> ## Content Index
> Fetch the complete content index at: https://www.coios.me/llms.txt
> Use this file to discover other available public pages before exploring further.

# Canada's 2025/26 flu vaccine cut A(H3N2) subclade K illness by about a third
- URL: https://www.coios.me/canada-s-2025-26-flu-vaccine-cut-a-h3n2-subclade-k-illness/
- Published: 2026-09-14T18:00:33.000Z
- Updated: 2026-09-14T18:00:33.000Z
- Description: In Canada's sentinel outpatient network for 2025/26, vaccine effectiveness was 38% against A(H3N2) overall and 33% against the antigenically mismatched subclade K, 28% against A(H1N1)pdm09 and 55% against B/Victoria.
- Author: Daniel Ryan
- Tags: Healthcare today, Infectious disease and pandemics, Canada, #preprint, #new, #item

The Canadian Sentinel Practitioner Surveillance Network estimated vaccine effectiveness by test-negative design among outpatients with medically attended acute respiratory illness from November to April, sequencing 59% of A(H3N2), 69% of A(H1N1)pdm09 and 82% of influenza B case viruses. A(H3N2) subclade K, antigenically mismatched to the J.2 vaccine component, accounted for 87% of sequenced A(H3N2) viruses, yet vaccine effectiveness against it was 33% (19%-45%). Effectiveness was 28% against A(H1N1)pdm09 despite antigenic match, and 55% against B/Victoria, with subclade-level estimates ranging from 10% to 68%.

*Why it is interesting: Antigenic and genetic match indicators diverged repeatedly from measured protection, including a mismatched subclade that still gave a third reduction in risk.*

Source

[medRxiv, 2026-09-12](https://doi.org/10.64898/2026.09.10.26362769?ref=coios.me)

DOI

10.64898/2026.09.10.26362769

Type

Preprint

Design

Test-negative design among outpatients with acute respiratory illness, 9,047 specimens sequenced and antigenically characterised, 2025/26 season

Verdict

New finding

Driver

[Infectious disease and pandemics](https://www.coios.me/d-infectious-disease/)