At a glance
The overall burden of influenza (flu) for the 2021-2022 season was an estimated 9.4 million flu illnesses, 4.3 million flu-related medical visits, 100,000 flu-related hospitalizations, and 4,900 flu deaths.
Background
Note
For the past several years, CDC has used a mathematical model to estimate the numbers of flu illnesses, medical visits, hospitalizations, and deaths that occur each year1234. The methods used to calculate the estimates have been described previously12. CDC uses the estimates of the burden of flu in the population to inform policy and communications related to flu prevention and control.
The overall burden of flu for the 2021-2022 season was an estimated 9.4 million flu illnesses, 4.3 million flu-related medical visits, 100,000 flu-related hospitalizations, and 4,900 flu deaths (Table 1).
2021–2022 Burden Estimates
With the emergence of SARS-CoV-2 (the virus that causes COVID-19), flu activity during 2021-2022 was lower than observed before the pandemic. Compared with flu seasons prior to pandemic, the 2021–2022 flu season was mild and occurred in two waves, with a higher number of hospitalizations in the second wave. Flu activity in the United States during the 2021–2022 season began to increase in November, declined in January 2022, increased again in March 2022 and remained elevated until mid-June 2022. The season was characterized by two distinct waves and predominately influenza A virus circulation. Overall, influenza A(H3N2) viruses were the most commonly detected influenza viruses this season. The 2021-2022 season is described as having low severity**; however, the effect of flu differed by age group and the severity of the season in some age groups was higher – hospitalization rates among older adults aged 65 years and older were higher compared with other age groups5.
CDC estimates that the burden of illness during the 2021–2022 season was low with an estimated 9.4 million people sick with flu, 4.3 million visits to a health care provider for flu, 100,000 hospitalizations for flu, and 4,900 flu deaths (Table 1). The number of cases of flu-related illness, medically attended illnesses, hospitalizations, and deaths were the lowest since the 2011-2012 season which was the first full season following the influenza A(H1N1)pdm09 pandemic6. Adoption of mitigation measures intended to prevent the spread of COVID-19 may have also impacted the timing or severity of flu activity during the 2021-2022 season.
CDC's estimates of hospitalizations and mortality associated with the 2021–2022 flu season show that even with the co-circulation of SARS-CoV-2 and other respiratory viruses, influenza viruses do still cause severe disease and death. Older adults accounted for 84% of deaths, which is similar to recent seasons before the COVID-19 pandemic. These findings continue to highlight the fact that older adults are particularly vulnerable to severe disease with influenza virus infection and that flu prevention measures such as vaccination are important to reducing the impact of the seasonal epidemics on the population and health care system.
Deaths in children with laboratory-confirmed influenza virus infection have been a reportable disease in the United States since 2004; 49 deaths were reported for the 2021-2022 season as of October 31, 2023. Although it is likely that reported deaths may under-estimate the true number of deaths for this age group, no deaths in children (younger than 18 years) were observed through the Influenza Hospital Surveillance Network (FluSurv-NET) during the 2021-2022 flu season. As a result, it was not possible to estimate deaths in this age group.
Conclusion
During the 2021-2022 flu season, CDC estimates that flu was associated with 9.4 million illnesses, 4.3 million medical visits, 100,000 hospitalizations, and 4,900 deaths. The flu burden was similar to the burden observed during the 2011-2012 season.
Why the estimates for the 2021-2022 flu season changed compared with previous estimates for this season
The model that CDC uses to estimate the burden of flu includes information collected about flu testing practices and vital records death data. Because current testing and death data were not available at the time of estimation (it takes approximately two years to finalize information on flu testing practices), the estimates that were previously published on the CDC website were made using testing information from prior flu seasons.
The 2021-2022 burden estimates are still preliminary and may change as more information for the season becomes available.
More information on How CDC Estimates Flu Burden is available, as well as answers to frequently asked questions about CDC's flu burden estimates.
Burden Estimates Limitations
These estimates are subject to some limitations. Keep reading to learn more.
2021-2022 Tables for Flu Burden and Burden-Prevented Estimates
*Some of the data used to calculate burden estimates are incomplete or not yet available. These estimates will change as those data become available and the estimates are updated.
**No pediatric deaths were reported in the surveillance system used to estimate the flu death burden; thus, our model was unable to estimate any pediatric deaths due to flu during the 2021-22 season [the Influenza Hospital Surveillance Network (FluSurv-NET)]. However, 49 pediatric deaths (as of October 31, 2023) were reported through a different surveillance system for reporting of pediatric deaths due to flu (the Influenza-associated Pediatric Mortality Surveillance System).
†Uncertainty interval: Adjusted estimates are presented in two parts: an uncertainty interval [UI] and a point estimate. The uncertainty interval provides a range in which the true number or rate of flu illnesses, medical visits, hospitalizations, or deaths would be expected to fall if the same study was repeated many times, and it gives an idea of the precision of the point estimate. A 95% uncertainty interval means that if the study were repeated 100 times, then 95 out of 100 times the uncertainty interval would contain the true point estimate. Conversely, in only 5 times out of a 100 would the uncertainty interval not contain the true point estimate.
- Reed C, Chaves SS, Daily Kirley P, Emerson R, Aragon D, Hancock EB, et al. Estimating influenza disease burden from population-based surveillance data in the United States. PLoS One. 2015;10(3):e0118369.
- Rolfes MA, Foppa IM, Garg S, et al. Annual estimates of the burden of seasonal influenza in the United States: A tool for strengthening influenza surveillance and preparedness. Influenza Other Respir Viruses 2018; 12(1): 132–7.
- Centers for Disease Control and Prevention. Estimated influenza illnesses and hospitalizations averted by influenza vaccination – United States, 2012-13 influenza season. MMWR Morb Mortal Wkly Rep. 2013 Dec 13;62(49):997-1000.
- Reed C, Kim IK, Singleton JA, Chaves SS, Flannery B, Finelli L, et al. Estimated influenza illnesses and hospitalizations averted by vaccination–United States, 2013-14 influenza season. MMWR Morb Mortal Wkly Rep. 2014 Dec 12;63(49):1151-4.
- Merced-Morales A, Daly P, Abd Elal AI, et al. Influenza Activity and Composition of the 2022–23 Influenza Vaccine — United States, 2021–22 Season. MMWR Morb Mortal Wkly Rep 2022;71:913–919. DOI: http://dx.doi.org/10.15585/mmwr.mm7129a1
- Update: Influenza Activity — United States, 2011–12 Season and Composition of the 2012–13 Influenza Vaccine https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6122a4.htm