What are the potential harmful effects of administering Beyfortus (nirsevimab) to adults?

Beyfortus (Nirsevimab) Should Not Be Administered to Adults

Beyfortus is not approved for use in adults, and its safety and effectiveness have not been established in this population. 1

FDA Approval and Indications

• Beyfortus (nirsevimab) is FDA-approved exclusively for the prevention of RSV-associated lower respiratory tract disease in infants and children up to 24 months of age 2, 1
• The product is specifically indicated for infants <8 months born during or entering their first RSV season, and for children 8-19 months at increased risk for severe RSV disease entering their second RSV season 2
• The FDA label explicitly states: "The safety and effectiveness of BEYFORTUS have not been established in adults" 1

Why Adults Should Not Receive Beyfortus

Lack of Safety and Efficacy Data

• While pharmacodynamic studies showed that RSV neutralizing antibody levels increased in adults following intramuscular administration, these were research studies only—not approval studies 1
• No clinical trials have evaluated the safety profile, appropriate dosing, or clinical efficacy of nirsevimab in adults for preventing RSV disease 1

Adults Have Alternative RSV Prevention Options

• Adults aged ≥75 years should receive RSV vaccination (age-based recommendation) 2
• Adults aged 60-74 years with specific risk factors should receive RSV vaccination through shared clinical decision-making 2
• Three RSV vaccines are FDA-approved for adults ≥60 years: RSVpreF3 (Arexvy, GSK), RSVpreF (Abrysvo, Pfizer), and mResvia (Moderna) 3

Mechanism and Pharmacokinetic Considerations

• Nirsevimab is designed as a passive immunization strategy with extended half-life (approximately 71 days) specifically optimized for infant physiology 1
• The dosing regimen (50 mg for infants <5 kg; 100 mg for infants ≥5 kg; 200 mg for second season) is weight-banded for pediatric patients only 2, 1, 4
• Adult dosing has never been established, and extrapolation from pediatric dosing would be inappropriate given differences in volume of distribution, clearance, and immune system maturity 1

Potential Harms if Administered Off-Label to Adults

Unknown Safety Profile

• Serious adverse events have not been characterized in adults receiving therapeutic doses 1
• The only contraindication established is history of severe allergic reaction (anaphylaxis) to nirsevimab or its components—but this is based on pediatric data 2

Theoretical Concerns

• While not studied in adults, there are theoretical concerns about antibody-dependent enhancement (ADE) with RSV F protein antibodies, as observed historically with inactivated RSV vaccine trials 5
• The extended half-life modification (YTE mutation in Fc region) increases binding to neonatal Fc receptor, which could theoretically disrupt immune homeostasis in ways not studied in adults 1, 5

Inappropriate Use of Limited Resources

• Nirsevimab has experienced supply constraints, and using it off-label in adults would divert critical resources from the approved pediatric population who have no alternative passive immunization options 6
• The cost-effectiveness of nirsevimab is established only for the pediatric indication 5

Critical Clinical Pitfall to Avoid

Do not confuse nirsevimab (Beyfortus) with RSV vaccines approved for adults. Nirsevimab is a long-acting monoclonal antibody for passive immunization of infants and young children, while RSVpreF vaccines (Arexvy, Abrysvo) and mResvia are active immunization products specifically studied and approved for adults ≥60 years 2, 3

What to Do Instead for Adult RSV Prevention

• For adults ≥75 years: Recommend RSV vaccination (age-based) 2
• For adults 60-74 years with risk factors (chronic cardiovascular disease, chronic lung disease, end-stage renal disease, complicated diabetes, severe immunocompromise, severe obesity [BMI ≥40], nursing home residence, or other conditions increasing risk): Recommend RSV vaccination through shared clinical decision-making 2
• Note that postlicensure surveillance suggests a potential increased risk for Guillain-Barré syndrome (GBS) after protein subunit RSV vaccination (GSK Arexvy and Pfizer Abrysvo), which influenced ACIP's risk-based approach for younger adults 2