, for example.</p> <!-- /wp:paragraph --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref}</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:html --> <iframe src="https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe> <!-- /wp:html --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"Tracking flu-like symptoms can be informative when testing is limited","slug":"tracking-flu-like-symptoms-can-be-informative-when-testing-is-limited"} --> <!-- wp:paragraph --> <p>Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases – and the global data shown in our data explorer – greatly underestimate the true number of cases.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Because of the lack of direct testing, it is useful to track flu-like symptoms.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>It is important to note however that these symptoms are not specific to the flu: people with other diseases – such as rhinovirus, COVID-19, common colds, malaria, and others – can also have these symptoms and meet the following criteria.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p><strong>Acute respiratory infections (ARIs)</strong> are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but <em>only if</em> they were judged by a doctor to be caused by an infection.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p><strong>Influenza-like illnesses (ILIs)</strong> are narrower – they include only people with a sudden respiratory infection with a fever above 38ºC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.<br>Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., … Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122–128.<br><a href="https://doi.org/10.2471/BLT.17.194514">https://doi.org/10.2471/BLT.17.194514</a><br>World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.<a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y"> https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. Scientific Reports, 11(1), 795. <a href="https://doi.org/10.1038/s41598-020-80842-9">https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p><strong>Severe acute respiratory infections (SARIs)</strong> are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38ºC, a cough, <em>and</em> required hospitalization.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases <em>does not</em> tell us about the total number of people with these infections.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a>  {/ref}</p> <!-- /wp:paragraph --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country.<br>But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. <a href="https://doi.org/10.2196/13403">https://doi.org/10.2196/13403</a><br>This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} <br><br></li><li>The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. <em>Scientific Reports</em>, <em>11</em>(1), 795.<a href="https://doi.org/10.1038/s41598-020-80842-9"> https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:html --> <iframe src="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe> <!-- /wp:html --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"Different flu strains circulate each year","slug":"different-flu-strains-circulate-each-year"} --> <!-- wp:paragraph --> <p>Each year, flu vaccines need to be updated because different viruses circulate in the population.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This happens for two reasons. One is that flu viruses gradually evolve, and can evade people’s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Another reason is that there are different types of flu that circulate each season. </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.<br>Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67–81 (2019). <a href="https://doi.org/10.1038/s41579-018-0115-z">https://doi.org/10.1038/s41579-018-0115-z</a> {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula Júnior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de Gámez, M. J., Pebody, R., Zambon, M., Higueros, R., … the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. <a href="https://doi.org/10.1371/journal.pone.0222381">https://doi.org/10.1371/journal.pone.0222381</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as ‘unknown subtype/lineage’.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. <a href="https://doi.org/10.1186/1741-7015-11-153">https://doi.org/10.1186/1741-7015-11-153</a> {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36–S43. <a href="https://doi.org/10.1093/cid/ciq020">https://doi.org/10.1093/cid/ciq020</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>To address this, the world needs more routine testing for the flu.</p> <!-- /wp:paragraph --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> <br>World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. <a href="https://www.who.int/publications/i/item/9789241549301">https://www.who.int/publications/i/item/9789241549301</a>  {/ref}<br><br></li><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.<br>See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a><br>Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670–2678. <a href="https://doi.org/10.1016/j.vaccine.2019.03.081">https://doi.org/10.1016/j.vaccine.2019.03.081</a> {/ref}</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:html --> <iframe src="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe> <!-- /wp:html --> <!-- /wp:owid/key-insight --> <!-- wp:owid/key-insight {"title":"The Spanish flu caused the largest influenza pandemic in history","slug":"the-spanish-flu-caused-the-largest-influenza-pandemic-in-history"} --> <!-- wp:paragraph --> <p>Some flu seasons are far more severe than usual seasonal influenza.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107–8112. https://doi.org/10.1073/pnas.1324197111{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>As you can see in the chart, it led to the <a href="https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history">most devastating</a> influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). “Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic”. American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMID 30202996. Online <a href="https://academic.oup.com/aje/article/187/12/2561/5092383">here</a>.{/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages <a href="https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history">declined</a> by more than ten years.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. <a href="https://doi.org/10.1371/journal.pone.0069586">https://doi.org/10.1371/journal.pone.0069586</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375–1378. <a href="https://doi.org/10.1086/322662">https://doi.org/10.1086/322662</a> </p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29–34. <a href="https://doi.org/10.1016/j.jtbi.2011.08.003">https://doi.org/10.1016/j.jtbi.2011.08.003</a> {/ref}</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>In this article, we provide more detail:</p> <!-- /wp:paragraph --> <!-- wp:owid/prominent-link {"linkUrl":"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history","className":"is-style-thin"} --> <!-- wp:paragraph --> <p></p> <!-- /wp:paragraph --> <!-- /wp:owid/prominent-link --> <!-- wp:owid/technical-text --> <!-- wp:heading {"level":5} --> <h5>What you should know about this data</h5> <!-- /wp:heading --> <!-- wp:list --> <ul><li>In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 “Spanish” Influenza Pandemic. <em>Bulletin of the History of Medicine</em>, <em>76</em>(1), 105–115. <a href="http://www.jstor.org/stable/44446153">http://www.jstor.org/stable/44446153<br></a>Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. <em>Bulletin of the History of Medicine</em>, <em>65</em>(1), 4–21. <a href="http://www.jstor.org/stable/44447656">http://www.jstor.org/stable/44447656</a><br>Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. <em>American Journal of Epidemiology</em>, <em>187</em>(12), 2561–2567.<a href="https://doi.org/10.1093/aje/kwy191"> https://doi.org/10.1093/aje/kwy191<br></a>Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf<br></a>Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., … Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687–695. <a href="https://doi.org/10.1016/S1473-3099(12)70121-4">https://doi.org/10.1016/S1473-3099(12)70121-4<br></a>Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. <a href="https://doi.org/10.1371/journal.pmed.1001558">https://doi.org/10.1371/journal.pmed.1001558</a> {/ref}<br><br></li><li>Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war. <br><br></li><li>There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher.</li></ul> <!-- /wp:list --> <!-- /wp:owid/technical-text --> <!-- wp:image {"id":57012,"sizeSlug":"large","linkDestination":"none"} --> <figure class="wp-block-image size-large"><img src="https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png" alt="" class="wp-image-57012"/></figure> <!-- /wp:image --> <!-- /wp:owid/key-insight --> <!-- wp:paragraph --> <p></p> <!-- /wp:paragraph --> <!-- /wp:owid/key-insights-slider --> <!-- wp:heading {"level":3} --> <h3>Explore our data on influenza</h3> <!-- /wp:heading --> <!-- wp:html --> <iframe class="wp-block-full-content-width" src="https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere" loading="lazy" style="width: 100%; min-height: 740px; max-height: 950px; height: 100vh; border: 0px none;"></iframe> <!-- /wp:html --> <!-- wp:columns {"className":"is-style-sticky-right"} --> <div class="wp-block-columns is-style-sticky-right"><!-- wp:column --> <div class="wp-block-column"><!-- wp:heading {"level":4} --> <h4>Why we provide this Influenza Data Explorer</h4> <!-- /wp:heading --> <!-- wp:paragraph --> <p>With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus.</p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>It differs from our widely-used infectious diseases projects, such as the <a href="https://ourworldindata.org/explorers/coronavirus-data-explorer">COVID-19 Explorer</a> and the <a href="https://ourworldindata.org/explorers/monkeypox">Mpox Explorer</a>. These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. <strong>This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.</strong></p> <!-- /wp:paragraph --> <!-- wp:paragraph --> <p>The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.</p> <!-- /wp:paragraph --></div> <!-- /wp:column --></div> <!-- /wp:columns --> <!-- wp:owid/research-and-writing --> <!-- wp:group {"className":"wp-block-research-and-writing"} --> <div class="wp-block-group wp-block-research-and-writing"><!-- wp:heading --> <h2 id="research-writing">Research & Writing</h2> <!-- /wp:heading --> <!-- wp:heading {"level":4} --> <h4>Key articles on Influenza</h4> <!-- /wp:heading --> <!-- wp:owid/grid {"className":"research-and-writing__sub-category"} --> <!-- wp:owid/card {"title":"How many people die from the flu?","linkUrl":"https://ourworldindata.org/influenza-deaths","mediaId":53551,"mediaUrl":"https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail.png","mediaAlt":""} --> <!-- wp:paragraph --> <p><em>Saloni Dattani</em></p> <!-- /wp:paragraph --> <!-- /wp:owid/card --> <!-- wp:owid/card {"title":"The Spanish flu: The global impact of the largest influenza pandemic in history","linkUrl":"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history","mediaId":50068,"mediaUrl":"https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27.png","mediaAlt":"Life expectancy - spanish flu"} --> <!-- wp:paragraph --> <p><em>Max Roser</em></p> <!-- /wp:paragraph --> <!-- /wp:owid/card --> <!-- /wp:owid/grid --> <!-- wp:owid/grid {"className":"research-and-writing__sub-category"} /--></div> <!-- /wp:group --> <!-- /wp:owid/research-and-writing --> <!-- wp:owid/all-charts /--> <p></p>	{ "id": "wp-56832", "slug": "influenza", "content": { "toc": [], "body": [ { "left": [ { "type": "text", "value": [ { "text": "Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "type": "sticky-right", "right": [ { "type": "text", "value": [ { "text": "Related topics", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "url": "https://ourworldindata.org/causes-of-death", "children": [ { "text": "Causes of Death", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "url": "https://ourworldindata.org/vaccination", "children": [ { "text": "Vaccination", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "url": "https://ourworldindata.org/coronavirus", "children": [ { "text": "Covid-19", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] } ], "parseErrors": [] } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The flu is estimated to cause ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/influenza-deaths", "children": [ { "text": "400,000 respiratory deaths each year", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ", "spanType": "span-simple-text" }, { "url": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "children": [ { "text": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This shows the mean estimate of annual influenza mortality between 2002\u20132011, excluding the 2009 \u201cSwine flu\u201d pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: ", "spanType": "span-simple-text" }, { "url": "https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf", "children": [ { "text": "https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013 but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate \u201cdoes not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.\u201d", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361\u20131369.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/j.vaccine.2021.11.080", "children": [ { "text": " https://doi.org/10.1016/j.vaccine.2021.11.080", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1001/jamanetworkopen.2022.8873", "children": [ { "text": " https://doi.org/10.1001/jamanetworkopen.2022.8873", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": "{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The flu is ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu", "children": [ { "text": "most severe", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O\u2019Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1098/rsos.211498", "children": [ { "text": "https://doi.org/10.1098/rsos.211498", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "But it\u2019s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In this article, we provide more detail:", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ", "spanType": "span-simple-text" }, { "url": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "children": [ { "text": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013\u00a0which are not included here.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "url": "https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map", "type": "chart", "parseErrors": [] }, { "type": "text", "value": [ { "text": "In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142\u2013151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/j.jet.2021.105293", "children": [ { "text": "https://doi.org/10.1016/j.jet.2021.105293", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., \u2026 Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1371/journal.pone.0253071", "children": [ { "text": "https://doi.org/10.1371/journal.pone.0253071", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This decline was very large because of the mathematics of epidemics.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases ", "spanType": "span-simple-text" }, { "children": [ { "text": "falls", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": " exponentially.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}\u200b\u200bBiggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. ", "spanType": "span-simple-text" }, { "children": [ { "text": "BMC Infectious Diseases", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": ", ", "spanType": "span-simple-text" }, { "children": [ { "text": "14", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": "(1), 480.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1186/1471-2334-14-480", "children": [ { "text": " https://doi.org/10.1186/1471-2334-14-480", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": "{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41467-022-29402-5", "children": [ { "text": "https://doi.org/10.1038/s41467-022-29402-5", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753", "children": [ { "text": "https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} We therefore show the share of tests that were positive for the influenza virus.", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "url": "https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true", "type": "chart", "parseErrors": [] }, { "type": "text", "value": [ { "text": "Over time, ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past", "children": [ { "text": "the severity of the flu has declined", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1007/s13524-019-00809-y", "children": [ { "text": "https://doi.org/10.1007/s13524-019-00809-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/sanitation", "children": [ { "text": "sanitation", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": ", neonatal healthcare, and ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/vaccination", "children": [ { "text": "childhood vaccination", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115\u2013E120. ", "spanType": "span-simple-text" }, { "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/", "children": [ { "text": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. ", "spanType": "span-simple-text" }, { "url": "https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm", "children": [ { "text": "https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1007/s13524-019-00809-y", "children": [ { "text": "https://doi.org/10.1007/s13524-019-00809-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} In lower-income countries, the flu could become a larger burden as ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age", "children": [ { "text": "their populations continue to age", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": ".", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In this article, we provide more detail:", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1007/s13524-019-00809-y", "children": [ { "text": "https://doi.org/10.1007/s13524-019-00809-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "alt": "People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980.", "size": "wide", "type": "image", "filename": "Historical-decline-in-influenza-deaths-by-birth-cohort.png", "parseErrors": [] }, { "type": "text", "value": [ { "text": "Several respiratory infections, including the flu, are more common in the winter.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/nrmicro.2017.118", "children": [ { "text": "https://doi.org/10.1038/nrmicro.2017.118", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In the chart, you can see the share of flu tests that were positive in different countries.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118\u2013124. ", "spanType": "span-simple-text" }, { "url": "https://pubmed.ncbi.nlm.nih.gov/36773717/", "children": [ { "text": "https://pubmed.ncbi.nlm.nih.gov/36773717/", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "spanType": "span-newline" }, { "text": "Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/nrmicro.2017.118", "children": [ { "text": "https://doi.org/10.1038/nrmicro.2017.118", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "spanType": "span-newline" }, { "text": "Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. ", "spanType": "span-simple-text" }, { "children": [ { "text": "Epidemiology & Infection", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": ", ", "spanType": "span-simple-text" }, { "children": [ { "text": "143", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": "(6), 1110\u20131118.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1017/S0950268814002702", "children": [ { "text": " https://doi.org/10.1017/S0950268814002702", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "spanType": "span-link" }, { "text": "Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010\u201319: A retrospective analysis. The Lancet Global Health, 11(5), e729\u2013e739. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/S2214-109X(23)00109-2", "children": [ { "text": "https://doi.org/10.1016/S2214-109X(23)00109-2", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "\u200b\u200bNewman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1371/journal.pone.0193263", "children": [ { "text": "https://doi.org/10.1371/journal.pone.0193263", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "You can see this in the chart for Singapore and Thailand, for example.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "url": "https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true", "type": "chart", "parseErrors": [] }, { "type": "text", "value": [ { "text": "Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases \u2013 and the global data shown in our data explorer \u2013 greatly underestimate the true number of cases.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Because of the lack of direct testing, it is useful to track flu-like symptoms.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "It is important to note however that these symptoms are not specific to the flu: people with other diseases \u2013 such as rhinovirus, COVID-19, common colds, malaria, and others \u2013 can also have these symptoms and meet the following criteria.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "children": [ { "text": "Acute respiratory infections (ARIs)", "spanType": "span-simple-text" } ], "spanType": "span-bold" }, { "text": " are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but ", "spanType": "span-simple-text" }, { "children": [ { "text": "only if", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": " they were judged by a doctor to be caused by an infection.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "children": [ { "text": "Influenza-like illnesses (ILIs)", "spanType": "span-simple-text" } ], "spanType": "span-bold" }, { "text": " are narrower \u2013\u00a0they include only people with a sudden respiratory infection with a fever above 38\u00baC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "text": "Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., \u2026 Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122\u2013128.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "url": "https://doi.org/10.2471/BLT.17.194514", "children": [ { "text": "https://doi.org/10.2471/BLT.17.194514", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "children": [ { "text": " https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. Scientific Reports, 11(1), 795. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41598-020-80842-9", "children": [ { "text": "https://doi.org/10.1038/s41598-020-80842-9", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "children": [ { "text": "Severe acute respiratory infections (SARIs)", "spanType": "span-simple-text" } ], "spanType": "span-bold" }, { "text": " are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38\u00baC, a cough, ", "spanType": "span-simple-text" }, { "children": [ { "text": "and", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": " required hospitalization.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases ", "spanType": "span-simple-text" }, { "children": [ { "text": "does not", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": " tell us about the total number of people with these infections.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": "\u00a0 {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "text": "But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.2196/13403", "children": [ { "text": "https://doi.org/10.2196/13403", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} ", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. ", "spanType": "span-simple-text" }, { "children": [ { "text": "Scientific Reports", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": ", ", "spanType": "span-simple-text" }, { "children": [ { "text": "11", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": "(1), 795.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41598-020-80842-9", "children": [ { "text": " https://doi.org/10.1038/s41598-020-80842-9", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "url": "https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true", "type": "chart", "parseErrors": [] }, { "type": "text", "value": [ { "text": "Each year, flu vaccines need to be updated because different viruses circulate in the population.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This happens for two reasons. One is that flu viruses gradually evolve, and can evade people\u2019s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41572-018-0002-y", "children": [ { "text": "https://doi.org/10.1038/s41572-018-0002-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Another reason is that there are different types of flu that circulate each season.\u00a0", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "text": "Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67\u201381 (2019). ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41579-018-0115-z", "children": [ { "text": "https://doi.org/10.1038/s41579-018-0115-z", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula J\u00fanior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de G\u00e1mez, M. J., Pebody, R., Zambon, M., Higueros, R., \u2026 the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1371/journal.pone.0222381", "children": [ { "text": "https://doi.org/10.1371/journal.pone.0222381", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as \u2018unknown subtype/lineage\u2019.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1186/1741-7015-11-153", "children": [ { "text": "https://doi.org/10.1186/1741-7015-11-153", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36\u2013S43. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1093/cid/ciq020", "children": [ { "text": "https://doi.org/10.1093/cid/ciq020", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "To address this, the world needs more routine testing for the flu.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41572-018-0002-y", "children": [ { "text": "https://doi.org/10.1038/s41572-018-0002-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. ", "spanType": "span-simple-text" }, { "url": "https://www.who.int/publications/i/item/9789241549301", "children": [ { "text": "https://www.who.int/publications/i/item/9789241549301", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": "\u00a0 {/ref}", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "text": "See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. ", "spanType": "span-simple-text" }, { "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "children": [ { "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "spanType": "span-newline" }, { "text": "Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670\u20132678. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/j.vaccine.2019.03.081", "children": [ { "text": "https://doi.org/10.1016/j.vaccine.2019.03.081", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] } ], "parseErrors": [] }, { "url": "https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true", "type": "chart", "parseErrors": [] }, { "type": "text", "value": [ { "text": "Some flu seasons are far more severe than usual seasonal influenza.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1038/s41572-018-0002-y", "children": [ { "text": "https://doi.org/10.1038/s41572-018-0002-y", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107\u20138112. https://doi.org/10.1073/pnas.1324197111{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "As you can see in the chart, it led to the ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history", "children": [ { "text": "most devastating", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). \u201cReassessing the Global Mortality Burden of the 1918 Influenza Pandemic\u201d. American Journal of Epidemiology. 187 (12): 2561\u20132567. doi:10.1093/aje/kwy191. PMID 30202996. Online ", "spanType": "span-simple-text" }, { "url": "https://academic.oup.com/aje/article/187/12/2561/5092383", "children": [ { "text": "here", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": ".{/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages ", "spanType": "span-simple-text" }, { "url": "https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history", "children": [ { "text": "declined", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " by more than ten years.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1371/journal.pone.0069586", "children": [ { "text": "https://doi.org/10.1371/journal.pone.0069586", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375\u20131378. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1086/322662", "children": [ { "text": "https://doi.org/10.1086/322662", "spanType": "span-simple-text" } ], "spanType": "span-link" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29\u201334. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/j.jtbi.2011.08.003", "children": [ { "text": "https://doi.org/10.1016/j.jtbi.2011.08.003", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "In this article, we provide more detail:", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "What you should know about this data", "spanType": "span-simple-text" } ], "type": "heading", "level": 5, "parseErrors": [] }, { "type": "list", "items": [ { "type": "text", "value": [ { "text": "In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 \u201cSpanish\u201d Influenza Pandemic. ", "spanType": "span-simple-text" }, { "children": [ { "text": "Bulletin of the History of Medicine", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": ", ", "spanType": "span-simple-text" }, { "children": [ { "text": "76", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": "(1), 105\u2013115. ", "spanType": "span-simple-text" }, { "url": "http://www.jstor.org/stable/44446153", "children": [ { "text": "http://www.jstor.org/stable/44446153", "spanType": "span-simple-text" }, { "spanType": "span-newline" } ], "spanType": "span-link" }, { "text": "Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. 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", "spanType": "span-simple-text" }, { "children": [ { "text": "American Journal of Epidemiology", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": ", ", "spanType": "span-simple-text" }, { "children": [ { "text": "187", "spanType": "span-simple-text" } ], "spanType": "span-italic" }, { "text": "(12), 2561\u20132567.", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1093/aje/kwy191", "children": [ { "text": " https://doi.org/10.1093/aje/kwy191", "spanType": "span-simple-text" }, { "spanType": "span-newline" } ], "spanType": "span-link" }, { "text": "Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ", "spanType": "span-simple-text" }, { "url": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "children": [ { "text": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf", "spanType": "span-simple-text" }, { "spanType": "span-newline" } ], "spanType": "span-link" }, { "text": "Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., \u2026 Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687\u2013695. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1016/S1473-3099(12)70121-4", "children": [ { "text": "https://doi.org/10.1016/S1473-3099(12)70121-4", "spanType": "span-simple-text" }, { "spanType": "span-newline" } ], "spanType": "span-link" }, { "text": "Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. ", "spanType": "span-simple-text" }, { "url": "https://doi.org/10.1371/journal.pmed.1001558", "children": [ { "text": "https://doi.org/10.1371/journal.pmed.1001558", "spanType": "span-simple-text" } ], "spanType": "span-link" }, { "text": " {/ref}", "spanType": "span-simple-text" }, { "spanType": "span-newline" }, { "spanType": "span-newline" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. 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These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. ", "spanType": "span-simple-text" }, { "children": [ { "text": "This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.", "spanType": "span-simple-text" } ], "spanType": "span-bold" } ], "parseErrors": [] }, { "type": "text", "value": [ { "text": "The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.", "spanType": "span-simple-text" } ], "parseErrors": [] }, { "text": [ { "text": "Research & Writing", "spanType": "span-simple-text" } ], "type": "heading", "level": 2, "parseErrors": [] }, { "text": [ { "text": "Key articles on Influenza", "spanType": "span-simple-text" } ], "type": "heading", "level": 4, "parseErrors": [] }, { "type": "text", "value": [ { "children": [ { "text": "Saloni Dattani", "spanType": "span-simple-text" } ], "spanType": "span-italic" } ], "parseErrors": [] }, { "type": "text", "value": [ { "children": [ { "text": "Max Roser", "spanType": "span-simple-text" } ], "spanType": "span-italic" } ], "parseErrors": [] } ], "type": "article", "title": "Influenza", "authors": [ "Saloni Dattani", "Fiona Spooner", "Edouard Mathieu", "Hannah Ritchie", "Max Roser" ], "excerpt": "Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with 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epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.	2023-05-10 09:34:09	2023-06-14 20:17:51	https://ourworldindata.org/wp-content/uploads/2023/05/flu-topic-page-thumbnail.png	{ "toc": false, "bodyClassName": "topic-page" }	Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher. Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population. This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world. It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus. Related topics * [Causes of Death](https://ourworldindata.org/causes-of-death) * [Vaccination](https://ourworldindata.org/vaccination) * [Covid-19](https://ourworldindata.org/coronavirus) The flu is estimated to cause [400,000 respiratory deaths each year](https://ourworldindata.org/influenza-deaths) on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf](https://jogh.org/documents/issue201902/jogh-09-020421.pdf) This shows the mean estimate of annual influenza mortality between 2002–2011, excluding the 2009 “Swine flu” pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: [https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf](https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf) {/ref} People also die from other complications of the flu – such as a stroke or heart attack – but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear. Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate “does not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.” In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly. This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu. Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361–1369.[ https://doi.org/10.1016/j.vaccine.2021.11.080](https://doi.org/10.1016/j.vaccine.2021.11.080) Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.[ https://doi.org/10.1001/jamanetworkopen.2022.8873](https://doi.org/10.1001/jamanetworkopen.2022.8873){/ref} The flu is [most severe](https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu) in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O’Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. [https://doi.org/10.1098/rsos.211498](https://doi.org/10.1098/rsos.211498) {/ref} Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map. But it’s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates. In this article, we provide more detail: ##### What you should know about this data The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf](https://jogh.org/documents/issue201902/jogh-09-020421.pdf) {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms. These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu – such as a stroke or heart attack – which are not included here. * Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records. <Chart url="https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map"/> In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing. You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low. Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142–151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. [https://doi.org/10.1016/j.jet.2021.105293](https://doi.org/10.1016/j.jet.2021.105293) Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., … Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. [https://doi.org/10.1371/journal.pone.0253071](https://doi.org/10.1371/journal.pone.0253071) {/ref} This decline was very large because of the mathematics of epidemics. The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref} However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases _falls_ exponentially. Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}Biggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. _BMC Infectious Diseases_, _14_(1), 480.[ https://doi.org/10.1186/1471-2334-14-480](https://doi.org/10.1186/1471-2334-14-480){/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata. Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. [https://doi.org/10.1038/s41467-022-29402-5](https://doi.org/10.1038/s41467-022-29402-5) Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). [https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753](https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753) {/ref} ##### What you should know about this data * Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} We therefore show the share of tests that were positive for the influenza virus. <Chart url="https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Over time, [the severity of the flu has declined](https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past) among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in [sanitation](https://ourworldindata.org/sanitation), neonatal healthcare, and [childhood vaccination](https://ourworldindata.org/vaccination) for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115–E120. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/) Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. [https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm](https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm) {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza. But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates. Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} In lower-income countries, the flu could become a larger burden as [their populations continue to age](https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age). In this article, we provide more detail: ##### What you should know about this data * These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} * The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population. * The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable. <Image filename="Historical-decline-in-influenza-deaths-by-birth-cohort.png" alt="People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980."/> Several respiratory infections, including the flu, are more common in the winter. This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. [https://doi.org/10.1038/nrmicro.2017.118](https://doi.org/10.1038/nrmicro.2017.118) {/ref} In the chart, you can see the share of flu tests that were positive in different countries. Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October. But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118–124. [https://pubmed.ncbi.nlm.nih.gov/36773717/](https://pubmed.ncbi.nlm.nih.gov/36773717/) Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. [https://doi.org/10.1038/nrmicro.2017.118](https://doi.org/10.1038/nrmicro.2017.118) Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. _Epidemiology & Infection_, _143_(6), 1110–1118.[ https://doi.org/10.1017/S0950268814002702 ](https://doi.org/10.1017/S0950268814002702)Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010–19: A retrospective analysis. The Lancet Global Health, 11(5), e729–e739. [https://doi.org/10.1016/S2214-109X(23)00109-2](https://doi.org/10.1016/S2214-109X(23)00109-2) Newman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. [https://doi.org/10.1371/journal.pone.0193263](https://doi.org/10.1371/journal.pone.0193263) {/ref} You can see this in the chart for Singapore and Thailand, for example. ##### What you should know about this data * Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases – and the global data shown in our data explorer – greatly underestimate the true number of cases. Because of the lack of direct testing, it is useful to track flu-like symptoms. It is important to note however that these symptoms are not specific to the flu: people with other diseases – such as rhinovirus, COVID-19, common colds, malaria, and others – can also have these symptoms and meet the following criteria. Acute respiratory infections (ARIs) are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but _only if_ they were judged by a doctor to be caused by an infection. Influenza-like illnesses (ILIs) are narrower – they include only people with a sudden respiratory infection with a fever above 38ºC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it. Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., … Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122–128. [https://doi.org/10.2471/BLT.17.194514](https://doi.org/10.2471/BLT.17.194514) World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.[ https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. Scientific Reports, 11(1), 795. [https://doi.org/10.1038/s41598-020-80842-9](https://doi.org/10.1038/s41598-020-80842-9) {/ref} Severe acute respiratory infections (SARIs) are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38ºC, a cough, _and_ required hospitalization. In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases _does not_ tell us about the total number of people with these infections. Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. [https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) {/ref} ##### What you should know about this data * Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country. But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. [https://doi.org/10.2196/13403](https://doi.org/10.2196/13403) This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} * The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. [https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. _Scientific Reports_, _11_(1), 795.[ https://doi.org/10.1038/s41598-020-80842-9](https://doi.org/10.1038/s41598-020-80842-9) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Each year, flu vaccines need to be updated because different viruses circulate in the population. This happens for two reasons. One is that flu viruses gradually evolve, and can evade people’s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) {/ref} Another reason is that there are different types of flu that circulate each season. You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref} There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2. Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67–81 (2019). [https://doi.org/10.1038/s41579-018-0115-z](https://doi.org/10.1038/s41579-018-0115-z) {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula Júnior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de Gámez, M. J., Pebody, R., Zambon, M., Higueros, R., … the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. [https://doi.org/10.1371/journal.pone.0222381](https://doi.org/10.1371/journal.pone.0222381) {/ref} Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as ‘unknown subtype/lineage’.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain. However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref} This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. [https://doi.org/10.1186/1741-7015-11-153](https://doi.org/10.1186/1741-7015-11-153) {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36–S43. [https://doi.org/10.1093/cid/ciq020](https://doi.org/10.1093/cid/ciq020) {/ref} To address this, the world needs more routine testing for the flu. ##### What you should know about this data * This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. [https://www.who.int/publications/i/item/9789241549301](https://www.who.int/publications/i/item/9789241549301) {/ref} * Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies. See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670–2678. [https://doi.org/10.1016/j.vaccine.2019.03.081](https://doi.org/10.1016/j.vaccine.2019.03.081) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Some flu seasons are far more severe than usual seasonal influenza. This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) {/ref} For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107–8112. https://doi.org/10.1073/pnas.1324197111{/ref} As you can see in the chart, it led to the [most devastating](https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history) influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). “Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic”. American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMID 30202996. Online [here](https://academic.oup.com/aje/article/187/12/2561/5092383).{/ref} The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages [declined](https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history) by more than ten years. But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. [https://doi.org/10.1371/journal.pone.0069586](https://doi.org/10.1371/journal.pone.0069586) Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375–1378. [https://doi.org/10.1086/322662](https://doi.org/10.1086/322662) Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29–34. [https://doi.org/10.1016/j.jtbi.2011.08.003](https://doi.org/10.1016/j.jtbi.2011.08.003) {/ref} In this article, we provide more detail: ##### What you should know about this data * In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 “Spanish” Influenza Pandemic. _Bulletin of the History of Medicine_, _76_(1), 105–115. [http://www.jstor.org/stable/44446153 ](http://www.jstor.org/stable/44446153)Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. _Bulletin of the History of Medicine_, _65_(1), 4–21. [http://www.jstor.org/stable/44447656](http://www.jstor.org/stable/44447656) Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. _American Journal of Epidemiology_, _187_(12), 2561–2567.[ https://doi.org/10.1093/aje/kwy191 ](https://doi.org/10.1093/aje/kwy191)Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf ](https://jogh.org/documents/issue201902/jogh-09-020421.pdf)Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., … Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687–695. [https://doi.org/10.1016/S1473-3099(12)70121-4 ](https://doi.org/10.1016/S1473-3099(12)70121-4)Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. [https://doi.org/10.1371/journal.pmed.1001558](https://doi.org/10.1371/journal.pmed.1001558) {/ref} Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war. * There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher. <Image filename="Influenza-pandemics-in-comparison-1.png" alt=""/> ### Explore our data on influenza <Chart url="https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere"/> #### Why we provide this Influenza Data Explorer With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus. It differs from our widely-used infectious diseases projects, such as the [COVID-19 Explorer](https://ourworldindata.org/explorers/coronavirus-data-explorer) and the [Mpox Explorer](https://ourworldindata.org/explorers/monkeypox). These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization. The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting. ## Research & Writing #### Key articles on Influenza _Saloni Dattani_ _Max Roser_	{ "id": 56832, "date": "2023-05-18T09:46:00", "guid": { "rendered": "https://owid.cloud/?page_id=56832" }, "link": "https://owid.cloud/influenza", "meta": { "owid_publication_context_meta_field": [], "owid_key_performance_indicators_meta_field": [] }, "slug": "influenza", "tags": [], "type": "page", "title": { "rendered": "Influenza" }, "_links": { "self": [ { "href": "https://owid.cloud/wp-json/wp/v2/pages/56832" } ], "about": [ { "href": "https://owid.cloud/wp-json/wp/v2/types/page" } ], "author": [ { "href": "https://owid.cloud/wp-json/wp/v2/users/47", "embeddable": true } ], "curies": [ { "href": "https://api.w.org/{rel}", "name": "wp", "templated": true } ], "replies": [ { "href": "https://owid.cloud/wp-json/wp/v2/comments?post=56832", "embeddable": true } ], "wp:term": [ { "href": "https://owid.cloud/wp-json/wp/v2/categories?post=56832", "taxonomy": "category", "embeddable": true }, { "href": "https://owid.cloud/wp-json/wp/v2/tags?post=56832", "taxonomy": "post_tag", "embeddable": true } ], "collection": [ { "href": "https://owid.cloud/wp-json/wp/v2/pages" } ], "wp:attachment": [ { "href": "https://owid.cloud/wp-json/wp/v2/media?parent=56832" } ], "version-history": [ { "href": "https://owid.cloud/wp-json/wp/v2/pages/56832/revisions", "count": 28 } ], "wp:featuredmedia": [ { "href": "https://owid.cloud/wp-json/wp/v2/media/56858", "embeddable": true } ], "predecessor-version": [ { "id": 57395, "href": "https://owid.cloud/wp-json/wp/v2/pages/56832/revisions/57395" } ] }, "author": 47, "parent": 0, "status": "publish", "content": { "rendered": "\n<!-- formatting-options toc:false bodyClassName:topic-page -->\n\n\n\t<div class=\"sticky-nav-contents\">\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#introduction\"><span class=\"wp-block-navigation-item__label\">Introduction</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#key-insights\"><span class=\"wp-block-navigation-item__label\">Key Insights</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#explore-our-data-on-influenza\" rel=\"\" title=\"\"><span class=\"wp-block-navigation-item__label\">Data Explorer</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#research-writing\"><span class=\"wp-block-navigation-item__label\">Research & Writing</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#interactive-charts-on-influenza\"><span class=\"wp-block-navigation-item__label\">Charts</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#endnotes\"><span class=\"wp-block-navigation-item__label\">Endnotes</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#citation\"><span class=\"wp-block-navigation-item__label\">Cite This Work</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\" href=\"#licence\"><span class=\"wp-block-navigation-item__label\">Reuse This Work</span></a></li>\n</div>\n\n\n\n<div class=\"wp-block-columns front-matter\">\n<div class=\"wp-block-column\" id=\"introduction\">\n<p>Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher. </p>\n\n\n\n<p>Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.</p>\n\n\n\n<p>This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.</p>\n\n\n\n<p>It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.</p>\n</div>\n\n\n\n<div class=\"wp-block-column\">\n<div class=\"wp-container-65e6e283dfe6c wp-block-group related-topics\">\n<p>Related topics</p>\n\n\n\n<ul><li><a href=\"https://ourworldindata.org/causes-of-death\">Causes of Death</a></li><li><a href=\"https://ourworldindata.org/vaccination\">Vaccination</a></li><li><a href=\"https://ourworldindata.org/coronavirus\">Covid-19</a></li></ul>\n</div>\n</div>\n</div>\n\n\n\n\t<block type=\"key-insights\">\n\t\t<title>Key insights on Influenza</title>\n <slug>key-insights</slug>\n <insights>\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Seasonal flu kills hundreds of thousands of people worldwide each year</title>\n <slug>seasonal-flu-kills-hundreds-of-thousands-of-people-worldwide-each-year</slug>\n <content>\n\n<p>The flu is estimated to cause <a href=\"https://ourworldindata.org/influenza-deaths\">400,000 respiratory deaths each year</a> on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> </p>\n\n\n\n<p>This shows the mean estimate of annual influenza mortality between 2002\u20132011, excluding the 2009 \u201cSwine flu\u201d pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: <a href=\"https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf\">https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf</a> {/ref} </p>\n\n\n\n<p>People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013 but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.</p>\n\n\n\n<p>Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate \u201cdoes not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.\u201d</p>\n\n\n\n<p>In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.</p>\n\n\n\n<p>This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu.</p>\n\n\n\n<p>Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361\u20131369.<a href=\"https://doi.org/10.1016/j.vaccine.2021.11.080\"> https://doi.org/10.1016/j.vaccine.2021.11.080</a></p>\n\n\n\n<p>Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.<a href=\"https://doi.org/10.1001/jamanetworkopen.2022.8873\"> https://doi.org/10.1001/jamanetworkopen.2022.8873</a>{/ref}</p>\n\n\n\n<p>The flu is <a href=\"https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu\">most severe</a> in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O\u2019Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. <a href=\"https://doi.org/10.1098/rsos.211498\">https://doi.org/10.1098/rsos.211498</a> {/ref}</p>\n\n\n\n<p>Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.</p>\n\n\n\n<p>But it\u2019s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n <block type=\"prominent-link\" style=\"is-style-thin\">\n <link-url>https://ourworldindata.org/influenza-deaths</link-url>\n <title></title>\n <content>\n\n<p></p>\n\n</content>\n <figure></figure>\n </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.<br><br></li><li>These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013 which are not included here.<br><br></li><li>Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Social distancing during COVID-19 had a large impact on the flu</title>\n <slug>social-distancing-during-covid-19-had-a-large-impact-on-the-flu</slug>\n <content>\n\n<p>In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing. </p>\n\n\n\n<p>You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.</p>\n\n\n\n<p>Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142\u2013151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. <a href=\"https://doi.org/10.1016/j.jet.2021.105293\">https://doi.org/10.1016/j.jet.2021.105293</a> </p>\n\n\n\n<p>Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., \u2026 Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. <a href=\"https://doi.org/10.1371/journal.pone.0253071\">https://doi.org/10.1371/journal.pone.0253071</a> {/ref}</p>\n\n\n\n<p>This decline was very large because of the mathematics of epidemics. </p>\n\n\n\n<p>The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}</p>\n\n\n\n<p>However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases <em>falls</em> exponentially.</p>\n\n\n\n<p>Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}\u200b\u200bBiggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. <em>BMC Infectious Diseases</em>, <em>14</em>(1), 480.<a href=\"https://doi.org/10.1186/1471-2334-14-480\"> https://doi.org/10.1186/1471-2334-14-480</a>{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.</p>\n\n\n\n<p>Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. <a href=\"https://doi.org/10.1038/s41467-022-29402-5\">https://doi.org/10.1038/s41467-022-29402-5</a><br>Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). <a href=\"https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753\">https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753</a> {/ref}</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} We therefore show the share of tests that were positive for the influenza virus.</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Seasonal flu used to be far more deadly</title>\n <slug>seasonal-flu-used-to-be-far-more-deatly</slug>\n <content>\n\n<p>Over time, <a href=\"https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past\">the severity of the flu has declined</a> among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}</p>\n\n\n\n<p>This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in <a href=\"https://ourworldindata.org/sanitation\">sanitation</a>, neonatal healthcare, and <a href=\"https://ourworldindata.org/vaccination\">childhood vaccination</a> for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115\u2013E120. <a href=\"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/\">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/</a> </p>\n\n\n\n<p>Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. <a href=\"https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm\">https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm</a> {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.</p>\n\n\n\n<p>But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates. </p>\n\n\n\n<p>Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref} In lower-income countries, the flu could become a larger burden as <a href=\"https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age\">their populations continue to age</a>.</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n <block type=\"prominent-link\" style=\"is-style-thin\">\n <link-url>https://ourworldindata.org/influenza-deaths</link-url>\n <title></title>\n <content>\n\n<p></p>\n\n</content>\n <figure></figure>\n </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}<br><br></li><li>The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.<br><br></li><li>The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.</li></ul>\n\n\n\t</div>\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"635\" height=\"550\" src=\"https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png\" alt=\"People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980.\" class=\"wp-image-56833\" srcset=\"https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png 635w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-400x346.png 400w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-150x130.png 150w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-768x665.png 768w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort.png 1252w\" sizes=\"(max-width: 635px) 100vw, 635px\" /></figure>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Flu seasons vary between countries</title>\n <slug>flu-seasons-vary-between-countries</slug>\n <content>\n\n<p>Several respiratory infections, including the flu, are more common in the winter. </p>\n\n\n\n<p>This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. <a href=\"https://doi.org/10.1038/nrmicro.2017.118\">https://doi.org/10.1038/nrmicro.2017.118</a> {/ref}</p>\n\n\n\n<p>In the chart, you can see the share of flu tests that were positive in different countries.</p>\n\n\n\n<p>Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October. </p>\n\n\n\n<p>But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118\u2013124. <a href=\"https://pubmed.ncbi.nlm.nih.gov/36773717/\">https://pubmed.ncbi.nlm.nih.gov/36773717/</a> <br><br>Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. <a href=\"https://doi.org/10.1038/nrmicro.2017.118\">https://doi.org/10.1038/nrmicro.2017.118</a><br><br>Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. <em>Epidemiology & Infection</em>, <em>143</em>(6), 1110\u20131118.<a href=\"https://doi.org/10.1017/S0950268814002702\"> https://doi.org/10.1017/S0950268814002702<br><br></a>Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010\u201319: A retrospective analysis. The Lancet Global Health, 11(5), e729\u2013e739. <a href=\"https://doi.org/10.1016/S2214-109X(23)00109-2\">https://doi.org/10.1016/S2214-109X(23)00109-2</a> </p>\n\n\n\n<p>\u200b\u200bNewman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. <a href=\"https://doi.org/10.1371/journal.pone.0193263\">https://doi.org/10.1371/journal.pone.0193263</a> {/ref}</p>\n\n\n\n<p>You can see this in the chart for Singapore and Thailand, for example.</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Tracking flu-like symptoms can be informative when testing is limited</title>\n <slug>tracking-flu-like-symptoms-can-be-informative-when-testing-is-limited</slug>\n <content>\n\n<p>Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases \u2013 and the global data shown in our data explorer \u2013 greatly underestimate the true number of cases.</p>\n\n\n\n<p>Because of the lack of direct testing, it is useful to track flu-like symptoms.</p>\n\n\n\n<p>It is important to note however that these symptoms are not specific to the flu: people with other diseases \u2013 such as rhinovirus, COVID-19, common colds, malaria, and others \u2013 can also have these symptoms and meet the following criteria.</p>\n\n\n\n<p><strong>Acute respiratory infections (ARIs)</strong> are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but <em>only if</em> they were judged by a doctor to be caused by an infection.</p>\n\n\n\n<p><strong>Influenza-like illnesses (ILIs)</strong> are narrower \u2013 they include only people with a sudden respiratory infection with a fever above 38\u00baC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.<br>Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., \u2026 Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122\u2013128.<br><a href=\"https://doi.org/10.2471/BLT.17.194514\">https://doi.org/10.2471/BLT.17.194514</a><br>World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.<a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y\"> https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. Scientific Reports, 11(1), 795. <a href=\"https://doi.org/10.1038/s41598-020-80842-9\">https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</p>\n\n\n\n<p><strong>Severe acute respiratory infections (SARIs)</strong> are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38\u00baC, a cough, <em>and</em> required hospitalization.</p>\n\n\n\n<p>In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases <em>does not</em> tell us about the total number of people with these infections.</p>\n\n\n\n<p>Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y\">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a>  {/ref}</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the ‘ILINet’ system, which is connected to many clinics across the country.<br>But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. <a href=\"https://doi.org/10.2196/13403\">https://doi.org/10.2196/13403</a><br>This means ‘non-sentinel’ data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} <br><br></li><li>The country’s sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y\">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. <em>Scientific Reports</em>, <em>11</em>(1), 795.<a href=\"https://doi.org/10.1038/s41598-020-80842-9\"> https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Different flu strains circulate each year</title>\n <slug>different-flu-strains-circulate-each-year</slug>\n <content>\n\n<p>Each year, flu vaccines need to be updated because different viruses circulate in the population.</p>\n\n\n\n<p>This happens for two reasons. One is that flu viruses gradually evolve, and can evade people\u2019s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>\n\n\n\n<p>Another reason is that there are different types of flu that circulate each season. </p>\n\n\n\n<p>You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}</p>\n\n\n\n<p>There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.<br>Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67\u201381 (2019). <a href=\"https://doi.org/10.1038/s41579-018-0115-z\">https://doi.org/10.1038/s41579-018-0115-z</a> {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula J\u00fanior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de G\u00e1mez, M. J., Pebody, R., Zambon, M., Higueros, R., \u2026 the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. <a href=\"https://doi.org/10.1371/journal.pone.0222381\">https://doi.org/10.1371/journal.pone.0222381</a> {/ref}</p>\n\n\n\n<p>Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as \u2018unknown subtype/lineage\u2019.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.</p>\n\n\n\n<p>However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}</p>\n\n\n\n<p>This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. <a href=\"https://doi.org/10.1186/1741-7015-11-153\">https://doi.org/10.1186/1741-7015-11-153</a> {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36\u2013S43. <a href=\"https://doi.org/10.1093/cid/ciq020\">https://doi.org/10.1093/cid/ciq020</a> {/ref}</p>\n\n\n\n<p>To address this, the world needs more routine testing for the flu.</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a> <br>World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. <a href=\"https://www.who.int/publications/i/item/9789241549301\">https://www.who.int/publications/i/item/9789241549301</a>  {/ref}<br><br></li><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.<br>See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a><br>Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670\u20132678. <a href=\"https://doi.org/10.1016/j.vaccine.2019.03.081\">https://doi.org/10.1016/j.vaccine.2019.03.081</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">The Spanish flu caused the largest influenza pandemic in history</title>\n <slug>the-spanish-flu-caused-the-largest-influenza-pandemic-in-history</slug>\n <content>\n\n<p>Some flu seasons are far more severe than usual seasonal influenza.</p>\n\n\n\n<p>This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>\n\n\n\n<p>For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107\u20138112. https://doi.org/10.1073/pnas.1324197111{/ref}</p>\n\n\n\n<p>As you can see in the chart, it led to the <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">most devastating</a> influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). \u201cReassessing the Global Mortality Burden of the 1918 Influenza Pandemic\u201d. American Journal of Epidemiology. 187 (12): 2561\u20132567. doi:10.1093/aje/kwy191. PMID 30202996. Online <a href=\"https://academic.oup.com/aje/article/187/12/2561/5092383\">here</a>.{/ref}</p>\n\n\n\n<p>The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">declined</a> by more than ten years.</p>\n\n\n\n<p>But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. <a href=\"https://doi.org/10.1371/journal.pone.0069586\">https://doi.org/10.1371/journal.pone.0069586</a> </p>\n\n\n\n<p>Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375\u20131378. <a href=\"https://doi.org/10.1086/322662\">https://doi.org/10.1086/322662</a> </p>\n\n\n\n<p>Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29\u201334. <a href=\"https://doi.org/10.1016/j.jtbi.2011.08.003\">https://doi.org/10.1016/j.jtbi.2011.08.003</a> {/ref}</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n <block type=\"prominent-link\" style=\"is-style-thin\">\n <link-url>https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history</link-url>\n <title></title>\n <content>\n\n<p></p>\n\n</content>\n <figure></figure>\n </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 \u201cSpanish\u201d Influenza Pandemic. <em>Bulletin of the History of Medicine</em>, <em>76</em>(1), 105\u2013115. <a href=\"http://www.jstor.org/stable/44446153\">http://www.jstor.org/stable/44446153<br></a>Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. <em>Bulletin of the History of Medicine</em>, <em>65</em>(1), 4\u201321. <a href=\"http://www.jstor.org/stable/44447656\">http://www.jstor.org/stable/44447656</a><br>Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. <em>American Journal of Epidemiology</em>, <em>187</em>(12), 2561\u20132567.<a href=\"https://doi.org/10.1093/aje/kwy191\"> https://doi.org/10.1093/aje/kwy191<br></a>Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf<br></a>Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., \u2026 Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687\u2013695. <a href=\"https://doi.org/10.1016/S1473-3099(12)70121-4\">https://doi.org/10.1016/S1473-3099(12)70121-4<br></a>Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. <a href=\"https://doi.org/10.1371/journal.pmed.1001558\">https://doi.org/10.1371/journal.pmed.1001558</a> {/ref}<br><br></li><li>Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war. <br><br></li><li>There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher.</li></ul>\n\n\n\t</div>\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"750\" height=\"550\" src=\"https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png\" alt=\"\" class=\"wp-image-57012\" srcset=\"https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png 750w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-400x293.png 400w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-150x110.png 150w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-768x563.png 768w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-1536x1126.png 1536w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-2048x1502.png 2048w\" sizes=\"(max-width: 750px) 100vw, 750px\" /></figure>\n\n</content>\n\t</block>\n\n\n<p></p>\n\n</insights>\n\t</block>\n\n\n<h3>Explore our data on influenza</h3>\n\n\n\n<iframe class=\"wp-block-full-content-width\" src=\"https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere\" loading=\"lazy\" style=\"width: 100%; min-height: 740px; max-height: 950px; height: 100vh; border: 0px none;\"></iframe>\n\n\n\n<div class=\"wp-block-columns is-style-sticky-right\">\n<div class=\"wp-block-column\">\n<h4>Why we provide this Influenza Data Explorer</h4>\n\n\n\n<p>With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus.</p>\n\n\n\n<p>It differs from our widely-used infectious diseases projects, such as the <a href=\"https://ourworldindata.org/explorers/coronavirus-data-explorer\">COVID-19 Explorer</a> and the <a href=\"https://ourworldindata.org/explorers/monkeypox\">Mpox Explorer</a>. These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. <strong>This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.</strong></p>\n\n\n\n<p>The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.</p>\n</div>\n</div>\n\n\n\n\n<div class=\"wp-container-65e6e284044bb wp-block-group wp-block-research-and-writing\">\n<h2 id=\"research-writing\">Research & Writing</h2>\n\n\n\n<h4>Key articles on Influenza</h4>\n\n\n\t<div class=\"wp-block-owid-grid research-and-writing__sub-category\">\n\t\t\n <div class=\"wp-block-owid-card with-image\" data-no-lightbox>\n <a href=\"https://ourworldindata.org/influenza-deaths\">\n <figure><img width=\"768\" height=\"303\" src=\"https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-768x303.png\" class=\"attachment-medium_large size-medium_large\" alt=\"\" loading=\"lazy\" srcset=\"https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-768x303.png 768w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-400x158.png 400w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-800x316.png 800w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-150x59.png 150w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-1536x606.png 1536w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail.png 1592w\" sizes=\"(max-width: 768px) 100vw, 768px\" /></figure>\n <div class=\"text-wrapper\">\n <div class=\"title\">How many people die from the flu?</div class=\"title\">\n <div class=\"description\">\n \n\n<p><em>Saloni Dattani</em></p>\n\n\n </div>\n </div>\n </a>\n </div>\n\n <div class=\"wp-block-owid-card with-image\" data-no-lightbox>\n <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">\n <figure><img width=\"768\" height=\"443\" src=\"https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-768x443.png\" class=\"attachment-medium_large size-medium_large\" alt=\"Life expectancy - spanish flu\" loading=\"lazy\" srcset=\"https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-768x443.png 768w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-400x231.png 400w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-800x461.png 800w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-150x87.png 150w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-1536x886.png 1536w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27.png 1720w\" sizes=\"(max-width: 768px) 100vw, 768px\" /></figure>\n <div class=\"text-wrapper\">\n <div class=\"title\">The Spanish flu: The global impact of the largest influenza pandemic in history</div class=\"title\">\n <div class=\"description\">\n \n\n<p><em>Max Roser</em></p>\n\n\n </div>\n </div>\n </a>\n </div>\n\n\t</div>\n\n\t<div class=\"wp-block-owid-grid research-and-writing__sub-category\">\n\t\t\n\t</div></div>\n\n\n\n\n\n\t<block type=\"all-charts\"></block>\n\n<p></p>", "protected": false }, "excerpt": { "rendered": "Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.", "protected": false }, "date_gmt": "2023-05-18T08:46:00", "modified": "2023-06-14T21:17:51", "template": "", "categories": [ 44, 46, 171 ], "menu_order": 17, "ping_status": "closed", "authors_name": [ "Saloni Dattani", "Fiona Spooner", "Edouard Mathieu", "Hannah Ritchie", "Max Roser" ], "modified_gmt": "2023-06-14T20:17:51", "comment_status": "closed", "featured_media": 56858, "featured_media_paths": { "thumbnail": "/app/uploads/2023/05/flu-topic-page-thumbnail-150x79.png", "medium_large": "/app/uploads/2023/05/flu-topic-page-thumbnail-768x403.png" } }

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<div class="wp-block-columns front-matter"> <div class="wp-block-column" id="introduction"> <p>Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher. </p>   <p>Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.</p>   <p>This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.</p>   <p>It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.</p> </div>   <div class="wp-block-column"> <div class="wp-block-group related-topics"> <p>Related topics</p>   <ul><li><a href="https://ourworldindata.org/causes-of-death">Causes of Death</a></li><li><a href="https://ourworldindata.org/vaccination">Vaccination</a></li><li><a href="https://ourworldindata.org/coronavirus">Covid-19</a></li></ul> </div> </div> </div>      <p>The flu is estimated to cause <a href="https://ourworldindata.org/influenza-deaths">400,000 respiratory deaths each year</a> on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> </p>   <p>This shows the mean estimate of annual influenza mortality between 2002–2011, excluding the 2009 “Swine flu” pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: <a href="https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf">https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf</a> {/ref} </p>   <p>People also die from other complications of the flu – such as a stroke or heart attack – but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.</p>   <p>Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate “does not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.”</p>   <p>In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.</p>   <p>This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu.</p>   <p>Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361–1369.<a href="https://doi.org/10.1016/j.vaccine.2021.11.080"> https://doi.org/10.1016/j.vaccine.2021.11.080</a></p>   <p>Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.<a href="https://doi.org/10.1001/jamanetworkopen.2022.8873"> https://doi.org/10.1001/jamanetworkopen.2022.8873</a>{/ref}</p>   <p>The flu is <a href="https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu">most severe</a> in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O’Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. <a href="https://doi.org/10.1098/rsos.211498">https://doi.org/10.1098/rsos.211498</a> {/ref}</p>   <p>Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.</p>   <p>But it’s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.</p>   <p>In this article, we provide more detail:</p>    <p></p>     <h5>What you should know about this data</h5>   <ul><li>The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.<br><br></li><li>These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu – such as a stroke or heart attack – which are not included here.<br><br></li><li>Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.</li></ul>    <iframe src="https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe>     <p>In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing. </p>   <p>You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.</p>   <p>Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142–151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. <a href="https://doi.org/10.1016/j.jet.2021.105293">https://doi.org/10.1016/j.jet.2021.105293</a> </p>   <p>Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., … Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. <a href="https://doi.org/10.1371/journal.pone.0253071">https://doi.org/10.1371/journal.pone.0253071</a> {/ref}</p>   <p>This decline was very large because of the mathematics of epidemics. </p>   <p>The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}</p>   <p>However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases <em>falls</em> exponentially.</p>   <p>Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}Biggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. <em>BMC Infectious Diseases</em>, <em>14</em>(1), 480.<a href="https://doi.org/10.1186/1471-2334-14-480"> https://doi.org/10.1186/1471-2334-14-480</a>{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.</p>   <p>Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. <a href="https://doi.org/10.1038/s41467-022-29402-5">https://doi.org/10.1038/s41467-022-29402-5</a><br>Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). <a href="https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753">https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753</a> {/ref}</p>    <h5>What you should know about this data</h5>   <ul><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} We therefore show the share of tests that were positive for the influenza virus.</li></ul>    <iframe src="https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe>     <p>Over time, <a href="https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past">the severity of the flu has declined</a> among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}</p>   <p>This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in <a href="https://ourworldindata.org/sanitation">sanitation</a>, neonatal healthcare, and <a href="https://ourworldindata.org/vaccination">childhood vaccination</a> for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115–E120. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/</a> </p>   <p>Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. <a href="https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm">https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm</a> {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.</p>   <p>But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates. </p>   <p>Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref} In lower-income countries, the flu could become a larger burden as <a href="https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age">their populations continue to age</a>.</p>   <p>In this article, we provide more detail:</p>    <p></p>     <h5>What you should know about this data</h5>   <ul><li>These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. <a href="https://doi.org/10.1007/s13524-019-00809-y">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}<br><br></li><li>The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.<br><br></li><li>The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.</li></ul>    <figure class="wp-block-image size-large"><img src="https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png" alt="People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980." class="wp-image-56833"/></figure>     <p>Several respiratory infections, including the flu, are more common in the winter. </p>   <p>This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. <a href="https://doi.org/10.1038/nrmicro.2017.118">https://doi.org/10.1038/nrmicro.2017.118</a> {/ref}</p>   <p>In the chart, you can see the share of flu tests that were positive in different countries.</p>   <p>Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October. </p>   <p>But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118–124. <a href="https://pubmed.ncbi.nlm.nih.gov/36773717/">https://pubmed.ncbi.nlm.nih.gov/36773717/</a> <br><br>Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. <a href="https://doi.org/10.1038/nrmicro.2017.118">https://doi.org/10.1038/nrmicro.2017.118</a><br><br>Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. <em>Epidemiology & Infection</em>, <em>143</em>(6), 1110–1118.<a href="https://doi.org/10.1017/S0950268814002702"> https://doi.org/10.1017/S0950268814002702<br><br></a>Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010–19: A retrospective analysis. The Lancet Global Health, 11(5), e729–e739. <a href="https://doi.org/10.1016/S2214-109X(23)00109-2">https://doi.org/10.1016/S2214-109X(23)00109-2</a> </p>   <p>Newman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. <a href="https://doi.org/10.1371/journal.pone.0193263">https://doi.org/10.1371/journal.pone.0193263</a> {/ref}</p>   <p>You can see this in the chart for Singapore and Thailand, for example.</p>    <h5>What you should know about this data</h5>   <ul><li>Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref}</li></ul>    <iframe src="https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe>     <p>Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases – and the global data shown in our data explorer – greatly underestimate the true number of cases.</p>   <p>Because of the lack of direct testing, it is useful to track flu-like symptoms.</p>   <p>It is important to note however that these symptoms are not specific to the flu: people with other diseases – such as rhinovirus, COVID-19, common colds, malaria, and others – can also have these symptoms and meet the following criteria.</p>   <p><strong>Acute respiratory infections (ARIs)</strong> are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but <em>only if</em> they were judged by a doctor to be caused by an infection.</p>   <p><strong>Influenza-like illnesses (ILIs)</strong> are narrower – they include only people with a sudden respiratory infection with a fever above 38ºC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.<br>Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., … Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122–128.<br><a href="https://doi.org/10.2471/BLT.17.194514">https://doi.org/10.2471/BLT.17.194514</a><br>World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.<a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y"> https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. Scientific Reports, 11(1), 795. <a href="https://doi.org/10.1038/s41598-020-80842-9">https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</p>   <p><strong>Severe acute respiratory infections (SARIs)</strong> are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38ºC, a cough, <em>and</em> required hospitalization.</p>   <p>In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases <em>does not</em> tell us about the total number of people with these infections.</p>   <p>Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a>  {/ref}</p>    <h5>What you should know about this data</h5>   <ul><li>Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country.<br>But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. <a href="https://doi.org/10.2196/13403">https://doi.org/10.2196/13403</a><br>This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} <br><br></li><li>The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href="https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. <em>Scientific Reports</em>, <em>11</em>(1), 795.<a href="https://doi.org/10.1038/s41598-020-80842-9"> https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</li></ul>    <iframe src="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe>     <p>Each year, flu vaccines need to be updated because different viruses circulate in the population.</p>   <p>This happens for two reasons. One is that flu viruses gradually evolve, and can evade people’s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>   <p>Another reason is that there are different types of flu that circulate each season. </p>   <p>You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}</p>   <p>There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.<br>Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67–81 (2019). <a href="https://doi.org/10.1038/s41579-018-0115-z">https://doi.org/10.1038/s41579-018-0115-z</a> {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula Júnior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de Gámez, M. J., Pebody, R., Zambon, M., Higueros, R., … the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. <a href="https://doi.org/10.1371/journal.pone.0222381">https://doi.org/10.1371/journal.pone.0222381</a> {/ref}</p>   <p>Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as ‘unknown subtype/lineage’.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.</p>   <p>However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}</p>   <p>This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. <a href="https://doi.org/10.1186/1741-7015-11-153">https://doi.org/10.1186/1741-7015-11-153</a> {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36–S43. <a href="https://doi.org/10.1093/cid/ciq020">https://doi.org/10.1093/cid/ciq020</a> {/ref}</p>   <p>To address this, the world needs more routine testing for the flu.</p>    <h5>What you should know about this data</h5>   <ul><li>This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> <br>World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. <a href="https://www.who.int/publications/i/item/9789241549301">https://www.who.int/publications/i/item/9789241549301</a>  {/ref}<br><br></li><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.<br>See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. <a href="https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a><br>Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670–2678. <a href="https://doi.org/10.1016/j.vaccine.2019.03.081">https://doi.org/10.1016/j.vaccine.2019.03.081</a> {/ref}</li></ul>    <iframe src="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true" loading="lazy" style="width: 100%; height: 600px; border: 0px none;"></iframe>     <p>Some flu seasons are far more severe than usual seasonal influenza.</p>   <p>This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href="https://doi.org/10.1038/s41572-018-0002-y">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>   <p>For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107–8112. https://doi.org/10.1073/pnas.1324197111{/ref}</p>   <p>As you can see in the chart, it led to the <a href="https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history">most devastating</a> influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). “Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic”. American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMID 30202996. Online <a href="https://academic.oup.com/aje/article/187/12/2561/5092383">here</a>.{/ref}</p>   <p>The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages <a href="https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history">declined</a> by more than ten years.</p>   <p>But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. <a href="https://doi.org/10.1371/journal.pone.0069586">https://doi.org/10.1371/journal.pone.0069586</a> </p>   <p>Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375–1378. <a href="https://doi.org/10.1086/322662">https://doi.org/10.1086/322662</a> </p>   <p>Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29–34. <a href="https://doi.org/10.1016/j.jtbi.2011.08.003">https://doi.org/10.1016/j.jtbi.2011.08.003</a> {/ref}</p>   <p>In this article, we provide more detail:</p>    <p></p>     <h5>What you should know about this data</h5>   <ul><li>In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 “Spanish” Influenza Pandemic. <em>Bulletin of the History of Medicine</em>, <em>76</em>(1), 105–115. <a href="http://www.jstor.org/stable/44446153">http://www.jstor.org/stable/44446153<br></a>Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. <em>Bulletin of the History of Medicine</em>, <em>65</em>(1), 4–21. <a href="http://www.jstor.org/stable/44447656">http://www.jstor.org/stable/44447656</a><br>Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. <em>American Journal of Epidemiology</em>, <em>187</em>(12), 2561–2567.<a href="https://doi.org/10.1093/aje/kwy191"> https://doi.org/10.1093/aje/kwy191<br></a>Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href="https://jogh.org/documents/issue201902/jogh-09-020421.pdf">https://jogh.org/documents/issue201902/jogh-09-020421.pdf<br></a>Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., … Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687–695. <a href="https://doi.org/10.1016/S1473-3099(12)70121-4">https://doi.org/10.1016/S1473-3099(12)70121-4<br></a>Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. <a href="https://doi.org/10.1371/journal.pmed.1001558">https://doi.org/10.1371/journal.pmed.1001558</a> {/ref}<br><br></li><li>Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war. <br><br></li><li>There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher.</li></ul>    <figure class="wp-block-image size-large"><img src="https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png" alt="" class="wp-image-57012"/></figure>    <p></p>    <h3>Explore our data on influenza</h3>   <iframe class="wp-block-full-content-width" src="https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere" loading="lazy" style="width: 100%; min-height: 740px; max-height: 950px; height: 100vh; border: 0px none;"></iframe>   <div class="wp-block-columns is-style-sticky-right"> <div class="wp-block-column"> <h4>Why we provide this Influenza Data Explorer</h4>   <p>With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus.</p>   <p>It differs from our widely-used infectious diseases projects, such as the <a href="https://ourworldindata.org/explorers/coronavirus-data-explorer">COVID-19 Explorer</a> and the <a href="https://ourworldindata.org/explorers/monkeypox">Mpox Explorer</a>. These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. <strong>This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.</strong></p>   <p>The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.</p> </div> </div>    <div class="wp-block-group wp-block-research-and-writing"> <h2 id="research-writing">Research & Writing</h2>   <h4>Key articles on Influenza</h4>     <p><em>Saloni Dattani</em></p>     <p><em>Max Roser</em></p>    </div>    <p></p>

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                                "text": "Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher.\u00a0",
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                                "text": "Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.",
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                                "text": "This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.",
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                                "text": "It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.",
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                                                "text": "Causes of Death",
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                        "text": "The flu is estimated to cause ",
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                                "text": "400,000 respiratory deaths each year",
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                        "text": " on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ",
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                        "text": "This shows the mean estimate of annual influenza mortality between 2002\u20132011, excluding the 2009 \u201cSwine flu\u201d pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: ",
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                        "text": "People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013 but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.",
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                        "text": "Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate \u201cdoes not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.\u201d",
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                        "text": "In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.",
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                        "text": "Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361\u20131369.",
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                        "text": "Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.",
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                        "text": "The flu is ",
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                        "text": " in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O\u2019Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. ",
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                        "text": "Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.",
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                        "text": "But it\u2019s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.",
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                        "text": "What you should know about this data",
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                            {
                                "text": "The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf",
                                "children": [
                                    {
                                        "text": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.",
                                "spanType": "span-simple-text"
                            },
                            {
                                "spanType": "span-newline"
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                                "text": "These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013\u00a0which are not included here.",
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                                "text": "Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.",
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                "url": "https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map",
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                        "text": "In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing.\u00a0",
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                "value": [
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                        "text": "You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.",
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                "value": [
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                        "text": "Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142\u2013151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1016/j.jet.2021.105293",
                        "children": [
                            {
                                "text": "https://doi.org/10.1016/j.jet.2021.105293",
                                "spanType": "span-simple-text"
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                        "text": "Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., \u2026 Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1371/journal.pone.0253071",
                        "children": [
                            {
                                "text": "https://doi.org/10.1371/journal.pone.0253071",
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                        "text": " {/ref}",
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                        "text": "This decline was very large because of the mathematics of epidemics.\u00a0",
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                        "text": "The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}",
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                        "text": "However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases ",
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                        "children": [
                            {
                                "text": "falls",
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                        "text": " exponentially.",
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                        "text": "Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}\u200b\u200bBiggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "children": [
                            {
                                "text": "BMC Infectious Diseases",
                                "spanType": "span-simple-text"
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                        "spanType": "span-italic"
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                    {
                        "text": ", ",
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                        "children": [
                            {
                                "text": "14",
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                    {
                        "text": "(1), 480.",
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                    {
                        "url": "https://doi.org/10.1186/1471-2334-14-480",
                        "children": [
                            {
                                "text": " https://doi.org/10.1186/1471-2334-14-480",
                                "spanType": "span-simple-text"
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                    {
                        "text": "{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.",
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                "value": [
                    {
                        "text": "Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/s41467-022-29402-5",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/s41467-022-29402-5",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753",
                        "children": [
                            {
                                "text": "https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
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                        "text": " {/ref}",
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                "text": [
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                        "text": "What you should know about this data",
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                            {
                                "text": "Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                "children": [
                                    {
                                        "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                        "spanType": "span-simple-text"
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                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref} We therefore show the share of tests that were positive for the influenza virus.",
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                "url": "https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true",
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                "type": "text",
                "value": [
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                        "text": "Over time, ",
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                    {
                        "url": "https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past",
                        "children": [
                            {
                                "text": "the severity of the flu has declined",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
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                    {
                        "text": " among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1007/s13524-019-00809-y",
                        "children": [
                            {
                                "text": "https://doi.org/10.1007/s13524-019-00809-y",
                                "spanType": "span-simple-text"
                            }
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                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
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                    {
                        "text": "This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in ",
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                    {
                        "url": "https://ourworldindata.org/sanitation",
                        "children": [
                            {
                                "text": "sanitation",
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                        "spanType": "span-link"
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                    {
                        "text": ", neonatal healthcare, and ",
                        "spanType": "span-simple-text"
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                    {
                        "url": "https://ourworldindata.org/vaccination",
                        "children": [
                            {
                                "text": "childhood vaccination",
                                "spanType": "span-simple-text"
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                        "spanType": "span-link"
                    },
                    {
                        "text": " for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115\u2013E120. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/",
                        "children": [
                            {
                                "text": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/",
                                "spanType": "span-simple-text"
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            {
                "type": "text",
                "value": [
                    {
                        "text": "Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm",
                        "children": [
                            {
                                "text": "https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.",
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                        "text": "But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates.\u00a0",
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                        "text": "Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1007/s13524-019-00809-y",
                        "children": [
                            {
                                "text": "https://doi.org/10.1007/s13524-019-00809-y",
                                "spanType": "span-simple-text"
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                        "spanType": "span-link"
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                    {
                        "text": " {/ref} In lower-income countries, the flu could become a larger burden as ",
                        "spanType": "span-simple-text"
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                    {
                        "url": "https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age",
                        "children": [
                            {
                                "text": "their populations continue to age",
                                "spanType": "span-simple-text"
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                        "spanType": "span-link"
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                        "text": ".",
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                        "text": "In this article, we provide more detail:",
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                "text": [
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                        "text": "What you should know about this data",
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                                "text": "These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. ",
                                "spanType": "span-simple-text"
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                            {
                                "url": "https://doi.org/10.1007/s13524-019-00809-y",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.1007/s13524-019-00809-y",
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                                "spanType": "span-link"
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                                "text": " {/ref}",
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                                "text": "The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.",
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                                "text": "The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.",
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                "alt": "People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980.",
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                "type": "image",
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                        "text": "Several respiratory infections, including the flu, are more common in the winter.\u00a0",
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                "value": [
                    {
                        "text": "This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/nrmicro.2017.118",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/nrmicro.2017.118",
                                "spanType": "span-simple-text"
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                        "spanType": "span-link"
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                        "text": " {/ref}",
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                "type": "text",
                "value": [
                    {
                        "text": "In the chart, you can see the share of flu tests that were positive in different countries.",
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                        "text": "Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October.\u00a0",
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                        "text": "But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118\u2013124. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://pubmed.ncbi.nlm.nih.gov/36773717/",
                        "children": [
                            {
                                "text": "https://pubmed.ncbi.nlm.nih.gov/36773717/",
                                "spanType": "span-simple-text"
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                        "spanType": "span-link"
                    },
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                        "spanType": "span-newline"
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                    {
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                    {
                        "text": "Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/nrmicro.2017.118",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/nrmicro.2017.118",
                                "spanType": "span-simple-text"
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                        ],
                        "spanType": "span-link"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "children": [
                            {
                                "text": "Epidemiology & Infection",
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                            }
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                        "spanType": "span-italic"
                    },
                    {
                        "text": ", ",
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                    {
                        "children": [
                            {
                                "text": "143",
                                "spanType": "span-simple-text"
                            }
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                        "spanType": "span-italic"
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                    {
                        "text": "(6), 1110\u20131118.",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1017/S0950268814002702",
                        "children": [
                            {
                                "text": " https://doi.org/10.1017/S0950268814002702",
                                "spanType": "span-simple-text"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "spanType": "span-newline"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": "Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010\u201319: A retrospective analysis. The Lancet Global Health, 11(5), e729\u2013e739. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1016/S2214-109X(23)00109-2",
                        "children": [
                            {
                                "text": "https://doi.org/10.1016/S2214-109X(23)00109-2",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    }
                ],
                "parseErrors": []
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            {
                "type": "text",
                "value": [
                    {
                        "text": "\u200b\u200bNewman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1371/journal.pone.0193263",
                        "children": [
                            {
                                "text": "https://doi.org/10.1371/journal.pone.0193263",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "You can see this in the chart for Singapore and Thailand, for example.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "text": [
                    {
                        "text": "What you should know about this data",
                        "spanType": "span-simple-text"
                    }
                ],
                "type": "heading",
                "level": 5,
                "parseErrors": []
            },
            {
                "type": "list",
                "items": [
                    {
                        "type": "text",
                        "value": [
                            {
                                "text": "Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                "children": [
                                    {
                                        "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref}",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "parseErrors": []
                    }
                ],
                "parseErrors": []
            },
            {
                "url": "https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true",
                "type": "chart",
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases \u2013 and the global data shown in our data explorer \u2013 greatly underestimate the true number of cases.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Because of the lack of direct testing, it is useful to track flu-like symptoms.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "It is important to note however that these symptoms are not specific to the flu: people with other diseases \u2013 such as rhinovirus, COVID-19, common colds, malaria, and others \u2013 can also have these symptoms and meet the following criteria.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
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            {
                "type": "text",
                "value": [
                    {
                        "children": [
                            {
                                "text": "Acute respiratory infections (ARIs)",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-bold"
                    },
                    {
                        "text": " are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "children": [
                            {
                                "text": "only if",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-italic"
                    },
                    {
                        "text": " they were judged by a doctor to be caused by an infection.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "children": [
                            {
                                "text": "Influenza-like illnesses (ILIs)",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-bold"
                    },
                    {
                        "text": " are narrower \u2013\u00a0they include only people with a sudden respiratory infection with a fever above 38\u00baC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.",
                        "spanType": "span-simple-text"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., \u2026 Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122\u2013128.",
                        "spanType": "span-simple-text"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "url": "https://doi.org/10.2471/BLT.17.194514",
                        "children": [
                            {
                                "text": "https://doi.org/10.2471/BLT.17.194514",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                        "children": [
                            {
                                "text": " https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. Scientific Reports, 11(1), 795. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/s41598-020-80842-9",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/s41598-020-80842-9",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
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            {
                "type": "text",
                "value": [
                    {
                        "children": [
                            {
                                "text": "Severe acute respiratory infections (SARIs)",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-bold"
                    },
                    {
                        "text": " are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38\u00baC, a cough, ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "children": [
                            {
                                "text": "and",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-italic"
                    },
                    {
                        "text": " required hospitalization.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "children": [
                            {
                                "text": "does not",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-italic"
                    },
                    {
                        "text": " tell us about the total number of people with these infections.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                        "children": [
                            {
                                "text": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": "\u00a0 {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "text": [
                    {
                        "text": "What you should know about this data",
                        "spanType": "span-simple-text"
                    }
                ],
                "type": "heading",
                "level": 5,
                "parseErrors": []
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            {
                "type": "list",
                "items": [
                    {
                        "type": "text",
                        "value": [
                            {
                                "text": "Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country.",
                                "spanType": "span-simple-text"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://doi.org/10.2196/13403",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.2196/13403",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "spanType": "span-newline"
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                        "type": "text",
                        "value": [
                            {
                                "text": "The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                                "children": [
                                    {
                                        "text": "https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "children": [
                                    {
                                        "text": "Scientific Reports",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-italic"
                            },
                            {
                                "text": ", ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "children": [
                                    {
                                        "text": "11",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-italic"
                            },
                            {
                                "text": "(1), 795.",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://doi.org/10.1038/s41598-020-80842-9",
                                "children": [
                                    {
                                        "text": " https://doi.org/10.1038/s41598-020-80842-9",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref}",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "parseErrors": []
                    }
                ],
                "parseErrors": []
            },
            {
                "url": "https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true",
                "type": "chart",
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Each year, flu vaccines need to be updated because different viruses circulate in the population.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "This happens for two reasons. One is that flu viruses gradually evolve, and can evade people\u2019s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/s41572-018-0002-y",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/s41572-018-0002-y",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Another reason is that there are different types of flu that circulate each season.\u00a0",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.",
                        "spanType": "span-simple-text"
                    },
                    {
                        "spanType": "span-newline"
                    },
                    {
                        "text": "Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67\u201381 (2019). ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/s41579-018-0115-z",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/s41579-018-0115-z",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula J\u00fanior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de G\u00e1mez, M. J., Pebody, R., Zambon, M., Higueros, R., \u2026 the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1371/journal.pone.0222381",
                        "children": [
                            {
                                "text": "https://doi.org/10.1371/journal.pone.0222381",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as \u2018unknown subtype/lineage\u2019.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1186/1741-7015-11-153",
                        "children": [
                            {
                                "text": "https://doi.org/10.1186/1741-7015-11-153",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36\u2013S43. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1093/cid/ciq020",
                        "children": [
                            {
                                "text": "https://doi.org/10.1093/cid/ciq020",
                                "spanType": "span-simple-text"
                            }
                        ],
                        "spanType": "span-link"
                    },
                    {
                        "text": " {/ref}",
                        "spanType": "span-simple-text"
                    }
                ],
                "parseErrors": []
            },
            {
                "type": "text",
                "value": [
                    {
                        "text": "To address this, the world needs more routine testing for the flu.",
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                    }
                ],
                "parseErrors": []
            },
            {
                "text": [
                    {
                        "text": "What you should know about this data",
                        "spanType": "span-simple-text"
                    }
                ],
                "type": "heading",
                "level": 5,
                "parseErrors": []
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                        "value": [
                            {
                                "text": "This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://doi.org/10.1038/s41572-018-0002-y",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.1038/s41572-018-0002-y",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://www.who.int/publications/i/item/9789241549301",
                                "children": [
                                    {
                                        "text": "https://www.who.int/publications/i/item/9789241549301",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": "\u00a0 {/ref}",
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                            },
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                                "spanType": "span-newline"
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                        "type": "text",
                        "value": [
                            {
                                "text": "Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                "children": [
                                    {
                                        "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.",
                                "spanType": "span-simple-text"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                "children": [
                                    {
                                        "text": "https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670\u20132678. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://doi.org/10.1016/j.vaccine.2019.03.081",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.1016/j.vaccine.2019.03.081",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref}",
                                "spanType": "span-simple-text"
                            }
                        ],
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                ],
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            {
                "url": "https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true",
                "type": "chart",
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                "type": "text",
                "value": [
                    {
                        "text": "Some flu seasons are far more severe than usual seasonal influenza.",
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                "type": "text",
                "value": [
                    {
                        "text": "This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1038/s41572-018-0002-y",
                        "children": [
                            {
                                "text": "https://doi.org/10.1038/s41572-018-0002-y",
                                "spanType": "span-simple-text"
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                        "text": " {/ref}",
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                        "text": "For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107\u20138112. https://doi.org/10.1073/pnas.1324197111{/ref}",
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                "value": [
                    {
                        "text": "As you can see in the chart, it led to the ",
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                        "url": "https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history",
                        "children": [
                            {
                                "text": "most devastating",
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                        "text": " influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). \u201cReassessing the Global Mortality Burden of the 1918 Influenza Pandemic\u201d. American Journal of Epidemiology. 187 (12): 2561\u20132567. doi:10.1093/aje/kwy191. PMID 30202996. Online ",
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                        "url": "https://academic.oup.com/aje/article/187/12/2561/5092383",
                        "children": [
                            {
                                "text": "here",
                                "spanType": "span-simple-text"
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                        "text": ".{/ref}",
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                        "text": "The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages ",
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                    {
                        "url": "https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history",
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                                "text": "declined",
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                    },
                    {
                        "text": " by more than ten years.",
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                "type": "text",
                "value": [
                    {
                        "text": "But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1371/journal.pone.0069586",
                        "children": [
                            {
                                "text": "https://doi.org/10.1371/journal.pone.0069586",
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                ],
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            {
                "type": "text",
                "value": [
                    {
                        "text": "Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375\u20131378. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1086/322662",
                        "children": [
                            {
                                "text": "https://doi.org/10.1086/322662",
                                "spanType": "span-simple-text"
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            {
                "type": "text",
                "value": [
                    {
                        "text": "Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29\u201334. ",
                        "spanType": "span-simple-text"
                    },
                    {
                        "url": "https://doi.org/10.1016/j.jtbi.2011.08.003",
                        "children": [
                            {
                                "text": "https://doi.org/10.1016/j.jtbi.2011.08.003",
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                        "text": " {/ref}",
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                        "text": "In this article, we provide more detail:",
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                "text": [
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                        "text": "What you should know about this data",
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                            {
                                "text": "In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 \u201cSpanish\u201d Influenza Pandemic. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "children": [
                                    {
                                        "text": "Bulletin of the History of Medicine",
                                        "spanType": "span-simple-text"
                                    }
                                ],
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                            },
                            {
                                "text": ", ",
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                            {
                                "children": [
                                    {
                                        "text": "76",
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                            },
                            {
                                "text": "(1), 105\u2013115. ",
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                            },
                            {
                                "url": "http://www.jstor.org/stable/44446153",
                                "children": [
                                    {
                                        "text": "http://www.jstor.org/stable/44446153",
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                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": "Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "children": [
                                    {
                                        "text": "Bulletin of the History of Medicine",
                                        "spanType": "span-simple-text"
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                                ],
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                            },
                            {
                                "text": ", ",
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                                "children": [
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                                        "text": "65",
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                            {
                                "text": "(1), 4\u201321. ",
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                            },
                            {
                                "url": "http://www.jstor.org/stable/44447656",
                                "children": [
                                    {
                                        "text": "http://www.jstor.org/stable/44447656",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "spanType": "span-newline"
                            },
                            {
                                "text": "Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "children": [
                                    {
                                        "text": "American Journal of Epidemiology",
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                            {
                                "text": ", ",
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                                        "text": "187",
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                            {
                                "text": "(12), 2561\u20132567.",
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                            },
                            {
                                "url": "https://doi.org/10.1093/aje/kwy191",
                                "children": [
                                    {
                                        "text": " https://doi.org/10.1093/aje/kwy191",
                                        "spanType": "span-simple-text"
                                    },
                                    {
                                        "spanType": "span-newline"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": "Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf",
                                "children": [
                                    {
                                        "text": "https://jogh.org/documents/issue201902/jogh-09-020421.pdf",
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                                    },
                                    {
                                        "spanType": "span-newline"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": "Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., \u2026 Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687\u2013695. ",
                                "spanType": "span-simple-text"
                            },
                            {
                                "url": "https://doi.org/10.1016/S1473-3099(12)70121-4",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.1016/S1473-3099(12)70121-4",
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                                    },
                                    {
                                        "spanType": "span-newline"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": "Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. ",
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                            },
                            {
                                "url": "https://doi.org/10.1371/journal.pmed.1001558",
                                "children": [
                                    {
                                        "text": "https://doi.org/10.1371/journal.pmed.1001558",
                                        "spanType": "span-simple-text"
                                    }
                                ],
                                "spanType": "span-link"
                            },
                            {
                                "text": " {/ref}",
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                                "text": "Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war.\u00a0",
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                                "text": "There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher.",
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                "text": [
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                        "text": "Explore our data on influenza",
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                "url": "https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere",
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                "text": [
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                        "text": "Why we provide this Influenza Data Explorer",
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                "value": [
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                        "text": "With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus.",
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                "type": "text",
                "value": [
                    {
                        "text": "It differs from our widely-used infectious diseases projects, such as the ",
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                    },
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                        "url": "https://ourworldindata.org/explorers/coronavirus-data-explorer",
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                                "text": "COVID-19 Explorer",
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                        "text": " and the ",
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                        "url": "https://ourworldindata.org/explorers/monkeypox",
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                            {
                                "text": "Mpox Explorer",
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                        "spanType": "span-link"
                    },
                    {
                        "text": ". These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. ",
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                    },
                    {
                        "children": [
                            {
                                "text": "This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.",
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                        "text": "The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.",
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                "text": [
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                        "text": "Research & Writing",
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                "text": [
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                        "text": "Key articles on Influenza",
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                "type": "text",
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                    {
                        "children": [
                            {
                                "text": "Saloni Dattani",
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                        "children": [
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                                "text": "Max Roser",
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        ],
        "type": "article",
        "title": "Influenza",
        "authors": [
            "Saloni Dattani",
            "Fiona Spooner",
            "Edouard Mathieu",
            "Hannah Ritchie",
            "Max Roser"
        ],
        "excerpt": "Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.",
        "dateline": "May 18, 2023",
        "subtitle": "Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.",
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    "createdAt": "2023-05-10T09:34:09.000Z",
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2023-05-18 08:46:00

2024-03-05 09:19:03

16tVxsM1-wzwm_wpno-Y-M_IEInOCPJuWZqHfMLywNJc

[
    "Saloni Dattani",
    "Fiona Spooner",
    "Edouard Mathieu",
    "Hannah Ritchie",
    "Max Roser"
]

Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.

2023-05-10 09:34:09

2023-06-14 20:17:51

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Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher. Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population. This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world. It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus. Related topics * [Causes of Death](https://ourworldindata.org/causes-of-death) * [Vaccination](https://ourworldindata.org/vaccination) * [Covid-19](https://ourworldindata.org/coronavirus) The flu is estimated to cause [400,000 respiratory deaths each year](https://ourworldindata.org/influenza-deaths) on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf](https://jogh.org/documents/issue201902/jogh-09-020421.pdf) This shows the mean estimate of annual influenza mortality between 2002–2011, excluding the 2009 “Swine flu” pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: [https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf](https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf) {/ref} People also die from other complications of the flu – such as a stroke or heart attack – but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear. Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate “does not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.” In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly. This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu. Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., & Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361–1369.[ https://doi.org/10.1016/j.vaccine.2021.11.080](https://doi.org/10.1016/j.vaccine.2021.11.080) Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., & Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.[ https://doi.org/10.1001/jamanetworkopen.2022.8873](https://doi.org/10.1001/jamanetworkopen.2022.8873){/ref} The flu is [most severe](https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu) in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O’Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., & Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. [https://doi.org/10.1098/rsos.211498](https://doi.org/10.1098/rsos.211498) {/ref} Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map. But it’s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates. In this article, we provide more detail: ##### What you should know about this data * The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf](https://jogh.org/documents/issue201902/jogh-09-020421.pdf) {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms. * These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu – such as a stroke or heart attack – which are not included here. * Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records. <Chart url="https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map"/> In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing. You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low. Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., & Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142–151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., & Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. [https://doi.org/10.1016/j.jet.2021.105293](https://doi.org/10.1016/j.jet.2021.105293) Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., … Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility & COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. [https://doi.org/10.1371/journal.pone.0253071](https://doi.org/10.1371/journal.pone.0253071) {/ref} This decline was very large because of the mathematics of epidemics. The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., & Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref} However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases _falls_ exponentially. Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}Biggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., & Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. _BMC Infectious Diseases_, _14_(1), 480.[ https://doi.org/10.1186/1471-2334-14-480](https://doi.org/10.1186/1471-2334-14-480){/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata. Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., & Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. [https://doi.org/10.1038/s41467-022-29402-5](https://doi.org/10.1038/s41467-022-29402-5) Paget, J., Caini, S., Del Riccio, M., van Waarden, W., & Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). [https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753](https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753) {/ref} ##### What you should know about this data * Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} We therefore show the share of tests that were positive for the influenza virus. <Chart url="https://ourworldindata.org/explorers/influenza?tab=chart&facet=none&country=CHN~CAN~GBR~USA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Over time, [the severity of the flu has declined](https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past) among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in [sanitation](https://ourworldindata.org/sanitation), neonatal healthcare, and [childhood vaccination](https://ourworldindata.org/vaccination) for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., & Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115–E120. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/) Centers for Disease Control and Prevention, & National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. [https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm](https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm) {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza. But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates. Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} In lower-income countries, the flu could become a larger burden as [their populations continue to age](https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age). In this article, we provide more detail: ##### What you should know about this data * These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., & Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959–2016. Demography, 56(5), 1723–1746. [https://doi.org/10.1007/s13524-019-00809-y](https://doi.org/10.1007/s13524-019-00809-y) {/ref} * The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population. * The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable. <Image filename="Historical-decline-in-influenza-deaths-by-birth-cohort.png" alt="People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980."/> Several respiratory infections, including the flu, are more common in the winter. This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. [https://doi.org/10.1038/nrmicro.2017.118](https://doi.org/10.1038/nrmicro.2017.118) {/ref} In the chart, you can see the share of flu tests that were positive in different countries. Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October. But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., & Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118–124. [https://pubmed.ncbi.nlm.nih.gov/36773717/](https://pubmed.ncbi.nlm.nih.gov/36773717/) Petrova, V. N., & Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47–60. [https://doi.org/10.1038/nrmicro.2017.118](https://doi.org/10.1038/nrmicro.2017.118) Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. _Epidemiology & Infection_, _143_(6), 1110–1118.[ https://doi.org/10.1017/S0950268814002702 ](https://doi.org/10.1017/S0950268814002702)Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., & Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010–19: A retrospective analysis. The Lancet Global Health, 11(5), e729–e739. [https://doi.org/10.1016/S2214-109X(23)00109-2](https://doi.org/10.1016/S2214-109X(23)00109-2) Newman, L. P., Bhat, N., Fleming, J. A., & Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. [https://doi.org/10.1371/journal.pone.0193263](https://doi.org/10.1371/journal.pone.0193263) {/ref} You can see this in the chart for Singapore and Thailand, for example. ##### What you should know about this data * Testing to confirm the flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?tab=chart&time=2014-01-01..2020-01-01&facet=none&country=SGP~THA&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Share+of+positive+tests+%28%25%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases – and the global data shown in our data explorer – greatly underestimate the true number of cases. Because of the lack of direct testing, it is useful to track flu-like symptoms. It is important to note however that these symptoms are not specific to the flu: people with other diseases – such as rhinovirus, COVID-19, common colds, malaria, and others – can also have these symptoms and meet the following criteria. **Acute respiratory infections (ARIs)** are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but _only if_ they were judged by a doctor to be caused by an infection. **Influenza-like illnesses (ILIs)** are narrower – they include only people with a sudden respiratory infection with a fever above 38ºC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it. Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., … Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122–128. [https://doi.org/10.2471/BLT.17.194514](https://doi.org/10.2471/BLT.17.194514) World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.[ https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. Scientific Reports, 11(1), 795. [https://doi.org/10.1038/s41598-020-80842-9](https://doi.org/10.1038/s41598-020-80842-9) {/ref} **Severe acute respiratory infections (SARIs)** are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38ºC, a cough, _and_ required hospitalization. In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases _does not_ tell us about the total number of people with these infections. Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. [https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) {/ref} ##### What you should know about this data * Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the 'ILINet' system, which is connected to many clinics across the country. But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., & Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. [https://doi.org/10.2196/13403](https://doi.org/10.2196/13403) This means 'non-sentinel' data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} * The country's sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. [https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y](https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&isAllowed=y) Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., & Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005–2019. _Scientific Reports_, _11_(1), 795.[ https://doi.org/10.1038/s41598-020-80842-9](https://doi.org/10.1038/s41598-020-80842-9) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=~Northern+Hemisphere&Confirmed+cases+or+Symptoms=Symptoms&Metric=Comparison+of+data+on+respiratory+infections&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Each year, flu vaccines need to be updated because different viruses circulate in the population. This happens for two reasons. One is that flu viruses gradually evolve, and can evade people’s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) {/ref} Another reason is that there are different types of flu that circulate each season. You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref} There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2. Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67–81 (2019). [https://doi.org/10.1038/s41579-018-0115-z](https://doi.org/10.1038/s41579-018-0115-z) {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula Júnior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de Gámez, M. J., Pebody, R., Zambon, M., Higueros, R., … the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. [https://doi.org/10.1371/journal.pone.0222381](https://doi.org/10.1371/journal.pone.0222381) {/ref} Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as ‘unknown subtype/lineage’.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain. However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref} This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., & Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. [https://doi.org/10.1186/1741-7015-11-153](https://doi.org/10.1186/1741-7015-11-153) {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., & Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36–S43. [https://doi.org/10.1093/cid/ciq020](https://doi.org/10.1093/cid/ciq020) {/ref} To address this, the world needs more routine testing for the flu. ##### What you should know about this data * This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) World Health Organization & others. (2015). A manual for estimating disease burden associated with seasonal influenza. [https://www.who.int/publications/i/item/9789241549301](https://www.who.int/publications/i/item/9789241549301) {/ref} * Testing to confirm flu is limited in many countries.{ref}World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies. See also: World Health Organization & others. (2019). Global influenza strategy 2019-2030. [https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf](https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf) Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., & Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670–2678. [https://doi.org/10.1016/j.vaccine.2019.03.081](https://doi.org/10.1016/j.vaccine.2019.03.081) {/ref} <Chart url="https://ourworldindata.org/explorers/influenza?uniformYAxis=0&country=Northern+Hemisphere~Southern+Hemisphere&Confirmed+cases+or+Symptoms=Confirmed+cases&Metric=Confirmed+cases+%28by+strain%29&Interval=Monthly&Surveillance+type=All+types&hideControls=true"/> Some flu seasons are far more severe than usual seasonal influenza. This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. [https://doi.org/10.1038/s41572-018-0002-y](https://doi.org/10.1038/s41572-018-0002-y) {/ref} For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., & Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107–8112. https://doi.org/10.1073/pnas.1324197111{/ref} As you can see in the chart, it led to the [most devastating](https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history) influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). “Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic”. American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMID 30202996. Online [here](https://academic.oup.com/aje/article/187/12/2561/5092383).{/ref} The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages [declined](https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history) by more than ten years. But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., & Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. [https://doi.org/10.1371/journal.pone.0069586](https://doi.org/10.1371/journal.pone.0069586) Luk, J., Gross, P., & Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375–1378. [https://doi.org/10.1086/322662](https://doi.org/10.1086/322662) Ma, J., Dushoff, J., & Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29–34. [https://doi.org/10.1016/j.jtbi.2011.08.003](https://doi.org/10.1016/j.jtbi.2011.08.003) {/ref} In this article, we provide more detail: ##### What you should know about this data * In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 “Spanish” Influenza Pandemic. _Bulletin of the History of Medicine_, _76_(1), 105–115. [http://www.jstor.org/stable/44446153 ](http://www.jstor.org/stable/44446153)Patterson, K. D., & Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. _Bulletin of the History of Medicine_, _65_(1), 4–21. [http://www.jstor.org/stable/44447656](http://www.jstor.org/stable/44447656) Spreeuwenberg, P., Kroneman, M., & Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. _American Journal of Epidemiology_, _187_(12), 2561–2567.[ https://doi.org/10.1093/aje/kwy191 ](https://doi.org/10.1093/aje/kwy191)Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. [https://jogh.org/documents/issue201902/jogh-09-020421.pdf ](https://jogh.org/documents/issue201902/jogh-09-020421.pdf)Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., … Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687–695. [https://doi.org/10.1016/S1473-3099(12)70121-4 ](https://doi.org/10.1016/S1473-3099(12)70121-4)Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., & the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. [https://doi.org/10.1371/journal.pmed.1001558](https://doi.org/10.1371/journal.pmed.1001558) {/ref} * Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war. * There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher. <Image filename="Influenza-pandemics-in-comparison-1.png" alt=""/> ### Explore our data on influenza <Chart url="https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere"/> #### Why we provide this Influenza Data Explorer With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus. It differs from our widely-used infectious diseases projects, such as the [COVID-19 Explorer](https://ourworldindata.org/explorers/coronavirus-data-explorer) and the [Mpox Explorer](https://ourworldindata.org/explorers/monkeypox). These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. **This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.** The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting. ## Research & Writing #### Key articles on Influenza _Saloni Dattani_ _Max Roser_

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        "rendered": "\n<!-- formatting-options toc:false bodyClassName:topic-page -->\n\n\n\t<div class=\"sticky-nav-contents\">\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#introduction\"><span class=\"wp-block-navigation-item__label\">Introduction</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#key-insights\"><span class=\"wp-block-navigation-item__label\">Key Insights</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#explore-our-data-on-influenza\" rel=\"\" title=\"\"><span class=\"wp-block-navigation-item__label\">Data Explorer</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#research-writing\"><span class=\"wp-block-navigation-item__label\">Research &amp; Writing</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#interactive-charts-on-influenza\"><span class=\"wp-block-navigation-item__label\">Charts</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#endnotes\"><span class=\"wp-block-navigation-item__label\">Endnotes</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#citation\"><span class=\"wp-block-navigation-item__label\">Cite This Work</span></a></li>\n\n<li class=\" wp-block-navigation-item wp-block-navigation-link\"><a class=\"wp-block-navigation-item__content\"  href=\"#licence\"><span class=\"wp-block-navigation-item__label\">Reuse This Work</span></a></li>\n</div>\n\n\n\n<div class=\"wp-block-columns front-matter\">\n<div class=\"wp-block-column\" id=\"introduction\">\n<p>Seasonal flu is a contagious illness caused by the influenza virus. It kills around 400,000 people from respiratory disease on average each year. In large pandemics, when new strains have evolved, the death toll has been much higher.&nbsp;</p>\n\n\n\n<p>Yet, data on the flu is limited. With better testing, countries could improve their response to flu epidemics. It could help to rapidly identify new strains, detect epidemics early, and design better-matched vaccines to target flu strains circulating in the population.</p>\n\n\n\n<p>This page therefore shows estimates of deaths during seasonal flu epidemics, historical flu pandemics, patterns of flu seasons in different countries, and confirmed cases of flu and flu-like symptoms across the world.</p>\n\n\n\n<p>It also includes our Flu Explorer, a resource for epidemiologists, infectious disease researchers, and public health experts to monitor the global spread of the influenza virus.</p>\n</div>\n\n\n\n<div class=\"wp-block-column\">\n<div class=\"wp-container-65e6e283dfe6c wp-block-group related-topics\">\n<p>Related topics</p>\n\n\n\n<ul><li><a href=\"https://ourworldindata.org/causes-of-death\">Causes of Death</a></li><li><a href=\"https://ourworldindata.org/vaccination\">Vaccination</a></li><li><a href=\"https://ourworldindata.org/coronavirus\">Covid-19</a></li></ul>\n</div>\n</div>\n</div>\n\n\n\n\t<block type=\"key-insights\">\n\t\t<title>Key insights on Influenza</title>\n        <slug>key-insights</slug>\n        <insights>\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Seasonal flu kills hundreds of thousands of people worldwide each year</title>\n        <slug>seasonal-flu-kills-hundreds-of-thousands-of-people-worldwide-each-year</slug>\n        <content>\n\n<p>The flu is estimated to cause <a href=\"https://ourworldindata.org/influenza-deaths\">400,000 respiratory deaths each year</a> on average across the world. These deaths come from pneumonia and other respiratory symptoms caused by the flu.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a>&nbsp;</p>\n\n\n\n<p>This shows the mean estimate of annual influenza mortality between 2002\u20132011, excluding the 2009 \u201cSwine flu\u201d pandemic influenza season. You can find estimated numbers for world regions in Table 2 of the paper. Rates for other age groups can be found here: <a href=\"https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf\">https://www.nivel.nl/sites/default/files/influenza-nieuwsbrief/GLaMOR%20project_seasonal%20estimates.pdf</a> {/ref}&nbsp;</p>\n\n\n\n<p>People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013 but global estimates have not been made of their death toll.{ref}The global number of people who die from other complications of the flu is unclear.</p>\n\n\n\n<p>Paget et al. (the authors of the Global Pandemic Mortality project, i.e. GLaMOR) state in their paper that their estimate \u201cdoes not cover cardiovascular deaths, something that could at least double the estimate of influenza-associated deaths.\u201d</p>\n\n\n\n<p>In recent meta-analyses, Behrouzi et al. found that influenza vaccination reduces the chances of major cardiovascular events (such as heart attacks and strokes) by around 34%, in clinical trials of the elderly.</p>\n\n\n\n<p>This suggests the death toll from other complications could be large. However, global estimates have not been made of these types of deaths from flu.</p>\n\n\n\n<p>Paget, J., Danielle Iuliano, A., Taylor, R. J., Simonsen, L., Viboud, C., &amp; Spreeuwenberg, P. (2022). Estimates of mortality associated with seasonal influenza for the European Union from the GLaMOR project. Vaccine, 40(9), 1361\u20131369.<a href=\"https://doi.org/10.1016/j.vaccine.2021.11.080\"> https://doi.org/10.1016/j.vaccine.2021.11.080</a></p>\n\n\n\n<p>Behrouzi, B., Bhatt, D. L., Cannon, C. P., Vardeny, O., Lee, D. S., Solomon, S. D., &amp; Udell, J. A. (2022). Association of Influenza Vaccination With Cardiovascular Risk: A Meta-analysis. JAMA Network Open, 5(4), e228873.<a href=\"https://doi.org/10.1001/jamanetworkopen.2022.8873\"> https://doi.org/10.1001/jamanetworkopen.2022.8873</a>{/ref}</p>\n\n\n\n<p>The flu is <a href=\"https://ourworldindata.org/influenza-deaths#which-factors-affect-the-number-of-deaths-from-the-flu\">most severe</a> in infants and the elderly.{ref}Metcalf, C. J. E., Paireau, J., O\u2019Driscoll, M., Pivette, M., Hubert, B., Pontais, I., Nickbakhsh, S., Cummings, D. A. T., Cauchemez, S., &amp; Salje, H. (2022). Comparing the age and sex trajectories of SARS-CoV-2 morbidity and mortality with other respiratory pathogens. Royal Society Open Science, 9(6), 211498. <a href=\"https://doi.org/10.1098/rsos.211498\">https://doi.org/10.1098/rsos.211498</a> {/ref}</p>\n\n\n\n<p>Among those over 65, the flu kills around 31 people per 100,000 each year from respiratory disease in Europe. You can see this on the map.</p>\n\n\n\n<p>But it\u2019s not only age that matters, as the map shows. Death rates from the flu are higher in countries in South America, Africa, and South Asia, than in Europe and North America, due to poverty, poorer underlying health, lower access to healthcare, and lower vaccination rates.</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n    <block type=\"prominent-link\" style=\"is-style-thin\">\n        <link-url>https://ourworldindata.org/influenza-deaths</link-url>\n        <title></title>\n        <content>\n\n<p></p>\n\n</content>\n        <figure></figure>\n    </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>The annual mortality rate of influenza was estimated by the Global Pandemic Mortality Project II using data between 2002 and 2011.{ref}Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf</a> {/ref} They made these estimates using data from routine surveillance metrics for the flu, along with the number of excess deaths that occurred during flu seasons and mortality records where deceased people had respiratory symptoms.<br><br></li><li>These are estimates of flu deaths due to respiratory symptoms. People also die from other complications of the flu \u2013 such as a stroke or heart attack \u2013&nbsp;which are not included here.<br><br></li><li>Estimates in low-income countries tend to be less certain due to lower levels of testing for influenza and limited mortality records.</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/grapher/annual-mortality-rate-from-seasonal-influenza-ages-65?tab=map\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Social distancing during COVID-19 had a large impact on the flu</title>\n        <slug>social-distancing-during-covid-19-had-a-large-impact-on-the-flu</slug>\n        <content>\n\n<p>In many countries, flu became much rarer during the COVID-19 pandemic, due to the impact of social distancing.&nbsp;</p>\n\n\n\n<p>You can see this in the chart. It shows the share of flu tests that were positive. In 2020 and 2021, there was a large decline in flu and the rates of positive tests were low.</p>\n\n\n\n<p>Because the influenza virus is spread between people, through respiratory droplets and human contact{ref}Kutter, J. S., Spronken, M. I., Fraaij, P. L., Fouchier, R. A., &amp; Herfst, S. (2018). Transmission routes of respiratory viruses among humans. Current Opinion in Virology, 28, 142\u2013151. https://doi.org/10.1016/j.coviro.2018.01.001{/ref}, social distancing led to a large reduction in contact between people and limited the virus from spreading.{ref}Farboodi, M., Jarosch, G., &amp; Shimer, R. (2021). Internal and external effects of social distancing in a pandemic. Journal of Economic Theory, 196, 105293. <a href=\"https://doi.org/10.1016/j.jet.2021.105293\">https://doi.org/10.1016/j.jet.2021.105293</a>&nbsp;</p>\n\n\n\n<p>Woskie, L. R., Hennessy, J., Espinosa, V., Tsai, T. C., Vispute, S., Jacobson, B. H., Cattuto, C., Gauvin, L., Tizzoni, M., Fabrikant, A., Gadepalli, K., Boulanger, A., Pearce, A., Kamath, C., Schlosberg, A., Stanton, C., Bavadekar, S., Abueg, M., Hogue, M., \u2026 Gabrilovich, E. (2021). Early social distancing policies in Europe, changes in mobility &amp; COVID-19 case trajectories: Insights from Spring 2020. PLOS ONE, 16(6), e0253071. <a href=\"https://doi.org/10.1371/journal.pone.0253071\">https://doi.org/10.1371/journal.pone.0253071</a> {/ref}</p>\n\n\n\n<p>This decline was very large because of the mathematics of epidemics.&nbsp;</p>\n\n\n\n<p>The reproductive number (also called the R number) can help to understand why. This refers to the average number of people who will be infected by someone with the virus. When the R number is greater than 1, the average person who is infected will spread the virus to more than one person, who spread it to more and more people; the number of cases rises exponentially and leads to an epidemic.{ref}Rothman, K. J., Lash, T. L., VanderWeele, T. J., &amp; Haneuse, S. (2021). Modern epidemiology (Fourth edition). Wolters Kluwer.{/ref}</p>\n\n\n\n<p>However, when the R number is lower than 1, the virus does not lead to an epidemic, and the number of cases <em>falls</em> exponentially.</p>\n\n\n\n<p>Seasonal flu viruses tend to have an R number that is slightly above 1 at the start of an epidemic.{ref}\u200b\u200bBiggerstaff, M., Cauchemez, S., Reed, C., Gambhir, M., &amp; Finelli, L. (2014). Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. <em>BMC Infectious Diseases</em>, <em>14</em>(1), 480.<a href=\"https://doi.org/10.1186/1471-2334-14-480\"> https://doi.org/10.1186/1471-2334-14-480</a>{/ref} Social distancing cut the number of contacts between people, and led to the R number of flu to dip much below 1 for a long time. This is why the spread of flu dwindled worldwide and was only seen in limited circumstances.{ref}This effect was so large that it may have led to the extinction of a lineage of flu called influenza B Yamagata.</p>\n\n\n\n<p>Dhanasekaran, V., Sullivan, S., Edwards, K. M., Xie, R., Khvorov, A., Valkenburg, S. A., Cowling, B. J., &amp; Barr, I. G. (2022). Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nature Communications, 13(1), 1721. <a href=\"https://doi.org/10.1038/s41467-022-29402-5\">https://doi.org/10.1038/s41467-022-29402-5</a><br>Paget, J., Caini, S., Del Riccio, M., van Waarden, W., &amp; Meijer, A. (2022). Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Eurosurveillance, 27(39). <a href=\"https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753\">https://doi.org/10.2807/1560-7917.ES.2022.27.39.2200753</a> {/ref}</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization &amp; others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} We therefore show the share of tests that were positive for the influenza virus.</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?tab=chart&amp;facet=none&amp;country=CHN~CAN~GBR~USA&amp;Confirmed+cases+or+Symptoms=Confirmed+cases&amp;Metric=Share+of+positive+tests+%28%25%29&amp;Interval=Monthly&amp;Surveillance+type=All+types&amp;hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Seasonal flu used to be far more deadly</title>\n        <slug>seasonal-flu-used-to-be-far-more-deatly</slug>\n        <content>\n\n<p>Over time, <a href=\"https://ourworldindata.org/influenza-deaths#what-did-influenza-mortality-look-like-in-the-past\">the severity of the flu has declined</a> among people of the same age, as the chart shows.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., &amp; Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}</p>\n\n\n\n<p>This is because of flu vaccination, which began in the 1930s and 1940s, as well as improvements in <a href=\"https://ourworldindata.org/sanitation\">sanitation</a>, neonatal healthcare, and <a href=\"https://ourworldindata.org/vaccination\">childhood vaccination</a> for other diseases.{ref}Barberis, I., Myles, P., Ault, S. K., Bragazzi, N. L., &amp; Martini, M. (2016). History and evolution of influenza control through vaccination: From the first monovalent vaccine to universal vaccines. Journal of Preventive Medicine and Hygiene, 57(3), E115\u2013E120. <a href=\"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/\">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139605/</a>&nbsp;</p>\n\n\n\n<p>Centers for Disease Control and Prevention, &amp; National Center for Immunization and Respiratory Diseases. (2021). Historical Reference of Seasonal Influenza Vaccine Doses Distributed. <a href=\"https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm\">https://www.cdc.gov/flu/prevent/vaccine-supply-historical.htm</a> {/ref} These benefits carried forward as people aged: they protected people from being vulnerable to diseases including influenza.</p>\n\n\n\n<p>But the flu still causes a large burden today, especially in countries that have poor sanitation, healthcare, and low vaccination rates.&nbsp;</p>\n\n\n\n<p>Another challenge is that populations have been aging rapidly.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., &amp; Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref} In lower-income countries, the flu could become a larger burden as <a href=\"https://ourworldindata.org/grapher/historic-and-un-pop-projections-by-age\">their populations continue to age</a>.</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n    <block type=\"prominent-link\" style=\"is-style-thin\">\n        <link-url>https://ourworldindata.org/influenza-deaths</link-url>\n        <title></title>\n        <content>\n\n<p></p>\n\n</content>\n        <figure></figure>\n    </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>These estimates come from a study by Enrique Acosta and colleagues, using data from the United States.{ref}Acosta, E., Hallman, S. A., Dillon, L. Y., Ouellette, N., Bourbeau, R., Herring, D. A., Inwood, K., Earn, D. J. D., Madrenas, J., Miller, M. S., &amp; Gagnon, A. (2019). Determinants of Influenza Mortality Trends: Age-Period-Cohort Analysis of Influenza Mortality in the United States, 1959\u20132016. Demography, 56(5), 1723\u20131746. <a href=\"https://doi.org/10.1007/s13524-019-00809-y\">https://doi.org/10.1007/s13524-019-00809-y</a> {/ref}<br><br></li><li>The authors use national data on deaths and routine surveillance data for flu to calculate the rate of excess deaths during flu seasons, while accounting for changes in the age structure of the population.<br><br></li><li>The chart shows that the risk that someone dies from influenza at a given age has declined over time. But, because the population is getting larger and older, the total number of flu deaths has remained stable.</li></ul>\n\n\n\t</div>\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"635\" height=\"550\" src=\"https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png\" alt=\"People born more recently have a lower risk of dying from influenza. Even when they reached the same age, people born in 1940 had a third of the risk of dying from flu than those born in 1900. This risk halved further for those born in 1980.\" class=\"wp-image-56833\" srcset=\"https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-635x550.png 635w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-400x346.png 400w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-150x130.png 150w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort-768x665.png 768w, https://owid.cloud/app/uploads/2023/05/Historical-decline-in-influenza-deaths-by-birth-cohort.png 1252w\" sizes=\"(max-width: 635px) 100vw, 635px\" /></figure>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Flu seasons vary between countries</title>\n        <slug>flu-seasons-vary-between-countries</slug>\n        <content>\n\n<p>Several respiratory infections, including the flu, are more common in the winter.&nbsp;</p>\n\n\n\n<p>This is because they transmit more efficiently at lower temperatures and humidity, and when there is more social contact between people indoors.{ref}Petrova, V. N., &amp; Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. <a href=\"https://doi.org/10.1038/nrmicro.2017.118\">https://doi.org/10.1038/nrmicro.2017.118</a> {/ref}</p>\n\n\n\n<p>In the chart, you can see the share of flu tests that were positive in different countries.</p>\n\n\n\n<p>Although the precise start and end of a flu season vary between years, flu epidemics tend to occur between November and May in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, they generally occur between June and October.&nbsp;</p>\n\n\n\n<p>But in countries closer to the equator, there tend to be multiple peaks each year, or flu is present throughout the year. This may be because of rainy seasons, when people have more indoor contact.{ref}Chen, C., Jiang, D., Yan, D., Pi, L., Zhang, X., Du, Y., Liu, X., Yang, M., Zhou, Y., Ding, C., Lan, L., &amp; Yang, S. (2023). The global region-specific epidemiologic characteristics of influenza: World Health Organization FluNet data from 1996 to 2021. International Journal of Infectious Diseases, 129, 118\u2013124. <a href=\"https://pubmed.ncbi.nlm.nih.gov/36773717/\">https://pubmed.ncbi.nlm.nih.gov/36773717/</a>&nbsp;<br><br>Petrova, V. N., &amp; Russell, C. A. (2018). The evolution of seasonal influenza viruses. Nature Reviews Microbiology, 16(1), 47\u201360. <a href=\"https://doi.org/10.1038/nrmicro.2017.118\">https://doi.org/10.1038/nrmicro.2017.118</a><br><br>Paynter, S. (2015). Humidity and respiratory virus transmission in tropical and temperate settings. <em>Epidemiology &amp; Infection</em>, <em>143</em>(6), 1110\u20131118.<a href=\"https://doi.org/10.1017/S0950268814002702\"> https://doi.org/10.1017/S0950268814002702<br><br></a>Igboh, L. S., Roguski, K., Marcenac, P., Emukule, G. O., Charles, M. D., Tempia, S., Herring, B., Vandemaele, K., Moen, A., Olsen, S. J., Wentworth, D. E., Kondor, R., Mott, J. A., Hirve, S., Bresee, J. S., Mangtani, P., Nguipdop-Djomo, P., &amp; Azziz-Baumgartner, E. (2023). Timing of seasonal influenza epidemics for 25 countries in Africa during 2010\u201319: A retrospective analysis. The Lancet Global Health, 11(5), e729\u2013e739. <a href=\"https://doi.org/10.1016/S2214-109X(23)00109-2\">https://doi.org/10.1016/S2214-109X(23)00109-2</a>&nbsp;</p>\n\n\n\n<p>\u200b\u200bNewman, L. P., Bhat, N., Fleming, J. A., &amp; Neuzil, K. M. (2018). Global influenza seasonality to inform country-level vaccine programs: An analysis of WHO FluNet influenza surveillance data between 2011 and 2016. PLOS ONE, 13(2), e0193263. <a href=\"https://doi.org/10.1371/journal.pone.0193263\">https://doi.org/10.1371/journal.pone.0193263</a> {/ref}</p>\n\n\n\n<p>You can see this in the chart for Singapore and Thailand, for example.</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Testing to confirm the flu is limited in many countries.{ref}World Health Organization &amp; others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?tab=chart&amp;time=2014-01-01..2020-01-01&amp;facet=none&amp;country=SGP~THA&amp;Confirmed+cases+or+Symptoms=Confirmed+cases&amp;Metric=Share+of+positive+tests+%28%25%29&amp;Interval=Monthly&amp;Surveillance+type=All+types&amp;hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Tracking flu-like symptoms can be informative when testing is limited</title>\n        <slug>tracking-flu-like-symptoms-can-be-informative-when-testing-is-limited</slug>\n        <content>\n\n<p>Direct testing for the presence of the influenza virus is limited in many countries. For this reason, flu cases recorded in public databases \u2013 and the global data shown in our data explorer \u2013 greatly underestimate the true number of cases.</p>\n\n\n\n<p>Because of the lack of direct testing, it is useful to track flu-like symptoms.</p>\n\n\n\n<p>It is important to note however that these symptoms are not specific to the flu: people with other diseases \u2013 such as rhinovirus, COVID-19, common colds, malaria, and others \u2013 can also have these symptoms and meet the following criteria.</p>\n\n\n\n<p><strong>Acute respiratory infections (ARIs)</strong> are the broadest type of metric. They can include anyone with sudden onset of at least one of the following symptoms: cough, sore throat, shortness of breath or rhinitis (inflammation of the mucous lining of the nose), but <em>only if</em> they were judged by a doctor to be caused by an infection.</p>\n\n\n\n<p><strong>Influenza-like illnesses (ILIs)</strong> are narrower \u2013&nbsp;they include only people with a sudden respiratory infection with a fever above 38\u00baC and a cough within the last 10 days.{ref}This definition has been used since 2011, after the Swine flu pandemic. Since then, most countries, but not all, have adopted it.<br>Fitzner, J., Qasmieh, S., Mounts, A. W., Alexander, B., Besselaar, T., Briand, S., Brown, C., Clark, S., Dueger, E., Gross, D., Hauge, S., Hirve, S., Jorgensen, P., Katz, M. A., Mafi, A., Malik, M., McCarron, M., Meerhoff, T., Mori, Y., \u2026 Vandemaele, K. (2018). Revision of clinical case definitions: Influenza-like illness and severe acute respiratory infection. Bulletin of the World Health Organization, 96(2), 122\u2013128.<br><a href=\"https://doi.org/10.2471/BLT.17.194514\">https://doi.org/10.2471/BLT.17.194514</a><br>World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021.<a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y\"> https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., &amp; Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. Scientific Reports, 11(1), 795. <a href=\"https://doi.org/10.1038/s41598-020-80842-9\">https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</p>\n\n\n\n<p><strong>Severe acute respiratory infections (SARIs)</strong> are severe cases of ILIs: they include only people with a sudden respiratory infection who had a fever above 38\u00baC, a cough, <em>and</em> required hospitalization.</p>\n\n\n\n<p>In many countries, only a fraction of clinics in the country report flu-like metrics to their national system. This means that the number of reported cases <em>does not</em> tell us about the total number of people with these infections.</p>\n\n\n\n<p>Since some countries provide data from a larger number of healthcare clinics than others, this needs to be kept in mind when comparing different countries.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y\">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y</a>&nbsp; {/ref}</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>Countries may use different sources for each metric. Some countries collect data for ARIs universally, i.e. from all hospitals and outpatient clinics in the country, while many do not.{ref}For example, the United States uses the &#8216;ILINet&#8217; system, which is connected to many clinics across the country.<br>But, in many countries including the US, clinics participate on a voluntary basis, so not all clinics are included. Clinics in some states and demographics are less likely to be part of the system. See also: Baltrusaitis, K., Vespignani, A., Rosenfeld, R., Gray, J., Raymond, D., &amp; Santillana, M. (2019). Differences in Regional Patterns of Influenza Activity Across Surveillance Systems in the United States: Comparative Evaluation. JMIR Public Health and Surveillance, 5(4), e13403. <a href=\"https://doi.org/10.2196/13403\">https://doi.org/10.2196/13403</a><br>This means &#8216;non-sentinel&#8217; data may not be representative of the cases across the country. They may also lack high-quality testing.{/ref} <br><br></li><li>The country&#8217;s sampling method may also be different for ILIs or SARIs. The sampling strategy for each metric and for each country is reported to the WHO.{ref}World Health Organization. (2022). Respiratory Viruses Surveillance Country, Territory and Area Profiles, 2021. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y\">https://apps.who.int/iris/bitstream/handle/10665/352183/WHO-EURO-2022-4760-44523-63025-eng.pdf?sequence=1&amp;isAllowed=y</a><br>Simpson, R. B., Gottlieb, J., Zhou, B., Hartwick, M. A., &amp; Naumova, E. N. (2021). Completeness of open access FluNet influenza surveillance data for Pan-America in 2005\u20132019. <em>Scientific Reports</em>, <em>11</em>(1), 795.<a href=\"https://doi.org/10.1038/s41598-020-80842-9\"> https://doi.org/10.1038/s41598-020-80842-9</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?uniformYAxis=0&amp;country=~Northern+Hemisphere&amp;Confirmed+cases+or+Symptoms=Symptoms&amp;Metric=Comparison+of+data+on+respiratory+infections&amp;Interval=Monthly&amp;Surveillance+type=All+types&amp;hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">Different flu strains circulate each year</title>\n        <slug>different-flu-strains-circulate-each-year</slug>\n        <content>\n\n<p>Each year, flu vaccines need to be updated because different viruses circulate in the population.</p>\n\n\n\n<p>This happens for two reasons. One is that flu viruses gradually evolve, and can evade people\u2019s immunity and cause reinfections.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., &amp; Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>\n\n\n\n<p>Another reason is that there are different types of flu that circulate each season.&nbsp;</p>\n\n\n\n<p>You can see this in the chart. It shows cases of two types of influenza: A and B, which commonly infect humans.{ref}There are four types of influenza viruses: A, B, C, and D. Influenza A and B tend to spread between people around the world each year. In contrast, influenza C and D mainly infect birds and other animals.{/ref}</p>\n\n\n\n<p>There are many subtypes of influenza A, including H1N1 and H3N2 among others.{ref}The subtypes are named according to two proteins the virus has on its surface: hemagglutinin (H) and neuraminidase (N). There are many subtypes of each of these two proteins (18 hemagglutinin subtypes and 11 neuraminidase subtypes), but only some of the combinations have been observed. For example, H3N2 is a type of influenza A virus which has hemagglutinin subtype 3 and neuraminidase subtype 2.<br>Long, J.S., Mistry, B., Haslam, S.M. et al. Host and viral determinants of influenza A virus species specificity. Nat Rev Microbiol 17, 67\u201381 (2019). <a href=\"https://doi.org/10.1038/s41579-018-0115-z\">https://doi.org/10.1038/s41579-018-0115-z</a> {/ref} In contrast, there are two lineages of influenza B, which are called Victoria and Yamagata.{ref}Caini, S., Kusznierz, G., Garate, V. V., Wangchuk, S., Thapa, B., de Paula J\u00fanior, F. J., Ferreira de Almeida, W. A., Njouom, R., Fasce, R. A., Bustos, P., Feng, L., Peng, Z., Araya, J. L., Bruno, A., de Mora, D., Barahona de G\u00e1mez, M. J., Pebody, R., Zambon, M., Higueros, R., \u2026 the Global Influenza B Study team. (2019). The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century. PLOS ONE, 14(9), e0222381. <a href=\"https://doi.org/10.1371/journal.pone.0222381\">https://doi.org/10.1371/journal.pone.0222381</a> {/ref}</p>\n\n\n\n<p>Unfortunately, testing for the flu is limited, and many countries lack testing to identify specific flu strains. This is why the number of confirmed cases shown in the chart greatly underestimates the actual number of infections, and why some cases are shown as \u2018unknown subtype/lineage\u2019.{ref}The subtype or lineage of a flu virus is not always determined during testing. This tends to be because some clinics do not test for all subtypes of influenza, due to a lack of testing resources. These are listed as unknown subtypes/lineages of influenza. For example, with influenza A, labs tend to test only whether they are the H1 or H3 strain.</p>\n\n\n\n<p>However, unknown subtypes/lineages can also include novel influenza strains, which have gone through significant evolution.{/ref}</p>\n\n\n\n<p>This lack of testing is a problem. When there is a mismatch between the strains in the vaccine and the viruses circulating in the population, vaccines tend to have lower efficacy and the flu season tends to be more severe.{ref}Tricco, A. C., Chit, A., Soobiah, C., Hallett, D., Meier, G., Chen, M. H., Tashkandi, M., Bauch, C. T., &amp; Loeb, M. (2013). Comparing influenza vaccine efficacy against mismatched and matched strains: A systematic review and meta-analysis. BMC Medicine, 11(1), 153. <a href=\"https://doi.org/10.1186/1741-7015-11-153\">https://doi.org/10.1186/1741-7015-11-153</a> {/ref} Additionally, limited testing also means the world is less able to detect new strains that may cause pandemics.{ref}Jernigan, D. B., Lindstrom, S. . L., Johnson, J. . R., Miller, J. D., Hoelscher, M., Humes, R., Shively, R., Brammer, L., Burke, S. A., Villanueva, J. M., Balish, A., Uyeki, T., Mustaquim, D., Bishop, A., Handsfield, J. H., Astles, R., Xu, X., Klimov, A. I., Cox, N. J., &amp; Shaw, M. W. (2011). Detecting 2009 Pandemic Influenza A (H1N1) Virus Infection: Availability of Diagnostic Testing Led to Rapid Pandemic Response. Clinical Infectious Diseases, 52(suppl_1), S36\u2013S43. <a href=\"https://doi.org/10.1093/cid/ciq020\">https://doi.org/10.1093/cid/ciq020</a> {/ref}</p>\n\n\n\n<p>To address this, the world needs more routine testing for the flu.</p>\n\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>This metric shows confirmed cases of flu: when people with flu symptoms have respiratory samples taken and tested to determine whether they have the influenza virus and whether they have influenza A or B. Some surveillance centers also test the samples to identify their subtype. These are described as strains for influenza A (e.g., A H7N9) and lineages for influenza B (e.g., B Yamagata). This testing can use molecular detection, virus culture, or immunological methods.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., &amp; Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a>&nbsp;<br>World Health Organization &amp; others. (2015). A manual for estimating disease burden associated with seasonal influenza. <a href=\"https://www.who.int/publications/i/item/9789241549301\">https://www.who.int/publications/i/item/9789241549301</a>&nbsp; {/ref}<br><br></li><li>Testing to confirm flu is limited in many countries.{ref}World Health Organization &amp; others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a> {/ref} In addition, not all confirmed flu cases are tested further to identify their strain. This is why many cases are shown as having an unknown subtype or lineage.{ref}Even in participating clinics, some data can be missing. For example, data collection forms may not be filled in or reported to the WHO for all patients with influenza-like illnesses who visit the clinics. Samples may not be packaged, stored, transported, or tested correctly, especially in regions with a lack of healthcare staff and supplies.<br>See also: World Health Organization &amp; others. (2019). Global influenza strategy 2019-2030. <a href=\"https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf\">https://apps.who.int/iris/bitstream/handle/10665/311184/9789241515320-eng.pdf</a><br>Gentile, A., Paget, J., Bellei, N., Torres, J. P., Vazquez, C., Laguna-Torres, V. A., &amp; Plotkin, S. (2019). Influenza in Latin America: A report from the Global Influenza Initiative (GII). Vaccine, 37(20), 2670\u20132678. <a href=\"https://doi.org/10.1016/j.vaccine.2019.03.081\">https://doi.org/10.1016/j.vaccine.2019.03.081</a> {/ref}</li></ul>\n\n\n\t</div>\n\n\n<iframe src=\"https://ourworldindata.org/explorers/influenza?uniformYAxis=0&amp;country=Northern+Hemisphere~Southern+Hemisphere&amp;Confirmed+cases+or+Symptoms=Confirmed+cases&amp;Metric=Confirmed+cases+%28by+strain%29&amp;Interval=Monthly&amp;Surveillance+type=All+types&amp;hideControls=true\" loading=\"lazy\" style=\"width: 100%; height: 600px; border: 0px none;\"></iframe>\n\n</content>\n\t</block>\n\n\t<block type=\"key-insight\">\n\t\t<title is-hidden=\"0\">The Spanish flu caused the largest influenza pandemic in history</title>\n        <slug>the-spanish-flu-caused-the-largest-influenza-pandemic-in-history</slug>\n        <content>\n\n<p>Some flu seasons are far more severe than usual seasonal influenza.</p>\n\n\n\n<p>This happens when influenza viruses combine with each other to make new strains which are more infectious and lethal, and lead to deadly pandemics.{ref}Krammer, F., Smith, G. J. D., Fouchier, R. A. M., Peiris, M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J., Webster, R. G., &amp; Garc\u00eda-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1), 3. <a href=\"https://doi.org/10.1038/s41572-018-0002-y\">https://doi.org/10.1038/s41572-018-0002-y</a> {/ref}</p>\n\n\n\n<p>For example, the Spanish flu pandemic was caused by a combination of human influenza and another animal influenza. Together, they formed the new H1N1 virus.{ref}Worobey, M., Han, G.-Z., &amp; Rambaut, A. (2014). Genesis and pathogenesis of the 1918 pandemic H1N1 influenza A virus. Proceedings of the National Academy of Sciences, 111(22), 8107\u20138112. https://doi.org/10.1073/pnas.1324197111{/ref}</p>\n\n\n\n<p>As you can see in the chart, it led to the <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">most devastating</a> influenza pandemic in recorded history. Estimates of the death toll vary: some studies estimate that 17.4 million people died globally from the Spanish flu between 1918 and 1920, while others estimate a much higher death toll of 50 to 100 million deaths.{ref}P. Spreeuwenberg; et al. (1 December 2018). \u201cReassessing the Global Mortality Burden of the 1918 Influenza Pandemic\u201d. American Journal of Epidemiology. 187 (12): 2561\u20132567. doi:10.1093/aje/kwy191. PMID 30202996. Online <a href=\"https://academic.oup.com/aje/article/187/12/2561/5092383\">here</a>.{/ref}</p>\n\n\n\n<p>The Spanish flu pandemic was most severe among children and young adults. Life expectancy at birth and at young ages <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">declined</a> by more than ten years.</p>\n\n\n\n<p>But surprisingly, it did not have a significant impact on older people. Research suggests that this is because older generations had been exposed to similar H1 influenza viruses decades before the pandemic began, which gave them some protection against the Spanish flu strain.{ref}Gagnon, A., Miller, M. S., Hallman, S. A., Bourbeau, R., Herring, D. A., Earn, D. J. D., &amp; Madrenas, J. (2013). Age-specific mortality during the 1918 influenza pandemic: Unravelling the mystery of high young adult mortality. PloS One, 8(8), e69586. <a href=\"https://doi.org/10.1371/journal.pone.0069586\">https://doi.org/10.1371/journal.pone.0069586</a>&nbsp;</p>\n\n\n\n<p>Luk, J., Gross, P., &amp; Thompson, W. W. (2001). Observations on Mortality during the 1918 Influenza Pandemic. Clinical Infectious Diseases, 33(8), 1375\u20131378. <a href=\"https://doi.org/10.1086/322662\">https://doi.org/10.1086/322662</a> </p>\n\n\n\n<p>Ma, J., Dushoff, J., &amp; Earn, D. J. D. (2011). Age-specific mortality risk from pandemic influenza. Journal of Theoretical Biology, 288, 29\u201334. <a href=\"https://doi.org/10.1016/j.jtbi.2011.08.003\">https://doi.org/10.1016/j.jtbi.2011.08.003</a> {/ref}</p>\n\n\n\n<p>In this article, we provide more detail:</p>\n\n\n    <block type=\"prominent-link\" style=\"is-style-thin\">\n        <link-url>https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history</link-url>\n        <title></title>\n        <content>\n\n<p></p>\n\n</content>\n        <figure></figure>\n    </block>\n\n\t<div class=\"wp-block-owid-technical-text\">\n\t\t\n\n<h5>What you should know about this data</h5>\n\n\n\n<ul><li>In the chart, we show a comparison of mortality estimates from different research groups for recent flu pandemics in history.{ref}Johnson, N. P. A. S., and Mueller, J. (2002). Updating the Accounts: Global Mortality of the 1918-1920 \u201cSpanish\u201d Influenza Pandemic. <em>Bulletin of the History of Medicine</em>, <em>76</em>(1), 105\u2013115. <a href=\"http://www.jstor.org/stable/44446153\">http://www.jstor.org/stable/44446153<br></a>Patterson, K. D., &amp; Pyle, G. F. (1991). The geography and mortality of the 1918 influenza pandemic. <em>Bulletin of the History of Medicine</em>, <em>65</em>(1), 4\u201321. <a href=\"http://www.jstor.org/stable/44447656\">http://www.jstor.org/stable/44447656</a><br>Spreeuwenberg, P., Kroneman, M., &amp; Paget, J. (2018). Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic. <em>American Journal of Epidemiology</em>, <em>187</em>(12), 2561\u20132567.<a href=\"https://doi.org/10.1093/aje/kwy191\"> https://doi.org/10.1093/aje/kwy191<br></a>Paget J, Spreeuwenberg P, Charu V, Taylor RJ, Iuliano AD, Bresee J, Simonsen L, Viboud C; Global Seasonal Influenza-associated Mortality Collaborator Network and GLaMOR Collaborating Teams*. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019 Dec;9(2):020421. doi: 10.7189/jogh.09.020421. <a href=\"https://jogh.org/documents/issue201902/jogh-09-020421.pdf\">https://jogh.org/documents/issue201902/jogh-09-020421.pdf<br></a>Dawood, F. S., Iuliano, A. D., Reed, C., Meltzer, M. I., Shay, D. K., Cheng, P.-Y., Bandaranayake, D., Breiman, R. F., Brooks, W. A., Buchy, P., Feikin, D. R., Fowler, K. B., Gordon, A., Hien, N. T., Horby, P., Huang, Q. S., Katz, M. A., Krishnan, A., Lal, R., \u2026 Widdowson, M.-A. (2012). Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: A modelling study. The Lancet Infectious Diseases, 12(9), 687\u2013695. <a href=\"https://doi.org/10.1016/S1473-3099(12)70121-4\">https://doi.org/10.1016/S1473-3099(12)70121-4<br></a>Simonsen, L., Spreeuwenberg, P., Lustig, R., Taylor, R. J., Fleming, D. M., Kroneman, M., Van Kerkhove, M. D., Mounts, A. W., Paget, W. J., &amp; the GLaMOR Collaborating Teams. (2013). Global Mortality Estimates for the 2009 Influenza Pandemic from the GLaMOR Project: A Modeling Study. PLoS Medicine, 10(11), e1001558. <a href=\"https://doi.org/10.1371/journal.pmed.1001558\">https://doi.org/10.1371/journal.pmed.1001558</a> {/ref}<br><br></li><li>Estimates for historical flu pandemics tend to come from data on mortality rates. Pandemics cause sudden shocks to mortality compared to typical years. Researchers can calculate the excess mortality during the pandemic to estimate the deaths they caused while adjusting for other known factors, such as famine and war.&nbsp;<br><br></li><li>There are still large uncertainties in each estimate, because historical mortality records are limited in many countries. However, the range of estimates for these pandemics is much higher than a typical flu season. For the Spanish flu pandemic, estimates are more than an order of magnitude higher.</li></ul>\n\n\n\t</div>\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"750\" height=\"550\" src=\"https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png\" alt=\"\" class=\"wp-image-57012\" srcset=\"https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-750x550.png 750w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-400x293.png 400w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-150x110.png 150w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-768x563.png 768w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-1536x1126.png 1536w, https://owid.cloud/app/uploads/2023/05/Influenza-pandemics-in-comparison-1-2048x1502.png 2048w\" sizes=\"(max-width: 750px) 100vw, 750px\" /></figure>\n\n</content>\n\t</block>\n\n\n<p></p>\n\n</insights>\n\t</block>\n\n\n<h3>Explore our data on influenza</h3>\n\n\n\n<iframe class=\"wp-block-full-content-width\" src=\"https://ourworldindata.org/explorers/influenza?country=Northern+Hemisphere~Southern+Hemisphere\" loading=\"lazy\" style=\"width: 100%; min-height: 740px; max-height: 950px; height: 100vh; border: 0px none;\"></iframe>\n\n\n\n<div class=\"wp-block-columns is-style-sticky-right\">\n<div class=\"wp-block-column\">\n<h4>Why we provide this Influenza Data Explorer</h4>\n\n\n\n<p>With this Flu Explorer, we aim to provide a helpful resource for epidemiologists, infectious disease researchers, and public health experts to understand the global spread of the influenza virus.</p>\n\n\n\n<p>It differs from our widely-used infectious diseases projects, such as the <a href=\"https://ourworldindata.org/explorers/coronavirus-data-explorer\">COVID-19 Explorer</a> and the <a href=\"https://ourworldindata.org/explorers/monkeypox\">Mpox Explorer</a>. These tools are designed for a broad audience. Unfortunately, flu data is incomplete in many ways, making it harder to communicate. <strong>This tool is therefore designed for users with pre-existing knowledge to navigate effectively the complex data published by the World Health Organization.</strong></p>\n\n\n\n<p>The explorer also highlights the significant gaps in influenza data. It is an important reminder of the need to improve data collection and reporting.</p>\n</div>\n</div>\n\n\n\n\n<div class=\"wp-container-65e6e284044bb wp-block-group wp-block-research-and-writing\">\n<h2 id=\"research-writing\">Research &amp; Writing</h2>\n\n\n\n<h4>Key articles on Influenza</h4>\n\n\n\t<div class=\"wp-block-owid-grid research-and-writing__sub-category\">\n\t\t\n      <div class=\"wp-block-owid-card  with-image\" data-no-lightbox>\n        <a href=\"https://ourworldindata.org/influenza-deaths\">\n            <figure><img width=\"768\" height=\"303\" src=\"https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-768x303.png\" class=\"attachment-medium_large size-medium_large\" alt=\"\" loading=\"lazy\" srcset=\"https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-768x303.png 768w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-400x158.png 400w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-800x316.png 800w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-150x59.png 150w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail-1536x606.png 1536w, https://owid.cloud/app/uploads/2022/10/Flu-deaths-thumbnail.png 1592w\" sizes=\"(max-width: 768px) 100vw, 768px\" /></figure>\n            <div class=\"text-wrapper\">\n              <div class=\"title\">How many people die from the flu?</div class=\"title\">\n              <div class=\"description\">\n                \n\n<p><em>Saloni Dattani</em></p>\n\n\n              </div>\n            </div>\n        </a>\n      </div>\n\n      <div class=\"wp-block-owid-card  with-image\" data-no-lightbox>\n        <a href=\"https://ourworldindata.org/spanish-flu-largest-influenza-pandemic-in-history\">\n            <figure><img width=\"768\" height=\"443\" src=\"https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-768x443.png\" class=\"attachment-medium_large size-medium_large\" alt=\"Life expectancy - spanish flu\" loading=\"lazy\" srcset=\"https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-768x443.png 768w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-400x231.png 400w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-800x461.png 800w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-150x87.png 150w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27-1536x886.png 1536w, https://owid.cloud/app/uploads/2020/03/Screenshot-2022-03-14-at-11.51.27.png 1720w\" sizes=\"(max-width: 768px) 100vw, 768px\" /></figure>\n            <div class=\"text-wrapper\">\n              <div class=\"title\">The Spanish flu: The global impact of the largest influenza pandemic in history</div class=\"title\">\n              <div class=\"description\">\n                \n\n<p><em>Max Roser</em></p>\n\n\n              </div>\n            </div>\n        </a>\n      </div>\n\n\t</div>\n\n\t<div class=\"wp-block-owid-grid research-and-writing__sub-category\">\n\t\t\n\t</div></div>\n\n\n\n\n\n\t<block type=\"all-charts\"></block>\n\n<p></p>",
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    "excerpt": {
        "rendered": "Flu epidemics kill hundreds of thousands of people globally each year, but countries can respond and save lives with better data.",
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    "date_gmt": "2023-05-18T08:46:00",
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