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RSV Prevention Products and Severe RSV-Associated Disease Among Infants
Julia C Bennett1,2, Elyse Bevers1, Sara Chronister1
1Washington State Department of Health, Shoreline.
Insights
New respiratory syncytial virus (RSV) prevention products, nirsevimab and RSVpreF vaccine, significantly reduced infant hospitalizations and ED visits in Washington state by their second year of use. Continued use and increased coverage are expected to further decrease severe RSV disease burden.
Area of Science:
- Public Health
- Pediatrics
- Infectious Diseases
Background:
- Respiratory syncytial virus (RSV) is a leading cause of hospitalization and emergency department (ED) visits in infants.
- Nirsevimab, a long-acting monoclonal antibody, and the RSVpreF vaccine (administered antenatally) became available in Washington state in 2023 for infant RSV prevention.
- Estimating the combined population-level impact of these new interventions is crucial for public health strategies.
Purpose of the Study:
- To estimate the combined population-level impact of nirsevimab and the antenatal RSVpreF vaccine on RSV-associated hospitalizations and ED visits among infants.
- To assess the relative rate of RSV-associated hospitalizations and ED visits during the first two respiratory seasons following the introduction of these prevention products.
Main Methods:
- A difference-in-differences cohort study design was employed using Washington state syndromic surveillance data.
- Data included RSV-associated hospitalizations and ED visits among infants from July 1, 2022, to June 30, 2025.
- Negative binomial models were used to compare the rate changes in children aged 7 months or younger (treatment group) versus those aged 8 to 24 months (comparison group) before and after product introduction.
Main Results:
- During the 2023-2024 season (first year of limited use), no significant decrease in RSV-associated hospitalizations and ED visits was observed in the treatment group compared to the comparison group.
- During the 2024-2025 season (second year of use), an estimated 43.0% relative decrease in RSV-associated hospitalizations and ED visits was observed among infants aged 7 months or younger, beyond temporal trends.
- The impact varied across racial groups, with a notably higher estimated benefit for White children compared to Native Hawaiian or Other Pacific Islander children.
Conclusions:
- Nirsevimab and the RSVpreF vaccine were associated with a reduced burden of RSV-associated hospitalizations and ED visits among infants in Washington state.
- The findings support the continued use of these RSV prevention products.
- Increased vaccine and antibody coverage are likely to lead to further reductions in severe RSV disease, particularly in at-risk populations.
Importance:
A 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.
Objective:
To 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 Participants:
This 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.
Exposure:
Introduction of nirsevimab for infants and antenatal RSVpreF.
Main Outcomes And Measures:
A 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.
Results:
During 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 Relevance:
In 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.
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