2021–2022 Estimated Flu Illnesses, Medical Visits, Hospitalizations, and Deaths Prevented by Flu Vaccination

This web page provides estimates on the burden of influenza (flu) and the effects of annual flu vaccination in the United States for the 2021–2022 season.

For more than a decade, CDC has used a model to estimate the numbers of flu illnesses, medical visits, hospitalizations, and deaths during the flu season, and to estimate the beneficial effects that flu vaccination had on outcomes in the United States (1-6). The methods used to calculate the estimates of flu burden prevented by flu vaccination have been described previously (1, 2, 6) and are outlined briefly below. CDC uses the estimates of the impact of flu vaccination to inform policy and communications promoting vaccination as the best way to prevent flu.

CDC estimates that during the 2021-2022 season, flu vaccination prevented 1.8 million flu-related illnesses, 1,000,000 medical visits, 22,000 hospitalizations, and nearly 1,000 deaths (Table 1).

2021-22 Flu Burden Prevented by Vaccination

Compared with flu seasons prior to the COVID-19 pandemic, the 2021–2022 season was considered a low severity season and occurred in two waves, with more hospitalizations occurring in the second wave. Seasonal flu activity began to increase in November 2021, declined in January 2022, increased again in March 2022, and remained elevated until mid-June 2022, far longer than is typical. The season had predominately influenza A virus circulation with subtype A(H3N2) viruses most commonly detected.

Despite limited circulation, CDC was able to calculate estimates of flu disease burden and estimates that during the 2021–2022 season 9 million people developed flu illness, 4 million people with flu went to a health care provider, 100,000 people with flu were hospitalized, and 5,000 people died with flu (7). ([refer to webpage with estimated burden]). The 2021-2022 season is described as being of low severity across all age groups and the three influenza indicators used to assess severity (outpatient visits, hospitalizations, and deaths); however, the burden of flu differed by the outcomes estimated by age group (community illnesses, medically attended illnesses, hospitalizations, and deaths) . The burden of flu was the lowest since the 2011-2012 season, which was the first full season following the 2009 H1N1 pandemic (8). Adoption of mitigation measures to prevent the spread of SARS-CoV-2 may have also impacted the timing or severity of flu activity during the 2021-2022 season.

CDC estimates that during the 2021-2022 season, 50% to 74% of people in the United States were vaccinated against flu. Flu vaccine effectiveness for the 2021-2022 season for all ages was 36% but varied by age group and was highest for children 6 months to 4 years (77%) and lowest among adults 50-64 years (8%) (10).  Based on the 2021-2022 flu disease burden estimates along with vaccine coverage and vaccine effectiveness data, CDC estimates that 1.8 million flu-related illnesses, 1,000,000 medical visits, 22,000 hospitalizations, and 1,000 deaths were prevented by flu vaccination. The number and proportion of flu-related hospitalizations prevented by vaccination during 2021–2022 varied by age group, due to age-specific differences in flu burden, vaccine coverage, and vaccine effectiveness. Flu vaccination prevented the fewest illnesses among adults 50 to 64 years, an age group with approximately 50% vaccine coverage and for whom flu vaccine effectiveness was lowest (8%) for the most common circulating influenza A(H3N2) virus. Flu vaccination prevented the highest number of community illnesses, medically attended illnesses, hospitalizations, and deaths among children 6 months to 4 years, an age group in which vaccine uptake was moderate (67%), and vaccine effectiveness was the greatest (77%).

Annual flu vaccination is the best way to protect against flu and associated complications. Several efforts are currently underway to understand and investigate factors that contribute to reduced or increased vaccine effectiveness and inform changes to vaccine composition, formulations, or production (11-13).

Improvements in vaccine coverage could also provide great public health benefit, producing even greater reductions in illnesses and lowering demands on the health system. During flu seasons, the health care system, including outpatient clinics, emergency departments, and hospitals, can experience high patient volumes. Any reduction in flu-associated medical visits and hospitalizations can reduce demands on the health care systems and improve health care providers’ abilities to provide the best care. Strategies to improve flu vaccine coverage include: using patient reminder/recall systems aided by immunization information systems; expanding access to flu vaccination at pharmacies, workplaces, and schools; and organizing mobile and/or outdoor vaccination clinics.

Conclusion

CDC estimates that during the 2021-2022 season flu vaccination contributed to meaningful reductions in the burden of flu. This report underscores the benefits of the current flu vaccination program but highlights areas where improvements in flu vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health.

Limitations

These estimates are subject to several limitations. First, influenza vaccination coverage estimates were derived from reports by survey respondents, not vaccination records, and are subject to recall bias. These coverage estimates are based on telephone surveys with relatively low response rates; non-response bias may remain after weighting for the survey design. Estimates of the number of people vaccinated based on these survey data have often exceeded the actual number of flu vaccine doses distributed, indicating that coverage estimates used in this report may overestimate the numbers of flu illnesses and hospitalizations prevented by flu vaccination. The model of prevented flu illness calculates outcomes directly prevented among people who were vaccinated. If indirect protection from decreased exposure to infectious people in a partially flu vaccinated population (i.e., herd immunity) also occurred, the model would underestimate the number of flu illnesses and hospitalizations prevented by flu vaccination. Estimates of the prevented burden in adults aged 65 years and older do not reflect the increasing use of high-dose or adjuvanted flu vaccines, which may have higher effectiveness compared with standard-dose, unadjuvanted vaccines; nor does the estimate reflect that flu vaccine effectiveness might continue to decrease with age, reaching very low levels among the oldest adults who also have the highest rates of flu vaccination. Finally, because the data and methods used to make these calculations are continually updated, current estimates are preliminary and will be updated when final data are available.

References

1. Kostova D, Reed C, Finelli L, Cheng PY, Gargiullo PM, Shay DK, et al. Influenza Illness and Hospitalizations Averted by Influenza Vaccination in the United States, 2005-2011. PLoS One. 2013;8(6):e66312.
2. 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.
3. 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.
4. 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.
5. Centers for Disease Control and Prevention. Estimated Influenza Illnesses and Hospitalizations Averted by Vaccination — United States, 2014–15 Influenza Season. 2015 December 10, 2015 [cited 2016 October 27]; Available from: http://www.cdc.gov/flu/about/disease/2014-15.htm
6. Tokars JI, Rolfes MA, Foppa IM, Reed C. An evaluation and update of methods for estimating the number of influenza cases averted by vaccination in the United States. Vaccine. 2018 Nov 19;36(48):7331-7.
7. https://www.cdc.gov/flu/about/burden/2021-2022.htm
8. 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
9. Centers for Disease Control and Prevention. Influenza Vaccination Coverage. 2022 October 18, 2022 [cited 2022 November 30]; Available from: https://www.cdc.gov/flu/fluvaxview/
10. Chung JR, Kim SS, Kondor RJ, et al., Interim Estimates of 2021–22 Seasonal Influenza Vaccine Effectiveness — United States, February 2022. MMWR Morb Mortal Wkly Rep. 11 Mar 2022, 71(10);365–370.
11. Hwang HS, Chang M, Kim YA. Influenza-Host Interplay and Strategies for Universal Vaccine Development. Vaccines. 2020 Sep 20;8(3).
12. Viboud C, Gostic K, Nelson MI, Price GE, Perofsky A, Sun K, et al. Beyond clinical trials: Evolutionary and epidemiological considerations for development of a universal influenza vaccine. PLoS Pathog. 2020 Sep;16(9):e1008583.
13. Centers for Disease Control and Prevention. CDC Seasonal Flu Vaccine Effectiveness Studies. 2022 October 22, 2022 [cited 2022 November 30]; Available from: https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htm