Lea Separovic, Suzana Sabaiduc, Yuping Zhan, Samantha E Kaweski, Romy Olsha, Maan Hasso, Richard Mather, Sara Carazo, Christine Lacroix, Isabelle Meunier, Lila Naouelle Salhi, James A Dickinson, Nathan Zelyas, Agatha N. Jassem, Katie Dover, Charlene Ranadheera, Ruimin Gao, Nathalie Bastien, Danuta M Skowronski
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.