What are the risks of administering Beyfortus (nirsevimab) to adults?

Beyfortus (Nirsevimab) Administration to Adults: Risks and Contraindications

Beyfortus is not approved for use in adults and should not be administered to this population, as safety and effectiveness have not been established beyond 24 months of age. 1

Approved Population and Age Restrictions

• Beyfortus is FDA-approved exclusively for infants and children up to 24 months of age for prevention of RSV lower respiratory tract disease 1
• The drug is specifically indicated for:
  • Infants <8 months born during or entering their first RSV season 2, 3
  • Children aged 8-19 months at increased risk for severe RSV disease entering their second RSV season 2, 3
• The FDA label explicitly states: "The safety and effectiveness of BEYFORTUS have not been established in children older than 24 months of age" and makes no mention of adult use 1

Why Adults Should Not Receive Beyfortus

• Nirsevimab is a passive immunization product designed for the pediatric immune system and RSV disease patterns specific to infants 1, 4
• The pharmacokinetic profile, dosing regimen (50 mg for <5 kg, 100 mg for ≥5 kg), and clinical trial data are entirely based on pediatric populations 1, 4, 5, 6
• Adults aged ≥60 years have separate FDA-approved RSV vaccines (RSVpreF3/Arexvy and RSVpreF/Abrysvo) specifically designed and tested for adult populations 2, 7

Potential Risks of Off-Label Adult Use

• Contraindication: History of severe allergic reaction (anaphylaxis) to a previous dose or product component 2, 3, 8, 1
• Unknown efficacy in adults, as the mechanism targets infant-specific RSV disease patterns and immune responses 1, 4
• Inappropriate dosing—adult body weight would require extrapolation far beyond studied parameters 1
• Risk of antidrug antibody formation (6.1% in pediatric trials) with unknown consequences in adults 5
• Potential for hypersensitivity reactions, though rare in pediatric populations 1, 6

Adverse Event Reporting if Administered

• Any adverse events from off-label adult use must be reported to FDA MedWatch (1-800-FDA-1088 or https://www.fda.gov/medwatch) 2, 8
• If co-administered with vaccines, report to VAERS instead of MedWatch 2, 8

Clinical Pitfall to Avoid

• Do not confuse pediatric RSV prophylaxis (nirsevimab) with adult RSV vaccination—these are entirely different products for different age groups with distinct mechanisms (passive monoclonal antibody vs. active immunization) 2, 7
• For adults requiring RSV protection, use age-appropriate RSV vaccines approved for those ≥60 years 2, 7