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. 2025 Jan;30(4):2500059.
doi: 10.2807/1560-7917.ES.2025.30.4.2500059.

Interim estimates of vaccine effectiveness against influenza A(H1N1)pdm09 and A(H3N2) during a delayed influenza season, Canada, 2024/25

Lea Separovic  1 , Yuping Zhan  1 , Samantha E Kaweski  1 , Suzana Sabaiduc  1 , Sara Carazo  2 , Romy Olsha  3 , Richard G Mather  4   3 , James A Dickinson  5 , Maan Hasso  3 , Isabelle Meunier  2 , Agatha N Jassem  1 , Nathan Zelyas  6 , Ruimin Gao  7 , Nathalie Bastien  7 , Danuta M Skowronski  8   1
Affiliations

Interim estimates of vaccine effectiveness against influenza A(H1N1)pdm09 and A(H3N2) during a delayed influenza season, Canada, 2024/25

Lea Separovic et al. Euro Surveill. 2025 Jan.

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.

Keywords: A(H1N1)pdm09; A(H3N2); influenza; test-negative design; vaccine effectiveness.

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Conflict of interest statement

Conflict of interest: DMS is Principal Investigator on grants received to her institution from the Public Health Agency of Canada in support of this work. She has received grants from Pacific Public Health Foundation and Canadian Institutes of Health Research and the Michael Smith Foundation for Health Research for unrelated work, also paid to her institution. SC reports funding from the Public Health Agency of Canada paid to her institution, but not pertaining to the current study. Other authors have no conflicts of interest to declare.

Figures

Figure
Figure
Influenza virus test-positive and test-negative specimens, by week of specimen collection, Canadian Sentinel Practitioner Surveillance Network, 27 October 2024–18 January 2025 (weeks 44–3) (n = 4,459)
See this image and copyright information in PMC

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