Effectiveness of inactivated and live-attenuated influenza vaccines in children during the 2025-2026 influenza season with genetically distinct influenza A and B viruses.

As part of ongoing surveillance since the 2013/14 season, we evaluated influenza vaccine performance among children in Japan during the 2025/26 season, when subclade K A(H3N2) predominated. A test-negative design was applied to hospitalized febrile children who underwent outpatient influenza testing at 16 hospitals. Among 1033 participants, vaccine uptake among test-negative controls was 38% (274/723) for inactivated influenza vaccine (IIV) and 5% (35/723) for live-attenuated influenza vaccine (LAIV). Adjusted VE against influenza A was estimated at 43% (95% CI: 15-62) for IIV (N = 922) and 81% (95% CI: 15-96) for LAIV (N = 640), although the number of LAIV recipients was relatively small. No significant protection was observed against influenza B. These findings indicate that both vaccines remained effective against influenza A, despite circulation of genetically distinct influenza A viruses. Continued surveillance is warranted to further clarify comparative effectiveness between vaccine types as LAIV uptake increases.