Neurology
Does seasonal influenza vaccination uptake increase after stroke? Evidence from a population-level administrative cohort Brian Steele* Brian Steele Michael D. Hill Jessalyn Holodinsky
Background. Observational evidence suggests that influenza vaccination may be associated with a reduction in stroke occurrence. This association is subject to treatment-selection bias; advancing age and the presence of stroke-related comorbidities are associated with both an increased risk for stroke and with an increased uptake of influenza vaccination. Using population-level data from Alberta, Canada, we estimated the effect of experiencing a stroke on the risk of receiving seasonal vaccination.
Methods. This study analyzed a cohort of linked administrative health records and registry data in a publicly funded healthcare system, covering the entire population of Alberta (n = 4.5 million) where seasonal influenza vaccinations are freely available to residents. Adults (aged 45-84) were eligible for inclusion and follow-up across 9 influenza seasons. For each season, individuals were ‘at-risk’ for vaccination between October and March and censored at first recorded seasonal vaccination, outmigration, or at season end. Cases were matched on demographic and medical comorbidity variables at each season using nearest neighbor matching without replacement. Marginal risk ratios were estimated with g-computation for the average treatment effect on the treated (ATT).
Results. The sample included 2.03 million adults (50.3% male) observed across one or more influenza season (2010-2011 to 2018-2019). Within the sample, 21,604 adults (1.1%) presented with a stroke event. Among cases, the risk of seasonal vaccination was 17% greater after experiencing a stroke (marginal RR: 1.17; 95% CI: 1.14 – 1.19).
Discussion. Our results suggest that seasonal influenza vaccination uptake increases following stroke. Sensitivity analyses are planned to examine incident cases alone and to evaluate alternative matching and weighting approaches. These findings will inform the design of studies evaluating vaccination for stroke-risk reduction and health promotion.