Adjuvanted Influenza Vaccine and Influenza Outbreaks in US Nursing Homes: Results From a Pragmatic Cluster-Randomized Clinical Trial

Abstract

Abstract Background Influenza outbreaks in nursing homes pose a threat to frail residents and occur even in vaccinated populations. We conducted a pragmatic cluster-randomized trial comparing adjuvanted trivalent influenza vaccine (aTIV) versus trivalent influenza vaccine (TIV). We report an exploratory analysis to compare the effect of aTIV versus TIV on facility-reported influenza outbreaks. Methods We evaluated the impact of the intent-to-treat vaccine assignment on outbreaks reported from November 2016 to March 2017. We collected data according to standard CDC definitions for both suspected outbreaks and those with a laboratory-confirmed case and adjusted for facility-level vaccination rates and resident characteristics in nursing homes. Results Of 823 randomized nursing homes, 777 (aTIV, n = 387; TIV, n = 390) reported information on influenza outbreaks. Treatment groups had similar characteristics at baseline except for race/ethnicity: homes assigned to TIV had a higher percentage of African-American residents (18.0% vs 13.7%). There were 133 versus 162 facility-reported suspected influenza outbreaks in aTIV versus TIV facilities, respectively; of these, 115 versus 140 were laboratory confirmed. The aTIV group experienced a 17% reduction in suspected (rate ratio, .83; 95% confidence interval, .65–1.05) and laboratory-confirmed (.83; .63–1.06) influenza outbreaks. Covariate adjustment increased the estimated reduction for suspected outbreaks to 21% (.79; .61–.99) and 22% for laboratory-confirmed outbreaks (.78; .60–1.02). Conclusions In an exploratory analysis of a cluster-randomized trial we observed 17–21% fewer outbreaks with aTIV than TIV. Clinical Trials Registration. (NCT02882100).