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

Abstract

Background The 2023-2024 influenza season included sustained elevated activity from December 2023 to February 2024 and continued activity through May 2024. Influenza A(H1N1), A(H3N2), and B viruses circulated during the season. Methods During 1 September 2023 to 31 May 2024, a multistate sentinel surveillance network of 24 medical centers in 20 US states enrolled adults aged ≥18 years hospitalized with acute respiratory illness. 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 severe acute respiratory syndrome coronavirus 2 (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% confidence interval [CI], 31%-48%) with varying estimates by age: 18-49 years, 53% (95% CI, 34%-67%); 50-64 years, 47% (95% CI, 31%-60%); ≥ 65 years, 31% (95% CI, 16%-43%). Protection was similar among immunocompetent patients (40%; 95% CI, 30%-49%) and immunocompromised patients (32%; 95% CI, 7%-50%). VE was statistically significant against influenza B (67%; 95% CI, 35%-84%) and A(H1N1) (36%; 95% CI, 21%-48%) and crossed the null against A(H3N2) (19%; 95% CI, -8% to 39%). VE was higher for patients 14-60 days from vaccination (54%; 95% CI, 40%-65%) than >120 days (18%; 95% CI, -1% to 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.