Effectiveness of RSV Prevention Strategies in US Infants: 2024–2025

OBJECTIVE: In 2023, 2 products, a long-acting monoclonal antibody (nirsevimab) and maternal respiratory syncytial virus (RSV) vaccination, were recommended in the United States to prevent severe RSV disease among infants in their first RSV season. Low uptake during the 2023 to 2024 RSV season limited effectiveness estimates. We estimated nirsevimab and maternal RSV vaccine effectiveness against RSV-associated emergency department (ED) encounters and hospitalization among US infants during the 2024 to 2025 RSV season, when uptake was higher. METHODS: We used electronic health record data from 5 health care systems in test-negative analyses of ED encounters and hospitalizations with RSV-like illness during October 1, 2024 to March 31, 2025, among infants in their first RSV season. Nirsevimab and maternal RSV vaccine effectiveness were estimated by comparing RSV-positive with RSV-negative encounters with respect to immunization status in separate logistic regression models adjusted for age, race and ethnicity, sex, calendar day, and geographic region. RESULTS: Among 3531 ED encounters and 470 hospitalizations included in the nirsevimab analyses, nirsevimab effectiveness against RSV-associated ED encounters and hospitalization was 62% (95% CI: 54-68) and 77% (95% CI: 61-87), respectively. Among 810 ED encounters and 187 hospitalizations included in the maternal RSV vaccine analyses, vaccine effectiveness against RSV-associated ED encounters and hospitalization was 49% (95% CI: 26-65) and 82% (95% CI: 54-93), respectively. CONCLUSION: Nirsevimab and maternal RSV vaccination were effective against RSV-associated ED encounters and hospitalization among infants in their first RSV season. As uptake increases and additional products are recommended, monitoring infant RSV immunization effectiveness remains important.