Importance: In 2023 new immunization strategies became available for preventing respiratory syncytial virus (RSV)-associated hospitalizations in infants and older adults. Modeling studies to understand the population-level impact of their use are important for public health planning. Objective: Estimate the hospitalizations averted in 2023-2024 due to new RSV immunizations and provide scenario projections for future seasons. Design: This modeling study used an RSV transmission model calibrated to RSV-diagnosed hospitalizations. Setting: King County, WA, October 2023-May 2025 Participants: Population of King County, WA (2.3 million individuals), disaggregated into infant, pediatric, adult, and older adult age groups. Exposures: RSV vaccination for adults aged ≥60 years, maternal RSV vaccination, and long-acting monoclonal antibodies for infants aged <8 months. Main Outcomes(s) and Measures(s): Proportion of RSV-diagnosed hospitalizations averted in adults ≥60 years and infants <1 year. Results: Approximately 25% of older adults and 33% of infants benefited from active or passive immunization during the 2023-2024 RSV season. We estimate that 108 (95% PI 89-154) RSV-diagnosed hospitalizations were averted, with most of the benefit observed in infants <6 months (23% fewer RSV-diagnosed hospitalizations than baseline) and adults ≥75 years (13% fewer RSV-diagnosed hospitalizations). For the 2024-2025 season, optimistic scenarios of high immunization coverage (50% in older adults and 80% in infants) project reductions of 28.9% (95% PI 28.0-29.7) in adults ≥75 years and 61.2% (95% PI 54.2-66.5) in infants <6 months compared to a counterfactual scenario with no immunizations. Targeting infants eligible for catch-up doses of nirsevimab early in the season increased the proportion of RSV-diagnosed hospitalizations averted in infants 6-11 months from 25.7% (95% CI 21.8-29.8) to 38.7% (95%PI 36.2-40.4). If vaccine protection in older adults wanes by 50% in the second year after immunization, the proportion of RSV-diagnosed hospitalizations averted would decrease to 21.1% (95% PI 20.1-22.0) in adults ≥75. Conclusions and Relevance: Our results suggest a modest reduction in RSV-diagnosed hospitalizations during the 2023-2024 season due to limited availability of immunization products, particularly for infants. We project that higher uptake earlier in the season will lead to substantial reductions in RSV hospitalizations in the 2024-2025 season.

Ms. Hansen reported receiving personal fees from Sanofi outside the submitted work and acknowledges PhD funding support from the Danish National Research Foundation (grant number DNRF170) for the PandemiX Center of Excellence. Dr. Englund reported receiving grants from Pfizer, AstraZeneca, Merck, and GlaxoSmithKline and receiving personal fees from Pfizer, AstraZeneca, Meissa Vaccines, Moderna, and Sanofi Pasteur outside the submitted work. Dr. Chu reported receiving personal fees from Roche, Abbvie, Merck outside the submitted work. Dr. Viboud reported receiving honoraria from Elsevier outside the submitted work. Dr. Chow received travel assistance from IDSA to attend IDWeek 2022 and from the Northwest Healthcare Response Network to attend the 2024 Common Health Coalition conference. No other disclosures were reported.

This work was funded by the Council of State and Territorial Epidemiologists and US Centers for Disease Control and Prevention through Cooperative Agreement number NU38OT000297 - Development of forecast, analytic, and visualization tools to improve outbreak response and support public health decision-making. Program Officers from CDC and CSTE were included in discussions pertaining to model development and implementation. Funders were not involved in the final analysis and interpretation of results presented here.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study was determined to be non-human subjects research by the University of Washington Institutional Review Board.

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.

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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).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

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The raw data used in this study is not publicly available. Aggregated datasets and model code have been provided on GitHub for reproducibility of results. Contact information is provided at the end of the manuscript for researchers wishing to access additional data.

https://github.com/chelsea-hansen/RSV-Projections-KingCounty