Influenza vaccine effectiveness among outpatients in Pittsburgh: combined results from five seasons of the US flu vaccine effectiveness network study 2011-2016

Background: Influenza has had a large impact in the US, with an estimated 30.9 million cases and 14.5 million influenza-related medical visits throughout 2016-2017. Vaccination is recommended for all individuals >6 months old, but vaccine effectiveness (VE) varies by age, circulating influenza strains, and high-risk medical conditions. VE also varies by season, and national estimates are reported each year by the CDC. VE results specific to Pittsburgh have not been evaluated. The objective of this study was to estimate influenza VE among outpatients in Pittsburgh with acute respiratory illness (ARI) during the 2011-2016 seasons and to compare estimates among different age groups and influenza subtypes. Methods: Data were gathered from the Pittsburgh site of the CDC’s US Flu VE Network study. Secondary analyses were conducted on 6,453 subjects >6 months old. Enrollees were outpatients with ARI and cough ≤7 days duration during 2011-2016. Vaccination status was defined as receipt of >1 dose of any influenza vaccine according to medical records, immunization registries, and/or self-report. Influenza status was determined by RT-PCR. A test-negative design was used. Chi-square statistics were calculated to compare baseline characteristics by vaccination status for categorical variables, and t-tests or Wilcoxon rank-sum statistics were used for continuous variables. VE estimates were calculated using odds ratios obtained from multivariable logistic regression models adjusted for age, sex, race/ethnicity, time from illness onset to enrollment, self-rated health status, high-risk conditions, and calendar time. Data were analyzed using SAS 9.4 software. Results: Overall VE was 39% (95% CI= 31%-45%). VE was highest for influenza A (H1N1)pdm09 (2009 H1N1 virus) (VE= 53%, 95% CI= 44%-61%) and lowest for influenza A (H3N2) (VE= 27%, 95% CI= 13%-38%). VE was highest in the >50 age group and varied by season. The 2013-2014 vaccine provided the most protection against influenza (VE= 51%, 95% CI= 34%-63%) and 2011-2012 provided the least (VE= 30%, 95% CI= -26%-61%). Conclusion: Results indicate influenza vaccination reduces the risk of infection with influenza A and B viruses. Although the level of protection varies by subtype, season, and age, this study provides support to the public health benefit of seasonal vaccination against influenza