Estimated Flu Illnesses, Medical Visits, Hospitalizations, and Deaths Prevented by Vaccination in the United States – 2021-2022 Flu Season

Español | Other Languages
Print
Related Pages
Related Pages
On This Page

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 illness 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, 970,000 flu-related medical visits, 22,000 flu-related hospitalizations, and 1,000 flu-related 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 (7).

Despite limited influenza virus circulation, 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, later than is typical. The season had predominately influenza A virus circulation with subtype A(H3N2) viruses most commonly detected (8).

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%) (10-11).  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, 970,000 flu-related medical visits, 23,000 flu-related hospitalizations, and 1,100 flu-related 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. 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-related 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.

Table 1: Estimated Number of Flu-Related Illnesses, Medical Visits, Hospitalizations, and Deaths Prevented by Vaccination, by Age Group—United States, 2021-2022 Flu Season
Influenza Vaccine Symptomatic Illnesses Medical Visits Hospitalizations Deaths
Age group Vaccine Coverage (%) Adjusted VE (95% CI) Estimate 95% UI Estimate 95% UI Estimate 95% UI Estimate 95% UI
Influenza, all flu
6 months-4 years 66.70% 77%
(44, 91)		 637,997 (240,883, 986,380) 427,458 (160,369, 660,943) 4,448 (1,679, 6,877) 0 (0, 0)
5-17 years 55.10% 32%
(0, 53)		 503,624 (835, 1,019,025) 261,884 (426, 532,732) 1,381 (2, 2,794) 0 (0, 0)
18-49 years 37.10% 32%
(3, 52)		 450,338 (24,603, 834,274) 166,625 (8,883, 309,046) 2,528 (138, 4,683) 12 (0, 37)
50-64 years 52.40% 8%
(-86, 54)		 58,873 (0, 525,764) 25,315 (0, 226,757) 624 (0, 5,576) 30 (0, 255)
65+ years 73.90% 32% (-79, 74) 152,716 (0, 573,968) 85,521 (0, 322,427) 13,883 (0, 52,179) 1,081 (0, 7,651)
All ages 36%
(20, 49)		 1,803,547 (1,052,143, 2,797,749) 966,803 (566,407, 1,472,052) 22,864 (6,544, 62,321) 1,122 (0, 7,790)

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.

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. 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.
  9. 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
  10. 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/
  11. 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.
  12. 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