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.