Vaccine effectiveness during an influenza B outbreak in a deployed military setting

Introduction Military deployments remain high-risk for acquisition and transmission of influenza-like illnesses. Outbreaks are associated with significant morbidity and reduced operational effectiveness. The impact of seasonal influenza vaccination uptake on vaccine effectiveness (VE) in this setting remains unclear. Methods VE was explored in a deployed military population during a large population transition period when a mostly unvaccinated relief cohort was introduced into a pre-existing partially vaccinated cohort in the context of an influenza B outbreak. A case definition was established, and symptomatic cases were included if confirmed (by a diagnostic test) or probable (epidemiologically linked). Data were collected on diagnosis, symptoms, symptom duration, isolation periods and vaccination status. The difference in vaccine status between groups was assessed by χ 2 , and the likelihood of infection in vaccinated or unvaccinated individuals was compared by OR. VE was estimated using screening methods for case-control studies, and the equation VE=1–(PCV/(1–PCV)/PNV/(1–PNV)), where PCV is the proportion of cases vaccinated and PNV is the proportion of controls vaccinated. Results 28 individuals (12 confirmed cases and 16 probable cases) were associated with the influenza B outbreak. Vaccination status between the pre-existing and relief cohorts differed significantly (p<0.00001). Cases were significantly less likely to occur in vaccinated compared with unvaccinated populations (OR 0.17, p<0.001). The mean symptom onset to decision to isolate was 3.0 days (IQR 3), and the mean duration of isolation was 2.7 days (IQR 1). Population vaccination status at the time of outbreak was 56.8% (619/1090). VE in the context of an influenza B outbreak was estimated at 83% (95% CI 56 to 94). Conclusions During a deployed military relief-in-place, vaccination gave significant protection against influenza infection in the context of an outbreak of influenza B. Further research is required to understand VE thresholds for outbreak risk in these settings and drivers of organisational and individual vaccine uptake hesitancy.