Beyfortus (nirsevimab) uses three dose amounts across two respiratory syncytial virus seasons: 50 mg for smaller infants, 100 mg for larger infants, and 200 mg for selected high-risk children entering a second season. The FDA prescribing information sets the first-season dose by body weight and the second-season dose by risk status, while current CDC recommendations further define which infants and young children should receive the antibody.
First-season dosing changes at a body weight of 5 kg
For neonates and infants born during or entering their first respiratory syncytial virus (RSV) season, the FDA label uses a simple weight threshold. Infants who weigh less than 5 kg receive 50 mg as a single intramuscular injection. Infants who weigh 5 kg or more receive 100 mg as a single intramuscular injection.
The threshold is based on body weight at the time of dosing, not birth weight. An infant who was born under 5 kg but weighs 5 kg or more when nirsevimab is administered receives the 100 mg first-season dose.
This weight-based approach was built into the phase 3 MELODY trial, which randomized 1,490 healthy late-preterm and term infants in a 2:1 ratio to nirsevimab or placebo before RSV season. Infants in the nirsevimab group received 50 mg if they weighed less than 5 kg and 100 mg if they weighed 5 kg or more.
MELODY established protection from a single first-season dose
Through 150 days after dosing, medically attended RSV-associated lower respiratory tract infection occurred in 12 of 994 infants receiving nirsevimab and 25 of 496 receiving placebo. That corresponded to an efficacy of 74.5% (95% confidence interval, 49.6% to 87.1%; P<0.001).
Hospitalization for RSV-associated lower respiratory tract infection occurred in 6 infants in the nirsevimab group and 8 in the placebo group, corresponding to an efficacy estimate of 62.1%, but the confidence interval crossed zero and the comparison was not statistically significant in that analysis.
The investigators concluded that a single injection before RSV season “protected healthy late-preterm and term infants” from medically attended RSV-associated lower respiratory tract infection.
Our nirsevimab vs clesrovimab comparison explains how this weight-based dosing differs from clesrovimab, which uses a fixed first-season dose.
The 200 mg dose is reserved for selected children entering a second RSV season
The FDA label also includes a 200 mg dose for children up to 24 months of age who remain at increased risk for severe RSV disease through their second RSV season. The 200 mg total is administered as two 100 mg intramuscular injections, rather than as one 200 mg injection.
CDC recommendations are narrower than the FDA label. Current CDC guidance recommends nirsevimab for children aged 8 through 19 months entering their second RSV season if they are at increased risk for severe RSV disease.
The groups include children with chronic lung disease of prematurity who required medical support during the previous six months, children with severe immunocompromise, certain children with cystic fibrosis who have severe lung disease or poor growth, and American Indian or Alaska Native children.
The second-season recommendation is therefore not a routine dose for every toddler. Most children who received protection during their first RSV season do not receive another dose the following year.
Second-season evidence relied heavily on pharmacokinetic bridging
The evidence base for the 200 mg second-season dose differs from the first-season efficacy trials. In the MEDLEY study, children at higher risk because of congenital heart disease, chronic lung disease of prematurity, or prematurity were evaluated against palivizumab. The second-season program provided safety and pharmacokinetic data, but direct efficacy data were limited.
CDC’s Advisory Committee on Immunization Practices therefore relied in part on pharmacokinetic bridging: the goal was to show that the 200 mg dose produced nirsevimab exposure in high-risk second-season children comparable to concentrations associated with protection in first-season trials.
This distinction matters when interpreting the evidence. The 200 mg recommendation is supported by regulatory pharmacology, safety data, and extrapolation from established first-season efficacy, rather than by a large placebo-controlled second-season efficacy trial.
Maternal RSV vaccination changes whether many first-season infants need nirsevimab
The FDA label describes who the product is approved to protect, but CDC recommendations determine how it is used in routine U.S. immunization practice. For most infants younger than 8 months, protection from severe RSV disease is recommended through either maternal RSV vaccination during pregnancy or a long-acting infant RSV antibody, not both.
CDC recommends an infant RSV antibody when the mother did not receive RSV vaccine during pregnancy, when maternal vaccination status is unknown, or when the infant was born within 14 days of maternal vaccination. Our nirsevimab vs maternal RSV vaccine guide explains how those two strategies differ.
Nirsevimab is also not a vaccine. It is a monoclonal antibody that supplies ready-made antibodies rather than training the infant’s immune system to generate its own response. Our original Beyfortus approval explainer covers the mechanism and the evidence behind the 2023 FDA approval.
The safety profile is generally mild, but serious hypersensitivity can occur
In the current FDA label, the most common adverse reactions were rash within 14 days of dosing, reported in 0.9% of nirsevimab recipients versus 0.6% of placebo recipients, and injection-site reactions within 7 days, reported in 0.3% versus 0%.
Beyfortus is contraindicated in infants and children with a history of serious hypersensitivity to nirsevimab or its excipients. The label also warns that serious hypersensitivity reactions have been reported after administration.
In 2026, FDA also listed immune thrombocytopenia as a potential safety signal under evaluation in its adverse-event monitoring system. A potential signal does not establish that the drug caused the event, and FDA had not concluded from that listing alone that a label change was required.
What the evidence cannot yet answer
The first-season efficacy evidence is strong because it comes from randomized trials with prespecified clinical endpoints. The second-season recommendation is less direct because efficacy was extrapolated in part from pharmacokinetic data in higher-risk children.
The major trials were sponsored by AstraZeneca, Sanofi, or both, and several investigators had industry affiliations. That does not negate the randomized results, but it is relevant when weighing sponsor-led pharmacokinetic modeling and exploratory analyses.
Real-world effectiveness studies now support infant RSV antibodies more broadly, but product choice, access, maternal vaccination status, local RSV timing, and the introduction of clesrovimab mean the practical decision is no longer simply whether an infant should receive Beyfortus.
The dosing logic is simpler than the eligibility rules
The dose itself follows a straightforward pattern: 50 mg below 5 kg and 100 mg at 5 kg or more for the first RSV season, then 200 mg for eligible high-risk children entering a second season. The harder question is whether a child should receive nirsevimab at all, which depends on age, RSV season, maternal vaccination history, risk status, and current CDC guidance.
References
- U.S. Food and Drug Administration. Beyfortus (nirsevimab-alip) prescribing information. BLA 761328. Revised 2025. FDA prescribing information.
- Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants. N Engl J Med. 2022;386:837-846. DOI: 10.1056/NEJMoa2110275.