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.external icon (1), or in the article by Tenforde, et al.external icon (2), on the impact of influenza vaccination during the 2019-2020 influenza season.

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

The rate of each outcome (illnesses, medical visits, hospitalizations, or deaths) among unprotected persons is 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 outcomes prevented by influenza 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. Tenforde MW, et al. Effect of Antigenic Drift on Influenza Vaccine Effectiveness in the United States—2019–2020external iconClinical Infectious Diseases, 2020;, ciaa1884