A Washington state cohort study estimated a 43% population-level relative decrease in the rate of respiratory syncytial virus–associated hospitalizations and emergency department visits among infants aged 7 months or younger during the second year of routine use of infant nirsevimab and maternal respiratory syncytial virus vaccination.

The controlled quasi-experimental difference-in-differences study, published in JAMA Network Open, used Washington state syndromic surveillance data from July 1, 2022, to June 30, 2025. Researchers compared infants aged 7 months or younger with children aged 8 to 24 months, a group largely ineligible for routine nirsevimab and used as a proxy for underlying seasonal trends.

The analysis included 16,775 respiratory syncytial virus–associated hospitalizations and emergency department visits among patients aged 24 months or younger. Of these, 6,840 occurred among infants aged 7 months or younger and 9,935 occurred among children aged 8 to 24 months. Most encounters were emergency department–only visits.

Cases were identified using respiratory syncytial virus–related International Statistical Classification of Diseases and Related Health Problems, Tenth Revision diagnosis codes in the state syndromic surveillance system. Researchers used negative binomial models with population denominator offsets and county-level random effects.

Prior postlicensure studies cited by the researchers have estimated nirsevimab effectiveness of 77% to 90% and maternal respiratory syncytial virus prefusion F protein–based vaccine effectiveness of 78% against respiratory syncytial virus–associated hospitalization. The lower 43% estimate in the Washington analysis reflects population-level impact rather than individual-level product effectiveness.

During the first season following product availability, from July 2023 to June 2024, the difference did not reach statistical significance (ratio of relative rates, 0.92; 95% CI, 0.80–1.03). Uptake during that season was limited by supply constraints; an estimated 39% of infants born from October 2023 to March 2024 were immunized through either nirsevimab or maternal vaccination. Preliminary Washington immunization data suggest coverage approximately doubled in the second season, which may help explain the observed population-level impact in 2024 to 2025.

During the second season, from July 2024 to June 2025, researchers estimated a 43% relative decrease in the rate of respiratory syncytial virus–associated hospitalizations and emergency department visits among infants aged 7 months or younger beyond the decrease estimated among children aged 8 to 24 months. Modeled rates among infants aged 7 months or younger decreased from 7.46 to 2.95 per 100 population. Observed annual rates declined from 6.1 per 100 population in 2022 to 2023 to 2.6 per 100 population in 2024 to 2025.

The median age of respiratory syncytial virus cases increased from 9 months in 2022 to 2023 to 12 months in 2024 to 2025, a finding consistent with a shift in disease burden away from the youngest infants.

The estimated population impact varied across Washington counties, ranging from a 15.5% to 57.7% decrease in respiratory syncytial virus–associated hospitalizations and emergency department visits. Some county estimates were imprecise, particularly in smaller populations.

Relative changes were generally similar across racial groups, except among Native Hawaiian or Other Pacific Islander infants. The estimated population impact in 2024 to 2025 was greater among White infants than among Native Hawaiian or Other Pacific Islander infants. Native Hawaiian or Other Pacific Islander children also had the highest observed annual rates across age groups and study years.

The researchers noted that the Centers for Disease Control and Prevention recommends nirsevimab for some children aged 8 to 19 months who are entering their second respiratory syncytial virus season and are at increased risk, including American Indian or Alaska Native children. No similar recommendation exists specifically for Native Hawaiian or Other Pacific Islander children, and the authors suggested further evaluation of whether such guidance should be considered.

A newly available option may further expand prevention strategies. Clesrovimab, a long-acting monoclonal antibody, offers fixed dosing regardless of neonatal weight and can be stored at room temperature for up to 48 hours, compared with 8 hours for nirsevimab. The product was approved by the US Food and Drug Administration in June 2025 and subsequently recommended by the Centers for Disease Control and Prevention.

Limitations included reliance on diagnosis codes rather than laboratory confirmation, creating potential misclassification. Race and ethnicity data were extracted from medical records, the method of ascertainment was unknown, and race was unknown for 11% of cases. The comparison group also may have included some children eligible for nirsevimab because the product is recommended for certain children aged 8 to 19 months at increased risk for severe disease.

The ecological design limited causal interpretation because researchers did not have individual-level data on receipt of respiratory syncytial virus prevention products.

“Finally, this was an ecological study without individual-level data on protection from RSV prevention products and thus causality cannot be established,” wrote lead study author Julia C. Bennett, PhD, of the Washington State Department of Health, and colleagues.

Disclosures can be found in the study.

Source: JAMA Network Open