Interim estimates of vaccine effectiveness against influenza A(H1N1)pdm09 and A(H3N2) during a delayed influenza season, Canada, 2024/25
Lea Separovic, Yuping Zhan, Samantha E Kaweski, Suzana Sabaiduc, Sara Carazo, Romy Olsha, Richard Mather, James A. Dickinson, Maan Hasso, Isabelle Meunier, Agatha N. Jassem, Nathan Zelyas, Ruimin Gao, Nathalie Bastien, Danuta M. Skowronski
Abstract
The Canadian Sentinel Practitioner Surveillance Network (SPSN) reports interim 2024/25 vaccine effectiveness (VE) against acute respiratory illness due to laboratory-confirmed influenza during a delayed season of predominant A(H1N1)pdm09 and lower A(H3N2) co-circulation. Through mid-January, the risk of outpatient illness due to influenza A is reduced by about half among vaccinated vs unvaccinated individuals. Adjusted VE is 53% (95% CI: 36-65) against A(H1N1)pdm09, comprised of clades 5a.2a and 5a.2a.1, and 54% (95% CI: 29-70) against A(H3N2), virtually all clade 2a.3a.1.