Late-Season Influenza Vaccine Effectiveness Against Medically Attended Outpatient Illness, United States, December 2022–April 2023

Abstract

Background: The 2022–23 US influenza season peaked early in fall 2022. Methods: Late-season influenza vaccine effectiveness (VE) against outpatient, laboratory-confirmed influenza was calculated among participants of the US Influenza VE Network using a test-negative design. Results: Of 2561 participants enrolled from December 12, 2022 to April 30, 2023, 91 laboratory-confirmed influenza cases primarily had A(H1N1)pdm09 (6B.1A.5a.2a.1) or A(H3N2) (3C.2a1b.2a.2b). Overall, VE was 30% (95% confidence interval −9%, 54%); low late-season activity precluded estimation for most subgroups. Conclusions: 2022–23 late-season outpatient influenza VE was not statistically significant. Genomic characterization may improve the identification of influenza viruses that circulate postinfluenza peak.