How CDC Estimates the Burden of Flu Illness Prevented 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 an influenza 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-related 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.

Estimates of flu vaccination coverage by month 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 (2).

Vaccine effectiveness estimates for each season are derived from existing CDC flu vaccine effectiveness networks (3). These networks estimate the effectiveness of vaccination for preventing flu 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 the United States.

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.

How are the numbers of flu-related illnesses, medical visits, hospitalizations and deaths estimated?

Rates of laboratory-confirmed flu-related 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 14 geographically distributed areas in the United States that conduct population-based flu 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 flu and the average sensitivity of flu tests used in the surveillance hospitals. The percentage of persons hospitalized and tested for flu was measured from data collected each season since the 2010-2011 flu season. When adjusting the hospitalization rates, testing data from prior seasons is used until testing data from that specific season is available usually 2 years after the season ends.

Adjusted rates were applied to the U.S. population by age group to calculate the numbers of flu-related hospitalizations. The numbers of flu 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 number of people seeking medical care for flu were then calculated using age group-specific data on the percentages of people with a respiratory illness who sought medical care, which was estimated from results of the 2010 Behavioral Risk Factor Surveillance Survey (14).

All estimates were rounded to two significant digits.