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

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• Lea Separovic¹ , Suzana Sabaiduc¹ , Yuping Zhan¹ , Samantha E Kaweski¹ , Romy Olsha² , Maan Hasso² , Richard G Mather^2,3 , Sara Carazo⁴ , Christine Lacroix⁴ , Isabelle Meunier⁴ , Lila N Salhi⁴ , James A Dickinson⁵ , Nathan Zelyas⁶ , Agatha N Jassem¹ , Katie Dover¹ , Charlene Ranadheera⁷ , Ruimin Gao⁷ , Nathalie Bastien⁷ , Danuta M Skowronski^1,8

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.

https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.5.2600068