2760. Accounting for Vaccination History in Estimates of Current Season Vaccine Effectiveness in the US Flu VE Network, 2012–2013 Through 2017–2018

BACKGROUND: Current season vaccine effectiveness (VE) and influenza risk may vary in persons based on vaccination history. United States Influenza Vaccine Effectiveness (US Flu VE) Network studies have explored prior vaccination effects using a single referent group of patients unvaccinated in both the prior and current seasons. We investigated vaccine benefit among those with and without prior season vaccination. METHODS: Our analysis included data from the US Flu VE Network among patients aged ≥9 years old with acute respiratory illness during 6 influenza seasons, 2012–2013 through 2017–2018. We determined current and prior season vaccination status from documented immunizations. Current season VE against laboratory confirmed influenza was estimated using multivariate logistic regression with an interaction term for prior and current season vaccination. Models were adjusted for age, calendar time, high-risk status, and site. RESULTS: Of 31,819 patients included in the analysis over 6 seasons, 9188 were influenza positive by RT–PCR. Percent flu positivity was greatest among those unvaccinated (34%), followed by those vaccinated in the prior season only (29%), those vaccinated in both seasons (25%), and those vaccinated in the current season only (23%). Among patients with prior season vaccination, current season VE against any influenza was 14% (95% CL: 5, 22) and against A(H3N2), A(H1N1)pdm09, and B was 10% (95% CL: 3, 17), 36% (95%CL: 25, 46), and 40% (95% CL: 33, 46), respectively. Among patients unvaccinated in the prior season, VE was 42% (95% CL: 37, 46) against any influenza in the current season and was 31% (95%CL: 22, 39), 57% (95% CL: 47, 65), and 55% (95% CL: 48, 61) against A(H3N2), A(H1N1)pdm09, and B, respectively. We observed significant interaction of prior season vaccination on current season VE in 4 of 6 seasons (P < 0.20). CONCLUSION: Current season vaccination was overall protective regardless of vaccination history. Among those vaccinated in the prior season, current season vaccination may provide some benefit in addition to residual protection from previous vaccination. DISCLOSURES: All authors: No reported disclosures.