Estimated Flu Illnesses, Medical Visits, and Hospitalizations Prevented by Vaccination in the United States — 2019–2020 Flu Season

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This web page provides estimates on the burden of influenza (flu) and the effects of annual flu vaccination in the United States for the 2019–2020 flu season. While previous updates to the prevented burden estimates for the 2019-2020 season included preliminary input data, this update provides the final estimates for flu vaccine prevented burden for the 2019-2020 season.

For the past several years, CDC has used a model to estimate the numbers of flu illnesses, medical visits, hospitalizations, and deaths and has also estimated the effect of flu vaccination on these outcomes in the United States (1-6). The methods used to calculate the estimates have been described previously (1,2,6). CDC uses the estimates of the impact of flu vaccination to inform policy and communications promoting influenza vaccination as the best way to prevent flu.

2019–2020 Estimates of Burden Prevented by Flu Vaccination

CDC estimates that during the 2019–2020 season 36 million people were ill, 16 million people went to a health care provider, 390,000 were hospitalized, and 25,000 died with flu (7). In March 2020, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) began circulating in the United States. As a result of measures implemented to reduce transmission of SARS-CoV-2, influenza virus circulation rapidly declined, and flu circulation ended earlier than in prior seasons. Overall, influenza A(H1N1)pdm09 viruses were the most commonly reported influenza viruses this season. The number of flu-associated illnesses, medically attended illnesses, hospitalizations, and deaths were lower than some more recent seasons and similar to other seasons during which influenza A(H1N1)pdm09 viruses dominated (8).

During the 2019-2020 season, flu vaccine coverage ranged from 38% among those aged 18-49 years to 76% among those aged 6 months-4 years. Vaccine effectiveness was 39% for all age groups and was lowest among children aged 5-17 years (32%) and highest among children aged 6 months-4 years (44%) (9-11). Applying this information, influenza vaccination prevented an estimated 7.1 million illnesses, 3.4 million medical visits, 100,000 hospitalizations, and 7,100 deaths due to influenza during the 2019-2020 season. Flu vaccination prevented an estimated 7.1 million illnesses, 3.45 million medical visits, 100,000 hospitalizations, and 7,200 deaths due to flu during the 2019-2020 season. The number and proportion of flu-associated hospitalizations prevented by vaccination during 2019–2020 varied by age group, due to age-specific differences in flu burden, vaccine coverage, and vaccine effectiveness. Overall, vaccination prevented 21% of hospitalizations across all age groups. Vaccination prevented the lowest proportion of flu-related hospitalizations among adults aged 18 to 49 years, the age group in which vaccination coverage is lowest. Vaccination prevented the greatest proportion of outcomes among children aged 6 months to 4 years, an age group in which there was high vaccine uptake and the vaccine effectiveness was greatest.

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.

Conclusion

CDC estimates that flu vaccination during the 2019–2020 flu season prevented 7.1 million illnesses, 3.4 million medical visits, 100,000 hospitalizations, and 7,200 deaths associated with flu. This report underscores the benefits of the current vaccination program, but highlights areas where improvements in vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health.

Limitations

These estimates are subject to several limitations. First, flu 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; nonresponse bias may remain after weighting for the survey design. Estimates of the number of persons vaccinated based on these survey data have often exceeded the actual number of doses distributed, indicating that coverage estimates used in this report may overestimate the numbers of illnesses and hospitalizations prevented by vaccination. The model of prevented illness calculates outcomes directly prevented among persons who were vaccinated. If indirect protection from decreased exposure to infectious persons in a partially vaccinated population (i.e., herd immunity) also occurred, the model would underestimate the number of illnesses and hospitalizations prevented by vaccination. Estimates of the prevented burden in older adults, aged ≥65 years does not reflect that 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.

Previous Estimates

Previous estimates of the burden of illness, medical visits, hospitalizations, and deaths related to flu and prevented by flu vaccination are available online and in publications (1-5). Past seasons’ estimates of flu disease burden prevented by vaccination, using the same methodology as for the 2019-2020 season, are available to provide context for the current season’s estimates.

2019-2020 Tables for Flu Burden and Burden-Prevented Estimates

Table 1: Flu vaccine coverage, by age group — United States 2019–2020 flu season
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 75.50% 44% (25, 58) 1,280,515 (608,392, 2,069,387) 857,945 (409,106, 1,382,468) 8,927 (4,241, 14,427) 124 (30, 578)
5-17 years 60.20% 32% (17, 44) 1,432,146 (683,095, 2,241,294) 744,716 (354,165, 1,175,369) 3,927 (1,873, 6,145) 33 (2.9, 160)
18-49 years 38.40% 34% (23, 44) 1,766,126 (1,088,102, 2,534,147) 653,466 (400,685, 943,805) 9,913 (6,108, 14,224) 306 (92, 1,237)
50-64 years 50.60% 40% (22, 54) 1,965,899 (973,031, 3,074,493) 845,337 (416,749, 1,322,469) 20,848 (10,319, 32,604) 1,399 (396, 5,766)
65+ years 69.80% 39% (9, 59) 624,309 (105,384, 1,188,492) 349,613 (59,687, 662,620) 56,755 (9,580, 108,045) 5,340 (743, 24,313)
All ages 39% (32, 44) 7,068,995 (5,419,108, 8,948,467) 3,451,077 (2,620,295, 4,398,353) 100,370 (52,249, 152,985) 7,202 (2,482, 27,118)

*Estimates from FluVaxView. (9)

†Estimates from US Flu VE Network. (11)

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: https://archive.cdc.gov/#/details?url=https://www.cdc.gov/flu/about/burden-averted/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. Centers for Disease Control and Prevention. Estimated Influenza Illnesses, Medical visits, Hospitalizations, and Deaths in the United States — 2019–2020 influenza season. 2020 October 1, 2020 [cited October 1, 2020]; Available from: https://www.cdc.gov/flu/about/burden/2019-2020.html
  8. Centers for Disease Control and Prevention. Disease Burden of Influenza. 2020 April 17, 2020 [cited September 28, 2020]; Available from: https://www.cdc.gov/flu/about/burden/index.html
  9. https://www.cdc.gov/flu/fluvaxview/
  10. Tenforde MW, Kondor RJG, Chung JR, Zimmerman RK, Nowalk MP, Jackson ML, Jackson LA, Monto AS, Martin ET, Belongia EA, McLean HQ, Gaglani M, Rao A, Kim SS, Stark TJ, Barnes JR, Wentworth DE, Patel MM, Flannery B. Effect of Antigenic Drift on Influenza Vaccine Effectiveness in the United States-2019-2020. Clin Infect Dis. 2021 Dec 6;73(11):e4244-e4250. doi: 10.1093/cid/ciaa1884. PMID: 33367650; PMCID: PMC8664438.
  11. https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htm