Infant RSV Hospitalization Rates Drop in First Season With Widespread Preventive Product Use

Respiratory syncytial virus (RSV)–associated hospitalization rates were significantly lower among infants aged 0 to 7 months during the 2024-2025 RSV season, the first with widespread availability of prevention products, compared with pre–COVID-19 pandemic seasons, according to a recent study in the CDC’s Morbidity and Mortality Weekly Report.¹

RSV remains the leading cause of hospitalization among US infants, with those aged 0 to 2 months at the highest risk.² The researchers highlighted that 2 effective prevention products were introduced during the 2023-2024 US RSV season.³

One is the maternal RSV vaccine, which is administered between weeks 32 and 36 of pregnancy, primarily from September through January in most of the US.¹ The other is nirsevimab, a long-acting monoclonal antibody for infants aged 0 to 7 months in their first RSV season and for high-risk children aged 8 to 19 months entering their second season.

To assess a potential link between product availability and RSV-associated hospitalizations, the researchers conducted an ecologic analysis, comparing pediatric hospitalization rates from 2 US active surveillance systems during the 2024-2025 and 2018-2020 seasons.

New respiratory syncytial virus (RSV) prevention products significantly reduce infant hospitalization rates, highlighting the importance of timely vaccination and nirsevimab administration. | Image Credit: kitsawet - stock.adobe.com

The analysis used data from the RSV-Associated Hospitalization Surveillance Network (RSV-NET) and the New Vaccine Surveillance Network (NVSN). RSV-NET tracks laboratory-confirmed RSV-associated hospitalizations at approximately 300 hospitals across 13 states. Additionally, NVSN monitors acute respiratory illness among hospitalized children at 7 US medical centers.

The researchers included RSV-associated hospitalization data for children younger than 5 years during the 2018-2019 and 2019-2020 RSV seasons (October-April), pooling these to represent pre-pandemic baseline rates (2018-2020). They also included data on children from the 2024-2025 season (October-February).

Hospitalization rates were analyzed across 3 groups based on eligibility for RSV prevention products. The first group included infants aged 0 to 7 months eligible for the maternal RSV vaccine or nirsevimab. It was further divided into infants aged 0 to 3 months and those aged 3 to 7 months. The second group included children aged 8 to 19 months, entering their second RSV season. Lastly, the third had children aged 20 to 59 months, who were ineligible for either prevention product.

RSV-associated hospitalizations per 1000 children under age 5 were calculated using US population denominators, with cumulative rates estimated from October to February for consistency across seasons.

The researchers identified 18,389 RSV-associated hospitalizations among children under age 5, with 15,405 in RSV-NET and 2984 in NVSN. Of these, 11,681 occurred during the 2018-2020 seasons, with median patient ages of 6.7 months and 6.3 months in RSV-NET and NVSN, respectively. In contrast, the 2024-2025 season saw 6708 hospitalizations, with higher median ages among patients in RSV-NET (14.7 months) and NVSN (12.7 months).

Among infants aged 0 to 7 months, cumulative RSV-associated hospitalization rates were significantly lower in the 2024-2025 season (RSV-NET, 8.5 per 1000 children; NVSN, 10.7 per 1000 children) than in the 2018-2020 season (RSV-NET, 15.0 per 1000 children; NVSN, 14.8 per 1000 children). These declines during the 2024-2025 season corresponded with estimated hospitalization rate reductions of 43% in RSV-NET (95% CI, 40-46) and 28% in NVSN (95% CI, 18-36; P < .001 for both), with the largest reductions observed between December and February.

The largest cumulative RSV-associated hospitalization rate differences were seen in infants aged 0 to 2 months, with estimated reductions of 52% in RSV-NET (95% CI, 49-46) and 45% in NVSN (95% CI, 32-57; P < .001 for both).

In contrast, hospitalization rates were higher in 2024-2025 for children aged 8 to 19 months (RSV-Net, 6.7 per 1000 children; NVSN, 5.9 per 1000 children) vs the 2018-2020 season (RSV-Net, 5.0 per 1000 children; NVSN, 4.7 per 1000 children). Compared with the 2018-2020 season (RSV-Net, 1.5 per 1000 children; NVSN, 2.5 per 1000 children), rates also increased during the 2024-2025 season for children aged 20 to 59 months (RSV-Net, 2.5 per 1000 children; NVSN, 1.7 per 1000 children). Weekly and monthly trends in these age groups mirrored the cumulative rate patterns.

The researchers acknowledged the study's limitations, including that it was an ecologic analysis without individual-level data on RSV prevention product coverage. Because of this, causality could not be assessed. Despite this, they expressed confidence in their findings.

“These findings highlight the importance of effective annual health care planning to implement Advisory Committee on Immunization Practices’ recommendations for RSV prevention products and ensure parents can protect infants as early as possible in the RSV season, either through maternal vaccination during pregnancy or infant receipt of nirsevimab,” the authors concluded.

References

1. Patton ME, Moline HL, Whitaker M, et al. Interim evaluation of respiratory syncytial virus hospitalization rates among infants and young children after introduction of respiratory syncytial virus prevention products — United States, October 2024–February 2025. MMWR Morb Mortal Wkly Rep. 2025;74:273–281. doi:10.15585/mmwr.mm7416a1
2. Hall CB, Weinberg GA, Blumkin AK, et al. Respiratory syncytial virus-associated hospitalizations among children less than 24 months of age. Pediatrics. 2013;132(2):e341-e348. doi:10.1542/peds.2013-0303
3. Jones JM, Fleming-Dutra KE, Prill MM, et al. Use of Nirsevimab for the prevention of respiratory syncytial virus disease among infants and young children: recommendations of the Advisory Committee on Immunization Practices - United States, 2023. MMWR Morb Mortal Wkly Rep. 2023;72(34):920-925. doi:10.15585/mmwr.mm7234a4