Influenza vaccine effectiveness against hospitalizations associated with influenza a in Hong Kong children aged 9 months to 17 years, 2024/25

Influenza began circulating in Hong Kong in mid-December 2024. As of 12 March 2025, 1279 children were enrolled in a prospective test negative study for influenza vaccine effectiveness (VE). Among 209 influenza-positive cases, 71 % had influenza A(H1N1), 11 % had influenza A(H3N2), 3 % had influenza A with unknown subtype, and 15 % had influenza B. Among the test-negative controls, 53 % had received influenza vaccination since August 2024. End of the winter season VE estimates against hospitalization associated with influenza A(H1N1) was 64.8 % (95 % CI: 38.1 % to 80.3 %) and for influenza A(H3N2) was 59.9 % (95 % CI: −42.0 % to 89.6 %). These findings indicate that influenza vaccination provided protection against hospitalization for influenza A(H1N1) and A(H3N2) in children, highlighting the importance of continued vaccination efforts in Hong Kong.