Nirsevimab Kept Infants With RSV Out of Intensive Care, 2024–25

Respiratory syncytial virus (RSV) is a leading cause of intensive care admission and acute respiratory failure among U.S. infants. In August 2023, CDC’s Advisory Committee on Immunization Practices recommended nirsevimab, a long-acting monoclonal antibody, for all infants younger than 8 months born during or entering their first RSV season whose mothers had not received an RSV vaccine more than 14 days before delivery. Studies after licensure showed it prevents RSV hospitalizations, but its effect against the most severe illness had not been rigorously measured across the country.

The study

The Overcoming RSV Network — an extension of CDC’s Overcoming COVID-19 Network — enrolled infants under 1 year admitted to an ICU with respiratory symptoms at 27 pediatric hospitals in 24 states between December 1, 2024, and April 15, 2025. It compared:

• case-patients: infants in the ICU with laboratory-confirmed RSV; and
• control patients: infants in the ICU who tested negative for RSV,

matched by hospital, admission date and age where possible. An infant counted as protected if they had received one dose of nirsevimab at least 7 days before falling ill. Infants whose mothers received an RSV vaccine in pregnancy, or who received the older antibody palivizumab, were excluded.

The analysis looked at two outcomes: ICU admission, and acute respiratory failure — illness needing CPAP, BiPAP or a ventilator beyond any support used at home.

Who was in it

Of 917 infants enrolled, 457 case-patients and 302 control patients remained after exclusions. Just 14% of the RSV patients had received nirsevimab, against 45% of the controls.

The RSV patients were mostly previously healthy: only 11% had an underlying medical condition (against 34% of controls), and 71% were born at term. About half in each group had acute respiratory failure. Median stays were 6 days in hospital and 4 in the ICU. Two RSV patients and two controls needed extracorporeal membrane oxygenation (ECMO); one RSV patient and four controls died.

How well it worked

The median time from dose to illness was about 52 days. An analysis of only the 226 fully matched pairs gave a similar result: 76%.

These figures fit earlier evidence: a single U.S. health system found 85% effectiveness against ICU admission, and a meta-analysis including studies from France and Spain estimated 81%.

What it means

Nirsevimab protected infants against the most severe outcomes of RSV in the 2024–25 season, even up to six months after the dose — and since most infants in intensive care for RSV had been healthy, protection matters for all babies. ACIP recommends that every infant be protected against severe RSV, either through a long-acting monoclonal antibody — nirsevimab, or clesrovimab, added in June 2025 — or through maternal RSV vaccination during pregnancy.

Limitations

• Testing for other respiratory viruses was not done consistently, so their contribution could not be assessed.
• Not every case-patient had a matched control, though the matched-pairs analysis gave similar results.
• Mothers’ RSV vaccination may have been missed, which would make nirsevimab look less effective than it is.
• The study could not precisely measure how long protection lasts.
• As an observational study, it may be affected by factors that were not measured.

Sources

Based on Zambrano LD, Simeone RM, Newhams MM, et al., "Nirsevimab Effectiveness Against Intensive Care Unit Admission for Respiratory Syncytial Virus in Infants — 24 States, December 2024–April 2025," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; written with the Overcoming RSV Investigators and rewritten in hubnx’s own words.