biostudies-literatureUnknownChung JRNCATS NIH HHSACL HHSUS Centers for Disease Control and PreventionNational Institutes of HealthNCIRD CDC HHSIntramural CDC HHS<h4>Background</h4>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.<h4>Methods</h4>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.<h4>Results</h4>Among 6,Clinical infectious diseases : an official publication of the Infectious Diseases Society of Americaciae658https://www.ebi.ac.uk/biostudies/studies/S-EPMC12287938biostudies-literatureInfluenza vaccine effectiveness against medically attended outpatient illness, United States, 2023-24 season.PMC12287938U01IP0011941U01 IP001180-01U01IP0011891U01 IP001181-011U01 IP001182-011U01 IP001184-01U01 IP001194U01 IP001184U01IP001181U01 IP001181U01 IP001182U01 IP001193UL1 TR001857U01 IP001180U01 IP0011911U01 IP001189-011U01 IP001191-01CC999999U01 IP0011891U01 IP001193-011U01 IP001194-01UL1TR001857Stiffler KASalata RAMurthy KWolfe CRWernli KJMartin ETLadikos CLyszkowicz EFigucia DARaiyani CWilliams OLFaryar KAHolland SCapasso-Gulve EOdame-Bamfo LPatel MFlannery BHossain MPhillips CHScotch MCheng CKChung JRConti TShirk PWeissman AHouse SLMills JPrice AMKondor RSaade EARamm JDKramer JDonskey CJVaughn IALamerato LWickersham BSheffield SRNordstrom LKeong LMEllington SRZimmerman RKGaglani MUS Flu VE Network InvestigatorsMamawala MMonto ASWalter EBFaleye TOCMurugan VMulrow MLava JAMoehling Geffel KRose SGlagola JJJackson JHearn DWilson MMWilliams JVCurley TMorrow DJBalasubramani GKTodd CAKirby MFrederick JCNguyen MDoud RAlbar ZNowalk MPSmith MDaSilva JBontrager NABRountree WKiniry ELehmkuhl MfalseInfluenza vaccine effectiveness against medically attended outpatient illness, United States, 2023-24 season.<h4>Background</h4>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.<h4>Methods</h4>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.<h4>Results</h4>Among 6,2025-01-01T00:00:00Z2025 Jan2026-03-31T10:48:29.058Z2025-08-28T03:05:45.493ZS-EPMC122879383976123010.1093/cid/ciae658