Interim 2025/26 influenza vaccine effectiveness estimates with immuno-epidemiological considerations for A(H3N2) subclade K protection, Canada, January 2026

In interim 2025/26 analyses, the Canadian Sentinel Practitioner Surveillance Network estimates influenza vaccine reduced the risk of medically-attended acute respiratory illness due to predominant influenza A(H3N2) viruses, including antigenically distinct subclade K, by about 40% relative to unvaccinated individuals. Vaccine effectiveness was about 30% against A(H1N1)pdm09, with insufficient case numbers for interim influenza B estimation. Meaningful protection against subclade K, despite substantial vaccine mismatch, is interpreted in the context of immuno-epidemiological considerations, including potential viral glycosylation, imprinting, and pre-immunity effects.