Differences in influenza vaccine effectiveness by sex among adults hospitalized with acute respiratory illness—IVY network, January 24, 2022–September 1, 2024

This analysis assessed differences in influenza vaccine effectiveness (VE) and severe in-hospital outcomes between U.S. male and female adults hospitalized with laboratory-confirmed influenza in a multi-center network during 2022–2024. Compared with men, women hospitalized with influenza were less likely to smoke (21.5 % vs 25.3 %, P = 0.02), to have COPD (21.9 % vs 22.7 %, P < 0.001), and to be admitted to an intensive care unit once hospitalized (17.3 % vs 20.7 %, P = 0.04). Influenza VE (95 % confidence interval [CI]) was significantly higher in women aged ≥50 years compared with men aged ≥50 years (48.5 % [39.2 %–56.4 %] vs 26.2 % [13.0 %–37.5 %]). VE was slightly lower in women aged 18–49 years compared with women ≥50 years (46.2 % [95 % CI: 24.2 %–61.8 % vs 61.3 % [41.0 %–74.6 %]) but significantly lower in men aged ≥50 years compared with men aged 18–49 years (61.3 % [41.0 %–74.6 %] vs 26.2 % [13.0 %–37.5 %]). Disaggregation of sex should be considered in future influenza VE studies.