Objective
This study examined the effect of the timing of administration of oseltamivir chemoprophylaxis for the control of influenza A H3N2 outbreaks among residents in long-term care facilities (LTCFs) in Manitoba, Canada, during the 2014–2015 influenza season.

Methods
A retrospective cohort study was conducted of all LTCF influenza A H3N2 outbreaks (n=94) using a hierarchical logistic regression analysis. The main independent variable was how many days passed between the start of the outbreak and commencement of oseltamivir chemoprophylaxis. The dependent variable was whether each person in the institution developed influenza-like illness (yes or no).

Results
Delay of oseltamivir chemoprophylaxis was associated with increased odds of infection in both univariate (t=5·41; df=51; P<·0001) and multivariable analyses (t=6·04; df=49; P<·0001) with an adjusted odds ratio of 1.3 (95% confidence interval [CI], 1·2–1·5) per day for influenza A H3N2.

Conclusions
The sooner chemoprophylaxis is initiated, the lower the odds of secondary infection with influenza in LTCFs during outbreaks caused by influenza A H3N2 in Manitoba. For every day that passed from the start of the outbreak to the initiation of oseltamivir, the odds of a resident at risk of infection in the facility developing symptomatic infection increased by 33%.