Before universal protection was recommended in 2023, respiratory syncytial virus (RSV) was the leading cause of hospitalization among U.S. infants. Each year an estimated 2%–3% of babies under 3 months were hospitalized with it, and RSV caused an estimated 58,000–80,000 hospitalizations and 100–300 deaths among children under 5. Risk is highest in the first 6 months, peaking at 1 month of age, and most babies hospitalized with RSV have no known risk factors — so every infant is at risk.
The options so far
Since 2023, the Advisory Committee on Immunization Practices (ACIP) has recommended that every infant be protected against RSV lower respiratory tract infection in one of two ways:
- Maternal vaccination during pregnancy — Abrysvo (Pfizer), at 32–36 weeks, recommended in September 2023; or
- Nirsevimab — Beyfortus (Sanofi with AstraZeneca), a long-acting monoclonal antibody given to the baby, recommended in August 2023.
Most infants shouldn't get both. In real-world studies they prevent roughly 70%–80% of RSV hospitalizations. In 2024–2025, the first full season with both widely available, two national surveillance networks estimated RSV hospitalization rates among infants 0–7 months were 43% and 28% lower than in pre-pandemic seasons.
A third choice
On June 9, 2025, the FDA approved clesrovimab (Enflonsia, Merck), a second long-acting RSV antibody, given as a single injection before or during a baby's first RSV season. In June 2025, ACIP recommended it as an alternative to nirsevimab for infants under 8 months born during or entering their first RSV season who aren't protected by maternal vaccination.
No option is preferred. The choice depends on the parents' preference, what's available, and when the baby is born relative to the RSV season.
The evidence
ACIP's RSV work group reviewed the evidence from September 2024 to June 2025 using the GRADE approach. It rested on one phase 2b/3 randomized, double-blind, placebo-controlled trial of 3,614 healthy preterm and term infants born at 29 weeks or later — 2,411 given clesrovimab and 1,203 a placebo.
| Through 150 days | Clesrovimab | Placebo | Efficacy |
|---|---|---|---|
| RSV illness needing medical care | 60 of 2,398 (2.5%) | 74 of 1,201 (6.2%) | 60.4% (95% CI 44.1%–71.9%) |
| RSV hospitalization | 5 (0.2%) | 27 (2.2%) | 90.9% (95% CI 76.2%–96.5%) |
Safety: over a year, serious adverse events were about as common with clesrovimab (11.5%) as with placebo (12.4%). Most reactions were mild or moderate — irritability and sleepiness were commonest — and fever and injection-site reactions were similar in both groups.
A separate trial in higher-risk infants and children compared clesrovimab with the older antibody palivizumab, which needed monthly doses; preliminary results showed similar side effects and similar RSV illness and hospitalization through 6 months, and drug levels suggest the same dose protects both groups.
Cost: updating the model used for nirsevimab, analysts from the University of Michigan and CDC estimated $105,000 per quality-adjusted life-year gained from a societal perspective (range: cost-saving to about $213,000), at a base price of $457 a dose.
Who gets an RSV antibody
"RSV antibody" here means nirsevimab or clesrovimab; the recommendations are the same, except that clesrovimab is one 105 mg dose for every baby while the nirsevimab dose depends on weight.
Give one dose to infants under 8 months, born during or entering their first RSV season (typically fall to spring in the continental U.S.), if:
- the mother didn't get an RSV vaccine in pregnancy;
- her vaccination status is unknown; or
- the baby was born within 14 days of her vaccination.
Most babies born 14 or more days after their mother's vaccination don't need it, except in rare cases. RSV antibody can be given at the same visit as routine childhood vaccines.
When
| Born | Give the dose |
|---|---|
| October 1–March 31 (in most of the continental U.S.) | within 1 week of birth, ideally before leaving the hospital |
| April–September | at the earliest opportunity from October |
The timing is deliberately flexible. RSV seasons are longer and less predictable in Alaska, and may differ in tropical climates; public health authorities may adjust the October–March window using local data, and providers should consult their state or territorial health department before changing it systematically.
Babies in the hospital: RSV can spread in health care settings, though how often is unknown, and standard and contact precautions help. Providers may consider giving the antibody to eligible infants during their hospital stay, using clinical judgment. Safety data are limited for babies under 32 weeks postmenstrual age or under 3.5 lb (1.6 kg).
Who shouldn't
Anyone with a history of severe allergic reaction, such as anaphylaxis, to any component of clesrovimab.
Reporting side effects
Report any clinically significant adverse event, even if you're not sure the product caused it:
- after an RSV antibody alone — to the FDA's MedWatch (online, fax, mail, or 800-332-1088);
- after an RSV antibody given with a vaccine — to VAERS (online, fax, mail, or 800-822-7967), naming the antibody; no separate MedWatch report is needed.
Full clinical guidance: RSV immunization for infants and young children and RSV vaccine guidance for pregnant women.
Sources
Based on Moulia DL, Link-Gelles R, Chu HY, et al., "Use of Clesrovimab for Prevention of Severe Respiratory Syncytial Virus–Associated Lower Respiratory Tract Infections in Infants: Recommendations of the Advisory Committee on Immunization Practices — United States, 2025," MMWR volume 74, number 32, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report dates ACIP's vote June 26, 2025, in its summary and June 25 in its recommendations section, so only the month is given here.
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