Abstract
During the COVID-19 pandemic early years, infection prevention measures suppressed transmission of seasonal influenza and other respiratory viruses. The early onset and moderate severity of the US 2022-2023 influenza season may have resulted from reduced use of nonpharmaceutical interventions or lower population immunity after 2 y of limited influenza virus circulation. We used a mathematical model of influenza virus transmission that incorporates vaccine-derived protection against both infection and severe disease to estimate the impact of influenza vaccines on healthcare burden. Assuming reported levels of past vaccine effectiveness (VE) against infection and hospitalization, we estimate that influenza vaccines prevented 69,886 (95% CI: 51,860 to 84,575) influenza-related hospitalizations nationwide during the 2022-2023 season, with 57% attributable to reduced susceptibility and onward transmission. Despite limited data on VE against infection, our analyses suggest substantial indirect protection, particularly from young adults to other age groups. This is supported by a significant negative correlation between young adult (aged 18 to 49 y) vaccination rates and observed hospital burden across US states. Among those aged ≧65 y, nearly half of averted hospitalizations resulted from vaccinating younger age groups. These findings highlight the need for better estimates of influenza VE against infection and the potential benefits of increasing young adult influenza vaccination rates to reduce both direct and indirect disease burden.