Influenza vaccine effectiveness among outpatients in the US Influenza Vaccine Effectiveness Network by study site 2011-2016

Background Influenza vaccination is recommended for all US residents aged >= 6 months. Vaccine effectiveness (VE) varies by age, circulating influenza strains, and the presence of high-risk medical conditions. We examined site-specific VE in the US Influenza VE Network, which evaluates annual influenza VE at ambulatory clinics in geographically diverse sites. Methods Analyses were conducted on 27 180 outpatients >= 6 months old presenting with an acute respiratory infection (ARI) with cough of <= 7-day duration during the 2011-2016 influenza seasons. A test-negative design was used with vaccination status defined as receipt of >= 1 dose of any influenza vaccine according to medical records, registries, and/or self-report. Influenza infection was determined by reverse-transcription polymerase chain reaction. VE estimates were calculated using odds ratios from multivariable logistic regression models adjusted for age, sex, race/ethnicity, time from illness onset to enrollment, high-risk conditions, calendar time, and vaccination status-site interaction. Results For all sites combined, VE was statistically significant every season against all influenza and against the predominant circulating strains (VE = 19%-50%) Few differences among four sites in the US Flu VE Network were evident in five seasons. However, in 2015-16, overall VE in one site was 24% (95% CI = -4%-44%), while VE in two other sites was significantly higher (61%, 95% CI = 49%-71%;P = .002, and 53%, 95% CI = 33,67;P = .034). Conclusion With few exceptions, site-specific VE estimates aligned with each other and overall VE estimates. Observed VE may reflect inherent differences in community characteristics of the sites and highlights the importance of diverse settings for studying influenza vaccine effectiveness.