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 description of the method can be found in an article by Tokars, et al. (1).
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 did not get a flu vaccine and were not protected from a flu virus infection (or were “unprotected” from flu) during the season. The rate of each outcome (illnesses, medical visits, hospitalizations, or deaths) among unprotected people is calculated and then used to estimate the number of flu-associated outcomes that would have been expected in the 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.