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

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 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 Statement Annual 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. Competing Interest Statement The authors have declared no competing interest. Funding Statement This research was supported by CDC grant U01 IP001136, CSTE grant NU38OT000297, and CDC CFA NU38FT000008. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes Competing Interest Statement: Disclose any competing interests here. Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. Data Availability All data for reproducing the analysis are publicly available at: https://github.com/bikaiming93/Flu_burden_avert