Northern Hemisphere 2025/26 influenza vaccine effectiveness during months with influenza A(H3N2) subclade K predominance- Suzhou, Eastern China.

During this 2025/26 Northern Hemisphere influenza season, the predominant influenza A(H3N2) subclade K virus has binding site mutations and postvaccine sera reduced inhibition, raising concerns about antigenic differences with available vaccines. During November 2025-January 2026, 8,455 patients with influenza-like illness (ILI) and 4,479 patients with severe acute respiratory illness (SARI) were identified through 25 sentinel surveillance hospitals in Suzhou City, Jiangsu Province, China. Among enrolled ILI and SARI patients, 4,032 (47.7%) and 697 (15.6%) were influenza positive by RT-PCR, respectively. Of all influenza positives, four were influenza B Victoria and three were influenza A(H1N1)pdm09; all remaining positives were influenza A(H3N2). Among gene sequenced viruses isolated from 44 randomly-selected positive samples during the study period, 95% were influenza A(H3N2) subclade K. Generalized linear regression models indicated Northern Hemisphere 2025/26 influenza vaccines, including A/Croatia/10136RV/2023 (H3N2)-like viruses, reduced risk for influenza-associated outpatient visits by approximately one-half during the November 2025-January 2026 subclade K dominant months. We did not observe statistical significance, although the point estimate suggested a possible one-fifth reduction in influenza-associated hospitalizations. Increasing seasonal influenza vaccine coverage could relieve stress on the medical system during respiratory disease season.