Scenario projections of RSV hospitalizations averted due to new immunization programs in King County, Washington, October 2023 to May 2025

ImportanceIn 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. ObjectiveEstimate the hospitalizations averted in 2023-2024 due to new RSV immunizations and provide scenario projections for future seasons. DesignThis modeling study used an RSV transmission model calibrated to RSV-diagnosed hospitalizations. SettingKing County, WA, October 2023-May 2025 ParticipantsPopulation of King County, WA (2.3 million individuals), disaggregated into infant, pediatric, adult, and older adult age groups. ExposuresRSV vaccination for adults aged [&ge;]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 [&ge;]60 years and infants <1 year. ResultsApproximately 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 [&ge;]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 [&ge;]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 [&ge;]75. Conclusions and RelevanceOur 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. KEY POINTSO_ST_ABSQuestionC_ST_ABSHow many respiratory syncytial virus (RSV)-diagnosed hospitalizations were averted in King County, WA during the 2023-2024 season due to new active and passive immunizations and how can we optimize disease reduction strategies in future seasons? FindingsWe found moderate reductions in RSV hospitalizations during the 2023-2024 season due to modest coverage. With higher levels of coverage earlier in the season more than half of RSV hospitalizations in infants and a quarter of RSV hospitalizations in older adults could be avoided. MeaningRSV immunizations are a powerful tool for preventing hospitalizations. Modeling studies can support public health strategies to optimize immunization coverage.