TOPLINE:

During the 2024-2025 respiratory syncytial virus (RSV) season in the US, maternal RSV vaccination and receipt of nirsevimab were effective against medically attended RSV-associated acute respiratory illness (ARI) and hospitalization, and rates of RSV-associated hospitalization among newborns and infants aged 0-11 months fell by up to half compared with rates in seasons before these products were introduced.

METHODOLOGY:

• Researchers conducted a population-based surveillance study across seven pediatric medical centers in the US to estimate the effectiveness of maternal RSV vaccination and nirsevimab against medically attended RSV-associated ARI and to assess their effect on RSV-associated hospitalizations during the October 2024-April 2025 season.
• They included 5029 children younger than 2 years (median age, 10 months; 43.3% girls) with medically attended ARI, defined as receiving care in inpatient, emergency department, urgent care, or outpatient settings.
• The effectiveness of maternal RSV vaccination was assessed among newborns and infants younger than 6 months at the medical encounter, and that of nirsevimab was evaluated among newborns and infants younger than 8 months as of October 2024 or born thereafter.
• The outcomes were medically attended RSV-associated ARI and RSV-associated hospitalization; RSV was confirmed using reverse transcription-polymerase chain reaction with RSV-A/RSV-B subtyping.

TAKEAWAY:

• Among 446 infants younger than 6 months, the effectiveness of maternal RSV vaccination was 64% (95% CI, 37%-79%) against medically attended RSV-associated ARI and 70% (95% CI, 37%-86%) against RSV-associated hospitalization.
• In the same-season analysis of 1765 children, the effectiveness of nirsevimab was 77% (95% CI, 69%-83%) against medically attended RSV-associated ARI and 81% (95% CI, 71%-87%) against RSV-associated hospitalization, with peak effectiveness in the first 30 days after administration.
• Nirsevimab continued to protect against RSV-associated hospitalization for 130-210 days post-receipt and was effective against both RSV-A and RSV-B subtypes.
• Rates of RSV-associated hospitalizations among infants aged 0-11 months declined by 41% during the 2024-2025 season vs observed rates in 2017-2020 and by 51% vs estimated counterfactual rates in 2024-2025, with the greatest reduction observed among those aged 0-2 months.

IN PRACTICE:

“Achieving sustained reductions in severe RSV disease burden in young infants will require consistent efforts each season to ensure early and high immunization coverage in new birth cohorts,” the authors wrote.

SOURCE:

This study was led by Heidi L. Moline, MD, MPH, National Center for Immunization and Respiratory Diseases, CDC, Atlanta. It was published online on December 22, 2025, in JAMA Pediatrics.

LIMITATIONS:

The findings came from children treated at large academic pediatric centers and may not have been nationally representative. Estimates of population-level effects relied on ecologic and quasi-experimental comparisons that could not establish causality. For a small subset of children for whom records were unavailable, immunization status was self-reported.

DISCLOSURES:

This study was funded by the CDC. Some authors disclosed receiving CDC grants to their institutions and having other financial relationships, including receipt of personal fees, honoraria for textbook chapters, travel support, and fees for serving on data and safety monitoring boards or in scientific advisory roles with various organizations and pharmaceutical companies.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.