Vaccine Effectiveness Against Influenza A(H1N1), A(H3N2), and B-Associated Hospitalizations, United States, 1 September 2023 to 31 May 2024
Author Department
Pulmonary/Critical Care Medicine
Document Type
Article, Peer-reviewed
Publication Date
10-2025
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.
Keywords: hospitalization; seasonal influenza; severe influenza; vaccine effectiveness; waning.
Recommended Citation
Lewis NM, Harker EJ, Cleary S, Zhu Y, Grijalva CG, Chappell JD, Rhoads JP, Baughman A, Casey JD, Blair PW, Jones ID, Johnson CA, Halasa NB, Lauring AS, Martin ET, Gaglani M, Ghamande S, Columbus C, Steingrub JS, Duggal A, Felzer JR, Prekker ME, Peltan ID, Brown SM, Hager DN, Gong MN, Mohamed A, Exline MC, Khan A, Ferguson SAN, Mosier J, Qadir N, Chang SY, Ginde AA, Zepeski A, Mallow C, Harris ES, Johnson NJ, Gibbs KW, Kwon JH, Vaughn IA, Ramesh M, Safdar B, Surie D, Dawood FS, Ellington S, Self WH. Vaccine Effectiveness Against Influenza A(H1N1), A(H3N2), and B-Associated Hospitalizations, United States, 1 September 2023 to 31 May 2024. J Infect Dis. 2025 Oct 15;232(4):e626-e636. doi: 10.1093/infdis/jiaf185.
PMID
40198276