Estimated Impact of 2022-2023 Influenza Vaccines on Annual Hospital Burden in the United States

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 non-pharmaceutical interventions or lower population immunity after two years 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, observed hospitalization burden, to estimate the impact of influenza vaccines on healthcare burden. Despite limited data on vaccine effectiveness 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 18-49 years) vaccination rates and observed hospital burden across US states. Assuming reported levels of past vaccine effectiveness against infection and hospitalization, we estimate that influenza vaccines prevented 67,931 [95% confidence interval (CI): 34,182, 95,842] influenza-related hospitalizations nationwide during the 2022-2023 season, with 61% attributable to reduced susceptibility and onward transmission. Among those aged >=65 years, nearly half of averted hospitalizations resulted from vaccinating younger age groups. These findings highlight the need for better estimates of influenza vaccine effectiveness against infection and the potential benefits of increasing young adult influenza vaccination rates to reduce both direct and indirect disease burden. Significance StatementAnnual influenza epidemics in the United States result in hundreds of thousands of hospitalizations. Quantifying the impact of influenza vaccines in reducing the burden of influenza is crucial, yet many analyses fail to consider the dual benefits of vaccines: directly protecting recipients and indirectly protecting their infectious contacts. Using a mathematical model that accounts for these effects, we estimate that influenza vaccines prevented nearly 68,000 hospitalizations during the 2022-2023 season, with an additional 26,500 hospitalizations potentially avoidable if coverage reached the national target of 70%. Although considerable uncertainty remains about the effectiveness of influenza vaccines in preventing infection, our findings suggest that vaccinating younger adults may offer significant indirect protection against influenza for older adults. Tailoring vaccine campaigns by both age group and US state could further enhance the public health impact of annual vaccination efforts.