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. 2025 Oct 6;81(3):667-678.
doi: 10.1093/cid/ciae597.

Influenza Vaccine Effectiveness Against Hospitalizations and Emergency Department or Urgent Care Encounters for Children, Adolescents, and Adults During the 2023-2024 Season, United States

Mark W Tenforde  1 , Emily L Reeves  1 , Zachary A Weber  2 , Sara Y Tartof  3 , Nicola P Klein  4 , Kristin Dascomb  5 , Malini B DeSilva  6 , Duck-Hye Yang  2 , Shaun J Grannis  7   8 , Stephanie A Irving  9 , Toan C Ong  10 , Ruth Link-Gelles  1   11 , S Bianca Salas  3 , Lina S Sy  3 , Bruno Lewin  3 , Richard Contreras  3 , Ousseny Zerbo  4 , Bruce Fireman  4 , John Hansen  4 , Julius Timbol  4 , Tamara Sheffield  5 , Daniel Bride  5 , Julie Arndorfer  5 , Josh VanOtterloo  5 , Charlene E McEvoy  6 , Omobosola O Akinsete  6 , Inih J Essien  6 , Brian E Dixon  7   12 , Colin Rogerson  7   8 , William F Fadel  7   12 , Thomas Duszynski  12 , Allison L Naleway  9 , Michelle A Barron  13 , Suchitra Rao  14 , David Mayer  10 , Catia Chavez  10 , Sarah W Ball  2 , Amanda B Payne  1 , Caitlin Ray  1 , Monica Dickerson  1 , Varsha Neelam  1 , Katherine Adams  1 , Brendan Flannery  1 , Jennifer DeCuir  1   11 , Shikha Garg  1   11
Affiliations

Influenza Vaccine Effectiveness Against Hospitalizations and Emergency Department or Urgent Care Encounters for Children, Adolescents, and Adults During the 2023-2024 Season, United States

Mark W Tenforde et al. Clin Infect Dis. .

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.

Keywords: influenza; test-negative design; vaccination; vaccine effectiveness.

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Conflict of interest statement

Potential conflicts of interest. All authors have completed and submitted the International Committee of Medical Journal Editors form for disclosure of potential conflicts of interest. During the conduct of the study, all Kaiser Permanente Southern California Department of Research and Evaluation and Kaiser Permanente Northern California Division of Research-affiliated authors reported receiving contractual support from the Centers for Disease Control and Prevention (CDC) via payments made to their respective institutions. Additionally, all authors affiliated with HealthPartners Institute, Intermountain Healthcare, Kaiser Permanente Center for Health Research, Regenstrief Institute, and University of Colorado Anschutz Medical Campus reported receiving contractual support from the CDC during the conduct of the study, via subcontracts from Westat, Inc., with payments made to their respective institutions. Unrelated to the submitted work, the following disclosures were reported from the past 36 months: S. Y. T. reports contracts from GlaxoSmithKline and Pfizer. N. P. K. reports support from Sanofi Pasteur, Merck, Pfizer, Seqirus, and GlaxoSmithKline. T. C. O. received consulting fees from Regenstrief Institute and support for travel from Patient-Centered Outcomes Research Institute (PCORI) and Regenstrief Institute and has a current patent, PCT/US2018/047961. S. J. G. reports contracts with National Institutes of Health (NIH) National Center for Advancing Translational Sciences and NIH National Institute of Mental Health. B. E. D. reports a contract with Vaccine Safety Datalink Project, Contract No. 75D30122D15424. W. F. F. reports a contract with Vaccine Safety Datalink Project, Contract No. 75D30122D15424. C. E. M. reports support from NIH, Department of Defense, PCORI, Astra Zeneca, and GlaxoSmithKline and is a member of the American Lung Association of Minnesota Board, Minnesota Department of Health Long COVID Advisory Committee, and Minnesota Department of Health Asthma Care Advisory Committee. T. S. is a member of CDC Advisory Committee on Immunization Practices Influenza Vaccine Work Group, chair of Utah Adult Immunization Coalition—volunteer vaccine quality improvement and advocacy group and a member of Utah Department of Health and Human Services Scientific Advisory Committee on Vaccines. O. Z. reports contract number R01AI168373 with the National Institute of Allergy and Infectious Diseases. L. S. S. reports contracts with GlaxoSmithKline, Moderna, and Dynavax. S. R. reports support from Biofire and GlaxoSmithKline. M. B. was a speaker bureau participant—Innoviva Specialty Therapeutics Oct 2023. No other disclosures were reported. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

Figures

Figure 1.
Figure 1.
Influenza vaccine effectiveness against influenza-associated hospitalizations and emergency department / urgent care encounters in children and adolescents aged 6 months-17 years. Abbreviations: CI = confidence interval; ED = emergency department; ICU = intensive care unit; UC = urgent care a Vaccine effectiveness was estimated using logistic regression models comparing the odds of vaccination between cases and controls; models adjusted for site, calendar day, patient age, sex, and race/ethnicity with calendar day and age treated as natural cubic splines with 4 degrees of freedom. b Influenza A and B coinfections were included in both influenza A and influenza B case counts.
Figure 2.
Figure 2.
Vaccine effectiveness against influenza-associated hospitalizations and emergency department / urgent care encounters in adults aged ≥18 years. Abbreviations: CI = confidence interval; ED = emergency department; ICU = intensive care unit; UC = urgent care. a Vaccine effectiveness was estimated using logistic regression models comparing the odds of vaccination between cases and controls; models adjusted for site, calendar day, patient age, sex, and race/ethnicity with calendar day and age treated as natural cubic splines with 4 degrees of freedom. b Influenza A and B coinfections were included in both influenza A and influenza B case counts.
Figure 3.
Figure 3.
A) Influenza vaccine effectivenessa by time since vaccination by influenza type and age groupb among adult inpatient encounters B) Influenza vaccine effectiveness by time since vaccination by influenza type and age group among emergency department / urgent care encounters. a Forest plots depict vaccine effectiveness point estimates (circles) and 95% confidence intervals (vertical lines). b Inpatient vaccine effectiveness estimates not shown for children aged 6 months-4 years or 517 years because of imprecision.
Figure 3.
Figure 3.
A) Influenza vaccine effectivenessa by time since vaccination by influenza type and age groupb among adult inpatient encounters B) Influenza vaccine effectiveness by time since vaccination by influenza type and age group among emergency department / urgent care encounters. a Forest plots depict vaccine effectiveness point estimates (circles) and 95% confidence intervals (vertical lines). b Inpatient vaccine effectiveness estimates not shown for children aged 6 months-4 years or 517 years because of imprecision.
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References

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