We compared effects of prior vaccination and added or lost protection from current season vaccination among those previously vaccinated.
Our analysis included data from the US Flu VE Network among participants ≥9 years old with acute respiratory illness from 2012-13 through 2017-18. Vaccine protection was estimated using multivariate logistic regression with an interaction term for effect of prior season vaccination on current season vaccine effectiveness. Models were adjusted for age, calendar time, high-risk status, site, and season for combined estimates. We estimated protection by combinations of current and prior vaccination compared to unvaccinated in both seasons or current vaccination compared to prior vaccinated.
31,819 participants were included. Vaccine protection against any influenza averaged 42% (38 to 47) among those vaccinated only the current season, 37% (33 to 40) among those vaccinated both seasons, and 26% (18 to 32) among those vaccinated only the prior season, compared to participants vaccinated neither season. Current season vaccination reduced the odds of any influenza among patients unvaccinated the prior season by 42% (37 to 46), including 57%, 27% and 55% against A(H1N1), A(H3N2) and influenza B, respectively. Among participants vaccinated the prior season, current season vaccination further reduced the odds of any influenza by 15% (7 to 23), including 29% against A(H1N1) and 26% against B viruses, but not against A(H3N2).
Our findings support ACIP recommendations for annual influenza vaccination. Benefits of current season vaccination varied among participants with and without prior season vaccination, by virus type/subtype and season.

Published by Oxford University Press for the Infectious Diseases Society of America 2020. This work is written by (a) US Government employee(s) and is in the public domain in the US.