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Clin Infect Dis
. 2025 Jan 6:ciae658.
doi: 10.1093/cid/ciae658. Online ahead of print. Influenza vaccine effectiveness against medically attended outpatient illness, United States, 2023-24 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 Kirby 1 , Lisa M Keong 1 , Rebecca Kondor 1 , Sascha R Ellington 1 , Brendan Flannery 1 ; US Flu VE Network Investigators
Collaborators, Affiliations
Background: The 2023-24 U.S. influenza season was characterized by a predominance of A(H1N1)pdm09 virus circulation with co-circulation 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 2023-24 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 (RT-PCR). 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 sub-type/lineage and A(H1N1)pdm09 genetic subclades.
Results: Among 6,589 enrolled patients, 1,770 (27%) tested positive for influenza including 796 A(H1N1)pdm09, 563 B/Victoria, and 323 A(H3N2). Vaccine effectiveness against any influenza illness was 41% (95% Confidence Interval [CI]: 32 to 49): 28% (95% CI: 13 to 40) against influenza A(H1N1)pdm09, 68% (95% CI: 59 to 76) against B/Victoria, and 30% (95% CI: 9 to 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 41% and provided protection overall against circulating influenza A and B viruses. Serologic studies would help inform differences observed by age groups.
Keywords: Influenza; vaccination; vaccine effectiveness.
. 2025 Jan 6:ciae658.
doi: 10.1093/cid/ciae658. Online ahead of print. Influenza vaccine effectiveness against medically attended outpatient illness, United States, 2023-24 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 Kirby 1 , Lisa M Keong 1 , Rebecca Kondor 1 , Sascha R Ellington 1 , Brendan Flannery 1 ; US Flu VE Network Investigators
Collaborators, Affiliations
- PMID: 39761230
- DOI: 10.1093/cid/ciae658
Background: The 2023-24 U.S. influenza season was characterized by a predominance of A(H1N1)pdm09 virus circulation with co-circulation 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 2023-24 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 (RT-PCR). 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 sub-type/lineage and A(H1N1)pdm09 genetic subclades.
Results: Among 6,589 enrolled patients, 1,770 (27%) tested positive for influenza including 796 A(H1N1)pdm09, 563 B/Victoria, and 323 A(H3N2). Vaccine effectiveness against any influenza illness was 41% (95% Confidence Interval [CI]: 32 to 49): 28% (95% CI: 13 to 40) against influenza A(H1N1)pdm09, 68% (95% CI: 59 to 76) against B/Victoria, and 30% (95% CI: 9 to 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 41% and provided protection overall against circulating influenza A and B viruses. Serologic studies would help inform differences observed by age groups.
Keywords: Influenza; vaccination; vaccine effectiveness.