Vaccine Effectiveness Against Influenza A(H1N1), A(H3N2), and B–Associated Hospitalizations—United States, September 1, 2023–May 31, 2024

Background The 2023–2024 influenza season included sustained elevated activity from December 2023–February 2024 and continued activity through May 2024. Influenza A(H1N1), A(H3N2), and B viruses circulated during the season. Methods During September 1, 2023–May 31, 2024, a multistate sentinel surveillance network of 24 medical centers in 20 U.S. states enrolled adults aged ≥18 years hospitalized with acute respiratory illness (ARI). Consistent with a test-negative design, cases tested positive for influenza viruses by molecular or antigen test, and controls tested negative for influenza viruses and SARS-CoV-2. Vaccine effectiveness (VE) against influenza–associated hospitalization was calculated as (1 − adjusted odds ratio for vaccination) × 100%. Results Among 7690 patients, including 1170 influenza cases (33% vaccinated) and 6520 controls, VE was 40% (95% CI: 31%–48%) with varying estimates by age (18–49 years: 53% [34%–67%]; 50–64 years: 47% [31%–60%]; ≥65 years: 31% [16%–43%]). Protection was similar among immunocompetent patients (40% [30%–49%]) and immunocompromised patients (32% [7–50%]). VE was statistically significant against influenza B (67% [35%–84%]) and A(H1N1) (36% [21%–48%]) and crossed the null against A(H3N2) (19% [-8%–39%]). VE was higher for patients 14–60 days from vaccination (54% [40%–65%]) than >120 days (18% [-1%–33%]). Conclusions During 2023–2024, influenza vaccination reduced the risk of influenza A(H1N1)– and influenza B–associated hospitalizations among adults; effectiveness was lower in patients vaccinated >120 days prior to illness onset compared with those vaccinated 14–60 days prior.