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. 2025 Nov 6;81(4):e184-e191.
doi: 10.1093/cid/ciae658.

Influenza Vaccine Effectiveness Against Medically Attended Outpatients Illness, United States, 2023-2024 Season

Jessie R Chung  1 , Ashley M Price  1 , Richard K Zimmerman  2 , Krissy Moehling Geffel  2 , Stacey L House  3 , Tara Curley  3 , Karen J Wernli  4   5 , C Hallie Phillips  4 , Emily T Martin  6 , Ivana A Vaughn  7 , Vel Murugan  8 , Matthew Scotch  8 , Elie A Saade  9 , Kiran A Faryar  9 , Manjusha Gaglani  10   11 , Jason D Ramm  10   11 , Olivia L Williams  12 , Emmanuel B Walter  12 , Marie K Kirby  1 , Lisa M Keong  1 , Rebecca Kondor  1 , Sascha R Ellington  1 , Brendan Flannery  1
Affiliations

Influenza Vaccine Effectiveness Against Medically Attended Outpatients Illness, United States, 2023-2024 Season

Jessie R Chung et al. Clin Infect Dis. .

Abstract

Background: The 2023-2024 US influenza season was characterized by a predominance of A(H1N1)pdm09 virus circulation with cocirculation of A(H3N2) and B/Victoria viruses. We estimated vaccine effectiveness (VE) in the United States against mild-to-moderate medically attended influenza illness in the 2023-2024 season.

Methods: We enrolled outpatients aged ≥8 months with acute respiratory illness in 7 states. Respiratory specimens were tested for influenza type/subtype by reverse-transcriptase polymerase chain reaction. Influenza VE was estimated with a test-negative design comparing odds of testing positive for influenza among vaccinated versus unvaccinated participants. We estimated VE by virus subtype/lineage and A(H1N1)pdm09 genetic subclades.

Results: Among 6629 enrolled patients, 1780 (27%) tested positive for influenza, including 806 with A(H1N1)pdm09, 567 with B/Victoria, and 328 with A(H3N2). VE against any influenza illness was 44% (95% confidence interval, 36%-51%): 29% (15%-41%) against influenza A(H1N1)pdm09, 74% (65%-81%) against B/Victoria, and 30% (8%-47%) against A(H3N2). Statistically significant protection against any influenza was found for all age groups except adults aged 50-64 years. Lack of protection in this age group was specific to influenza A-associated illness. We observed differences in VE by birth cohort and A(H1N1)pdm09 virus genetic subclade.

Conclusions: Vaccination reduced outpatient medically attended influenza overall by 44% and provided protection overall against circulating influenza A and B viruses. Serologic studies would help inform differences observed by age groups.

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

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Conflict of interest statement

Potential conflicts of interest. R. K. Z. has received grants from Sanofi Pasteur. S. L. H. has received grants from Seegene, Abbott, Healgen, Roche, CorDx, Hologic, Cepheid, Janssen, and Wondfo Biotech. E. T. M. has received grants from Merck. E. A. S. has received grants from Protein Sciences Corporation and consulting fees from Johnson & Johnson. E. B. W. has received research funding from Pfizer, Moderna, Seqirus, Najit Technologies, and Clinetic for the conduct of clinical research studies, and he has also received support as an advisor to Vaxcyte and Pfizer consultant to ILiAD Biotechnologies, and DSMB member for Shionogi. All other authors report no potential conflicts. 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.
Adjusted vaccine effectiveness (VE) against outpatient influenza A(H1N1)pdm09 genetic subclade-associated illness visits among patients aged ≥8 months enrolled at US Flu VE Network sites, October 2023 through April 2024; models were adjusted for study site, age, presence of ≥1 underlying health condition, and month of illness onset (data in the 2a group in children [aged 8 months to 17 years] were unadjusted due to small sample size). Note that 95% confidence intervals (CIs) that exclude 0% are considered statistically significant. Abbreviation: NR, not reported due to small sample size.
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References

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