Influenza vaccination status ascertainment and vaccine effectiveness estimation: Validity of self-report for current and prior season.

Accurate vaccination status ascertainment is fundamental to valid vaccine effectiveness (VE) estimation. We evaluated the accuracy of current and prior season's self-reported influenza vaccination status among outpatients with acute respiratory illness recruited by the Canadian Sentinel Practitioner Surveillance Network (SPSN) during the 2023-2024 season in Quebec, Canada. Vaccination status was self-reported at specimen collection for influenza testing and validated against the provincial vaccination registry. Self-report showed high registry agreement, including sensitivity, specificity, accuracy and kappa statistics for current (91%, 98%, 96%, 0.89) and prior (85%, 95%, 92%, 0.78) season's vaccination. Metrics were similar by influenza case status, age, sex and comorbidity with sensitivity varying more than specificity. Crude test-negative design estimates of relative reduction in the odds of influenza test-positivity among vaccinated versus unvaccinated individuals, derived as (1-odds ratio)x100, were similar for self-reported (61%) and registry-based (63%) vaccination status. Findings support self-reported influenza vaccination for timely and valid VE estimation.