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

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This web page provides estimates on the burden of influenza and the effects of annual influenza vaccination in the United States for the 2019–2020 influenza 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 influenza vaccine prevented burden for the 2019-2020 season.

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

2019–2020 Estimate

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 influenza (7). The season was characterized by two consecutive waves of activity, beginning with influenza B viruses and followed by A(H1N1)pdm09 viruses. 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 influenza 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 influenza-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).

Influenza vaccination prevented an estimated 7.09 million illnesses, 3.46 million medical visits, 100,000 hospitalizations, and 7,100 deaths due to influenza during the 2019-2020 season. The number and proportion of influenza-associated hospitalizations prevented by vaccination during 2019–2020 varied by age group, due to age-specific differences in influenza burden, vaccine coverage, and vaccine effectiveness.  Overall, vaccination prevented 21% of hospitalizations across all age groups.  Vaccination prevented the lowest proportion of influenza-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.

Influenza vaccination is the best way to protect against influenza and associated complications; however, more effective or longer lasting influenza vaccines are needed. 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 (9-11).

Improvements in vaccine coverage could also provide a greater public health benefit, producing even greater reductions in illnesses and demands for resources from the health system. During influenza season, the health care system, including outpatient clinics, emergency departments, and hospitals, experience high patient volumes. Any reduction in influenza-associated medical visits and hospitalizations will reduce demands on the healthcare system stressed by the COVID-19 pandemic and improve healthcare providers’ abilities to provide the best care. Strategies to improve influenza vaccine coverage include: using patient reminder/recall systems aided by immunization information systems; expanding access to vaccination at pharmacies, workplaces, and schools; and organizing mobile and/or outdoor vaccination clinics that would allow for social distancing among patients and healthcare providers.

Conclusion

CDC estimates that influenza vaccination during the 2019–2020 influenza season prevented 7.09 million illnesses, 3.46 million medical visits, 100,000 hospitalizations, and 7,100 deaths associated with influenza. 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, 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; 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 influenza vaccination.

Previous Estimates

Previous estimates of the burden of illness, medical visits, hospitalizations, and deaths related to influenza and prevented by influenza vaccination are available online and in publications (1-5).  Past seasons’ estimates of influenza disease burden averted 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 Influenza Burden and Burden-Prevented Estimates

Table 1: Influenza vaccine coverage, by age group — United States 2019–2020 influenza season
Vaccine coverage *  Vaccine effectiveness against influenza A(H1N1)pdm09† Vaccine effectiveness against influenza B/Victoria†
Age Group % 95% CI % 95% CI % 95% CI

6 months-4 years

 75.5 74.5–76.5 28 -6–52 57 34–72

5-17 yrs

 60.2 59.5–60.9 22 -4–41 35 18–48

18-49 yrs

 38.4 37.6–39.2 26 9–39 45 30–56

50-64 yrs

 50.6 49.2–52.0 40 20–56 44 0–68

≥65 years

 69.8 68.6–71.0 42 9–64 34 -40–69

*Estimates from FluVaxView. (12)

†Estimates from US Flu VE Network. (13)

Table 2: Estimated number and fraction of influenza illnesses, medical visits, hospitalizations, and deaths prevented by vaccination, by age group — United States, 2019–2020 influenza season
Prevented Illnesses Prevented Medical Visits Prevented Hospitalizations Prevented Deaths
Age Group No. 95% CI* No. 95% CI* No. 95% CI* Fraction prevented (%) 95% CI* No. 95% CI*

6 months-4 yrs

 1,292,086 598,248– 2,072,214 865,697 400,598–1,390,669 9,008 4,171– 14,447 29 17–37 110 24– 488

5-17 yrs

 1,459,139 695,207– 2,313,353 758,753 359,224–1,204,830 4,001 1,906–6,343 17 10–22 33 3– 149

18-49 yrs

 1,767,451 1,094,507– 2,537,308 653,957 399,358–947,629 9,921 6,143– 14,242 11 8–13 315 85– 1,244

50-64 yrs

 1,951,848 957,368–3,041,647 839,294 412,662– 1,316,896 20,699 10,153– 32,256 18 11–24 1,384 372– 5,569

≥65 yrs

 615,537 114,456–1,176,545 344,701 64,598– 661,486 55,958 10,405– 106,959 25 6–37 5,212 731– 24,606

All ages

 7,086,061 5,413,208– 8,956,756 3,462,402 2,617,677–4,394,485 99,586 52,474– 153,680 21 14–26 7,055 2,504– 27,091

Questions & Answers

How does CDC estimate the number of hospitalizations, illnesses, and medically-attended illnesses associated with influenza that occurred in the United States?

Rates of laboratory-confirmed influenza-associated hospitalization by age group were obtained from FluSurv-NET, a collaboration between CDC, the Emerging Infections Program Network, and selected state and local health departments in 13 geographically distributed areas in the United States that conduct population-based influenza surveillance. Reported hospitalization rates were adjusted using a multiplier to correct for underreporting, which is calculated from the percent of persons hospitalized with respiratory illness who were tested for influenza and the average sensitivity of influenza tests used in the surveillance hospitals. These values were measured from data collected during eight post-pandemic seasons; estimates of underreporting of influenza-related hospitalizations

Adjusted rates were applied to the U.S. population by age group to calculate the numbers of influenza-associated hospitalizations. The numbers of influenza illnesses were then estimated from hospitalizations based on previously measured ratios that reflect the estimated number of ill persons per hospitalization in each age group (5).

The numbers of persons seeking medical care for influenza were then calculated using age group-specific data on the percentages of persons with a respiratory illness who sought medical care, which were estimated from results of the 2010 Behavioral Risk Factor Surveillance Survey (14).

All estimates were rounded to three significant digits.

 

How does CDC estimate the number of influenza-associated outcomes that were prevented with influenza vaccination?

The annual estimates of influenza vaccination coverage by month during each season and the final end-of-season vaccine effectiveness measurements were used to estimate how many persons were not protected by vaccination during the season and thus were at risk for these outcomes.

The rate of each outcome among persons at risk was then used to estimate the number of influenza-associated outcomes that would have been expected in the same population if no one had been protected by vaccination. Finally, the averted outcomes attributable to vaccination were calculated as the difference between outcomes in the hypothetical unvaccinated population and the observed vaccinated population.

Estimates of 2019–2020 influenza vaccination coverage by month from July 2019 through April 2020, were based on self-report or parental report of vaccination status using data from the National Immunization Survey for children aged 6 months-17 years and Behavioral Risk Factor Surveillance Survey data for adults aged ≥18 years (12).

Vaccine effectiveness estimates for the 2019–2020 season were derived from the US Influenza Vaccine Effectiveness Network, a group of five academic institutions that conduct annual vaccine effectiveness studies. (13) The network estimates the effectiveness of vaccination for preventing influenza diagnosed by real-time reverse transcription polymerase chain reaction-positive testing among persons with acute respiratory illness of ≤7 days duration seen in hospitals, emergency departments, or outpatient clinics in communities in

Calculations were stratified by month of the year to account for annual variations in the timing of disease and vaccination and then summed across the whole season. The prevented fraction was calculated as the number of prevented illnesses divided by the total illnesses that would have been expected in an unvaccinated population.

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/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. Flannery B, Chung JR, Belongia EA, McLean HQ, Gaglani M, Murthy K, et al. Interim Estimates of 2017-18 Seasonal Influenza Vaccine Effectiveness – United States, February 2018. MMWR Morb Mortal Wkly Rep. 2018 Feb 16;67(6):180-5.
  10. Hwang HS, Chang M, Kim YA. Influenza-Host Interplay and Strategies for Universal Vaccine Development. Vaccines. 2020 Sep 20;8(3).
  11. 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.
  12. Centers for Disease Control and Prevention. Influenza Vaccination Coverage. 2019 September 26, 2019 [cited 2020 September 30]; Available from: https://www.cdc.gov/flu/fluvaxview/
  13. Centers for Disease Control and Prevention. CDC Seasonal Flu Vaccine Effectiveness Studies. 2020 July 1, 2020 [cited 2020 September 30]; Available from: https://www.cdc.gov/flu/professionals/vaccination/effectiveness-studies.htm
  14. Biggerstaff M, Jhung M, Kamimoto L, Balluz L, Finelli L. Self-reported influenza-like illness and receipt of influenza antiviral drugs during the 2009 pandemic, United States, 2009-2010. Am J Public Health. 2012 Oct;102(10):e21-6.