Effectiveness of Influenza Vaccine in Preventing Death among Ontario Residents Aged ≥65 Years during 20 Seasons

BACKGROUND: Estimation of influenza vaccine effectiveness (VE) among older adults at high risk of serious complications of influenza infection is challenging. These challenges include identifying and adjusting for potential confounders. The rarity of documented influenza deaths, even among individuals at greatest risk, makes VE assessment for this outcome particularly difficult. Thus, population-level linkages of public health system records are helpful in estimating VE for preventing death. METHODS: We conducted a retrospective cohort study from 1993 through 2013 among community-dwelling residents of Ontario aged ≥65 years. Eligible subjects were registered with Ontario’s publicly funded health insurance program, which provided universal, free access to physician services, hospital care, and vaccines; we excluded those with no contact with the health system during the prior 3 years. We assessed influenza VE for preventing death from any cause and death for which the primary cause was a respiratory or circulatory condition. Influenza vaccination was determined using physician and pharmacist billing claims. Temperature and influenza virus detection data were collected from provincial sources. Ratio-of-ratios models were used to estimate VE, by comparing marginal changes in outcome rates among vaccinated and unvaccinated groups at various levels of influenza circulation. We evaluated several methods of characterizing temporal variation in influenza circulation and several geographic scales for using influenza and temperature data in VE estimation models. RESULTS: During the 20-year study period, we estimated VE among a mean of 1.39 million subjects each season, of whom a mean of 51.9% were vaccinated. During weeks of high influenza A circulation, we estimated that VE for the prevention of any death was 16% (95% CI -6% to 38%) and for the prevention of a respiratory or circulatory death was 21% (95% CI 2% to 40%). The precision of VE estimates improved when regional-level rather than Ontario-wide temperature data were used. CONCLUSION: During weeks of influenza A circulation over 20 seasons, we found that modest but significant reductions in deaths with an underlying respiratory or circulatory cause occurred among Ontario residents aged ≥65 who received an influenza vaccine. DISCLOSURES: All authors: No reported disclosures.