Abstract
BACKGROUND: The 2023-2024 influenza season had predominant influenza A(H1N1)pdm09 virus activity, but A(H3N2) and B viruses cocirculated. Seasonal influenza vaccine strains were well-matched to these viruses. METHODS: Using healthcare encounters data from health systems in 8 US states, we evaluated influenza vaccine effectiveness (VE) against influenza-associated medical encounters from October 2023 to April 2024. Using a test-negative design, we compared the odds of vaccination between patients with an acute respiratory illness who tested positive (cases) versus negative (controls) for influenza by molecular assay, adjusting for confounders. VE was stratified by age group, influenza type (overall, influenza A, influenza B), and care setting (hospitalization, emergency department or urgent care [ED/UC] encounter). RESULTS: Overall, 74 000 encounters in children and adolescents aged 6 months-17 years (3479 hospitalizations, 70 521 ED/UC encounters) and 267 606 in adults aged ≥18 years (66 828 hospitalizations, 200 778 ED/UC encounters) were included. Across care settings, among children and adolescents, 15% (2758/17 833) of cases versus 32% (18 240/56 167) of controls had received vaccination. Among adults, 25% (11 632/46 614) of cases versus 44% (97 811/220 992) of controls across care settings had received vaccination. VE was 58% (95% confidence interval [95% CI], 44-69) against hospitalization and 58% (95% CI, 56-60) against ED/UC encounters for children and adolescents, and 39% (95% CI, 35-43) against hospitalization and 47% (95% CI, 46-49) against ED/UC encounters for adults. Across age groups, VE was higher against influenza B than influenza A. CONCLUSIONS: Influenza vaccines provided protection against influenza-associated illness across health care settings and age groups during the 2023-2024 influenza season.