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

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

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.

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Canada / epidemiology
  • Child
  • Child, Preschool
  • Female
  • Humans
  • Infant
  • Influenza A Virus, H1N1 Subtype* / immunology
  • Influenza A Virus, H3N2 Subtype* / genetics
  • Influenza A Virus, H3N2 Subtype* / immunology
  • Influenza A Virus, H3N2 Subtype* / isolation & purification
  • Influenza Vaccines* / administration & dosage
  • Influenza Vaccines* / immunology
  • Influenza, Human* / epidemiology
  • Influenza, Human* / prevention & control
  • Male
  • Middle Aged
  • Seasons*
  • Sentinel Surveillance*
  • Vaccination* / statistics & numerical data
  • Vaccine Efficacy*
  • Young Adult

Substances

  • Influenza Vaccines