A large cohort study yesterday of nirsevimab (Beyfortus) use during the 2024-25 respiratory syncytial virus (RSV) season shows the monoclonal antibody was 77% effective against hospitalization in treated infants.
The study was published as a research letter in JAMA Network Open and is the first analysis of nirsevimab during the 2024-25 season. The drug was first recommended for use in the United States in 2023 for all infants younger than 8 months entering their first RSV season and high-risk infants aged 8 to 19 months, excluding those born to vaccinated mothers.
The current study used electronic records from more than 1,700 hospitals and 40,000 clinics in the United States, Canada, Lebanon, and Saudi Arabia. Infants born from February 1, 2024, to January 31, 2025, were included in the study, and the 2024-25 RSV season was defined as October 1, 2024, to March 31, 2025.
The authors excluded infants with missing maternal information, infants born to mothers who received an RSV vaccine 14 to 280 days before delivery, infants who died without hospitalization, infants who received nirsevimab or had RSV-associated hospitalization during the 2023-24 RSV season, and infants who did not have well-child visits after September 30, 2025, to reduce loss to follow-up.
Reduced ICU admissions
The study included 409,723 infants with an average age of 8 months, and an average gestational age of 39 weeks. The study authors reported 3,385 RSV hospital admissions, 1,089 RSV-associated intensive care unit (ICU) admissions, and 139 RSV-associated admissions requiring intubation.
A total of 215,301 infants in the study did not receive nirsevimab (52.5%), compared with 194,422 (47.5%) who got the treatment beginning in October 2024. Among those not treated, 2,535 had RSV-associated admissions, compared with 850 in the treated group.
The authors recorded 765 ICU admissions among untreated infants and 324 among treated infants, as well as 90 intubations in the untreated group and 49 in the treated group.
The maximum estimated daily hospitalization rate was 2.90 (95% confidence interval [CI], 2.42 to 3.38) per 100,000 treated infants versus 13.84 (95% CI, 13.16 to 14.53) per 100 ,000 untreated infants.
Public health groups should continue to recommend nirsevimab.
The unadjusted and adjusted hazard ratios for RSV-associated hospitalization after treatment with nirsevimab were 0.29 (95% CI, 0.26 to 0.32) and 0.23 (95% CI, 0.21 to 0.26), respectively, for an adjusted nirsevimab effectiveness of 77% against hospitalization.
“This study and prior analyses offer strong evidence that nirsevimab remains an effective tool to reduce RSV-associated hospitalization,” the authors concluded. “Public health groups should continue to recommend nirsevimab.”