Background:
The severity of the 2017-2018 influenza season in the U.S. was high with influenza A(H3N2) viruses predominating. We report influenza vaccine effectiveness (VE) and estimate the number of vaccine prevented influenza-associated illnesses, medical visits, hospitalizations, and deaths for the 2017-2018 influenza season.
Methods:
We used national age-specific estimates of 2017-2018 influenza vaccine coverage and disease burden. We estimated VE, and 95% confidence intervals (CI), against medically-attended RT-PCR confirmed influenza virus infection, in the ambulatory setting, using a test-negative design. We estimated influenza type/subtype-specific burden using multipliers applied to population-based rates of influenza-associated hospitalizations. We used a compartmental model to estimate numbers, with 95% credible intervals (CrI), of influenza-associated outcomes prevented by vaccination.
Results:
The VE against outpatient medically-attended, laboratory-confirmed influenza was 38% (95% CI: 31-43%) including 22% (95% CI: 12-31%) against influenza A(H3N2), 62% (95% CI: 50-71%) against influenza A(H1N1)pdm09, and 50% (95% CI: 41-57%) against influenza B. We estimated that influenza vaccination prevented 7.1 million (95% CrI: 5.4 million-9.3 million) illnesses, 3.7 million (95% CrI: 2.8 million-4.9 million) medical visits, 109,000 (95% CrI: 39,000-231,000) hospitalizations, and 8,000 (95% CrI: 1,100-21,000) deaths. Vaccination prevented 10% of expected hospitalizations overall and 41% among young children (6 months-4 years).
Conclusions:
Despite 38% VE, influenza vaccination reduced a substantial burden of influenza-associated illness, medical visits, hospitalizations, and deaths in the U.S. during the 2017-2018 season. Our results demonstrate the benefit of current influenza vaccination and the need for improved vaccines.