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Respiratory syncytial virus (RSV)–associated hospitalization rates among infants in the United States were significantly lower in the first year when prevention products were widely available, based on surveillance network data from the 2024-2025 season.

Both a maternal vaccine for RSV and a long-acting monoclonal antibody for infants and young children aged 0-19 months (nirsevimab) were in use during the 2024-2025 RSV season, wrote Monica Patton, MD, an epidemiologist at the Centers for Disease Control and Prevention’s National Center for Immunization and Respiratory Diseases, and colleagues.

In a study published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report, the researchers reviewed data from the RSV-Associated Hospitalization Surveillance Network (RSV-NET) and New Vaccine Surveillance Network (NVSN). They compared RSV-associated hospitalizations in children aged 5 years or younger during the first year of availability of RSV prevention products in 2024-2025 with RSV-associated hospitalizations for a pooled period of 2018-2019 and 2019-2020, before the COVID-19 pandemic.

The study included 11,681 RSV-associated hospitalizations from 2018 to 2020 and 6708 from 2024 to 2025. The median age of the children with RSV-associated hospitalizations was significantly older in the later time period than in the early period for both networks (14.7 months vs 6.7 months in RSV-NET and 12.7 months vs 6.3 months in NVSN).

RSV-associated hospitalization rates among infants aged 0-7 months were reduced by 43% in RSV-NET and 28% in the NVSN for the 2024-2025 season vs the pre-pandemic period. RSV-associated hospitalizations for children aged 8-19 months or 20-59 months were not significantly different between the two time periods, and slightly higher in the 2024-2025 period.

Notably, the largest estimated reductions in both networks were among infants aged 0-2 months (52% and 45% for RSV-NET and NVSN, respectively). The largest reduction in RSV-association hospitalizations overall occurred during the RSV peak months of December-February.

“Higher RSV-associated hospitalization rates during 2024-25 compared with 2018-20 among children in older age groups, who were largely ineligible for RSV prevention products, suggest a more severe 2024-25 season overall and indicate that observed reductions in hospitalization rates among younger infants might be underestimated,” the researchers wrote in their discussion.

The findings were limited by several factors including the lack of individual-level data to assess causality, concerns that RSV-NET and NVSN data might not be nationally representative, and the use of interim data that may not reflect the entire RSV season, the researchers noted.

However, the results support the current recommendations of the Advisory Committee on Immunization Practices (ACIP)’s recommendations for RSV prevention products in the form of maternal vaccination or nirsevimab for infants, they said.

Early Data Support Effectiveness

“RSV prevention products such as maternal RSV vaccine and nirsevimab are relatively new tools in the protection against RSV disease,” said Shirin A. Mazumder, MD, an infectious diseases specialist in Memphis, Tennessee, in an interview.

“Collecting data from the most recent RSV season is important to gauge the effectiveness of the RSV prevention products and the impact of these products on severe RSV infection resulting in hospitalization among infants and young children,” said Mazumder, who was not involved in the study.

The findings of the interim evaluation were not surprising and confirmed the effectiveness of maternal RSV vaccine and use of nirsevimab in infants shown in previous studies, Mazumder told Medscape Medical News.

Barriers and Next Steps

“A single dose of the maternal RSV vaccine administered in the last trimester of pregnancy is effective in producing antibodies that are passed along to the fetus and provides protection from RSV during the first 6 months of a newborn’s life; however, timing is a potential barrier,” Mazumder told Medscape Medical News. “There is a limited window for mothers to receive the vaccine as it can only be given during weeks 32-36 of pregnancy and between the months of September through January,” she said. Issues of access related to product availability and insurance coverage are evolving barriers for some patients as well, said Mazumder. “Prenatal counseling and education, along with continued support from ACIP can help to overcome some of the existing barriers,” she said.

Research on data from more geographic areas and among different populations may help to provide a more comprehensive picture of the impact of RSV prevention products, Mazumder added.

This study received no outside funding. Patton had no financial conflicts to disclose. Mazumder had no financial conflicts to disclose.