Effectiveness of seasonal influenza vaccine 2025/26 among persons 65 years and older: A cohort study in Stockholm County, Sweden.
OBJECTIVES To estimate vaccine effectiveness (VE) against any influenza, outpatient visits and hospitalisations for influenza, and 30-day all-cause mortality among community-dwelling adults aged ≥65 years during a period dominated by influenza A(H3N2) subclade K in the 2025/26 season in Stockholm County, Sweden. METHODS A population-based cohort study was conducted among all residents aged ≥65 years in Stockholm County (n = 415,779) from 3 November 2025 to 23 February 2026. Individuals residing in nursing homes or with recent influenza diagnoses were excluded. Influenza vaccinations (from Vaccinera), healthcare encounters and date of death (VAL), were linked using pseudonymised identifiers. VE was estimated using Cox proportional hazards models, adjusted for age, sex, socioeconomic status, healthcare seeking behaviour and specific comorbidity. VE was calculated as (1 - hazard ratio) × 100% with 95% confidence intervals (CI), and analyses were stratified by age groups 65-74 and ≥75 years. RESULTS Approximately two-thirds of the cohort were vaccinated, and 54% were aged ≥75 years. Vaccinated individuals tended to be older and have higher socioeconomic status, specific comorbidities, and prior vaccination history. VE (95% CI) during the study period was 38% (32-44) against any influenza, 27% (8-42) against outpatient visits, 39% (32-45) against hospitalisations, and 56% (34-70) against 30-day all-cause mortality after an influenza-related hospitalisation. Age-stratified analyses yielded similar estimates. No vaccinated individuals aged 65-74 years experienced 30-day mortality. CONCLUSIONS During a season with influenza A(H3N2) subclade K dominance, seasonal influenza vaccination provided low to moderate protection against outpatient visits and hospitalisations, and moderate protection against 30-day all-cause mortality among older adults. These findings reinforce the value of seasonal influenza vaccination for older populations and highlight the need for continued viral surveillance and VE monitoring.