Effectiveness of the 2025–2026 trivalent influenza vaccine against laboratory-confirmed influenza among older adults in Eastern China: A test-negative case–control study

Evidence on the real-world effectiveness of seasonal influenza vaccination among older adults in China remains limited and varies by season and circulating virus. We used a test-negative case-control design to estimate the effectiveness of the 2025-2026 trivalent influenza vaccine against medically attended, laboratory-confirmed influenza among adults aged ≥60 years attending two sentinel hospitals in Shengzhou, Zhejiang Province. Participants vaccinated ≥14 days before symptom onset were considered vaccinated; those vaccinated 0-13 days before onset were excluded from the primary analysis. Vaccine effectiveness (VE) was calculated as (1 - adjusted odds ratio) × 100% using multivariable logistic regression. Among 532 participants in the primary analysis, 156 were influenza-positive cases and 376 were test-negative controls. Effective current-season vaccination was recorded in 32.7% of cases and 48.4% of controls. Adjusted VE against any influenza was 46% (95% confidence interval [CI], 18-64). Type-specific VE was 42% (95% CI, 8-63) against influenza A and 60% (95% CI, 14-81) against influenza B. Subtype-specific estimates were less precise, and interaction analyses did not identify differences by age group or chronic-disease status. The overall estimate remained stable across diagnostic restrictions and alternative model specifications. The 2025-2026 trivalent influenza vaccine was associated with moderate protection against medically attended influenza in older adults under routine program conditions, supporting continued seasonal vaccination and multi-season surveillance.