How CDC Estimates the Burden of Flu Illness Averted by Vaccination

CDC estimates the total number of influenza (flu) illnesses, medical visits, hospitalizations, and deaths prevented by vaccination using a model that incorporates season-specific data on burden of disease, vaccine coverage and vaccine effectiveness (VE) for five age groups. More detailed descriptions of the method can be found in an article by Tokars, et al. (1), or in the article by  Chung, et al. (2), on the impact of flu vaccination during the 2021-2022 flu season.

CDC uses the annual estimates of flu vaccination coverage by month during each season, and the final end-of-season vaccine effectiveness measurements to estimate how many people were not protected by flu vaccination during the season.

The rate of each outcome (illnesses, medical visits, hospitalizations, or deaths) among unprotected people is then used to estimate the number of flu-associated outcomes that would have been expected in the same population if no one had been protected by flu vaccination. Finally, the outcomes prevented by flu vaccination are calculated as the difference between outcomes in the hypothetical unvaccinated population and the observed vaccinated population.

Calculations are 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.

  1. 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; 36(48): 7331-7.
  2. 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 2022;71:365–370. DOI: