RSV Prevention Products and Severe RSV-Associated Disease Among Infants

ImportanceA long-acting monoclonal antibody (nirsevimab) for use in infants aged 7 months or younger and an antenatal respiratory syncytial virus (RSV) vaccine (RSVpreF) became available in Washington state in 2023.ObjectiveTo estimate the combined population-level impact of nirsevimab and antenatal RSV vaccine on the relative rate of RSV-associated hospitalizations and emergency department (ED) visits among infants during the first 2 respiratory seasons of use.Design, Setting, and ParticipantsThis difference-in-diferrences cohort study used data from the Washington state syndromic surveillance program on RSV-associated hospitalizations and ED visits among infants from July 1, 2022, to June 30, 2025.ExposureIntroduction of nirsevimab for infants and antenatal RSVpreF.Main Outcomes and MeasuresA difference-in-differences analysis was conducted to estimate mean relative changes in the rate of RSV-associated hospitalizations and ED visits between the period before and the period after introduction of RSV prevention products for children aged 7 months or younger (treatment group) beyond that for children aged 8 to 24 months (comparison group) using negative binomial models. Relative changes were estimated separately for July 1, 2023, to June 30, 2024, and July 1, 2024, to June 30, 2025, vs July 1, 2022, to June 30, 2023.ResultsDuring July 2022 to June 2025, there were 16 775 RSV-associated hospitalizations and ED visits among children aged 24 months or younger in Washington state (median age, 10 months IQR, 4-19 months; 9228 male 55%). The median age increased from 9 months (IQR, 3-19 months) during 2022-2023 to 12 months (IQR, 6-21 months) during 2024-2025. During the first year of limited use of RSV prevention products (2023-2024), estimates did not indicate a significant decrease in the rate of RSV-associated hospitalizations and ED visits among children aged 7 months or younger beyond temporal trends also observed for children aged 8 to 24 months (ratio of relative rates, 0.92 95% CI, 0.80-1.03). In the second year of use (2024-2025), there was an estimated 43.0% (95% CI, 32.0%-52.1%) relative decrease in the rate of RSV-associated hospitalizations and ED visits among children aged 7 months or younger beyond that for children aged 8 to 24 months. Relative changes were similar across racial groups, except for Native Hawaiian or Other Pacific Islander children. The estimated impact in 2024-2025 was 1.77 times (95% CI, 1.17-2.66 times) higher for White vs Native Hawaiian or Other Pacific Islander children.Conclusions and RelevanceIn this cohort study of children aged 24 months or younger, nirsevimab and antenatal RSVpreF vaccine were associated with a reduced burden of RSV-associated hospitalization and ED visits among infants aged 7 months or younger. Results support continued use of these products in the state. Increased coverage will likely be associated with additional decreases in severe disease burden, particularly among populations at increased risk.