Harm of Beyfortus (Nirsevimab) if Given to Adults
Beyfortus has not been approved for use in adults, and the FDA explicitly states that safety and effectiveness have not been established in adult populations. 1
Lack of Safety and Efficacy Data in Adults
The FDA has not approved nirsevimab for use in adult populations, as no safety or efficacy data supports its use in this group. 2
All clinical trials establishing the safety profile of Beyfortus were conducted exclusively in pediatric populations (infants and children ≤24 months of age), including preterm infants, term infants, and immunocompromised children. 1, 3, 4, 5
The FDA drug label explicitly states: "The safety and effectiveness of BEYFORTUS have not been established in adults." 1
Theoretical Risks Based on Pediatric Data
While adult-specific harm data does not exist, extrapolating from pediatric safety profiles and pharmacokinetic studies reveals potential concerns:
Allergic and Hypersensitivity Reactions
Beyfortus is contraindicated in persons with a history of severe allergic reaction (e.g., anaphylaxis) to a previous dose or product component, which would apply to any age group including adults. 2, 6, 1
Immediate allergic reactions including hives, angioedema, and anaphylaxis are possible, though rare in pediatric populations. 6
Pharmacokinetic Considerations
In adults who received intramuscular nirsevimab during pharmacodynamic studies, RSV neutralizing antibody levels reached approximately 4 times baseline at 8 hours post-dosing, with maximum levels by day 6. 1
However, these pharmacokinetic observations were conducted solely to understand the drug's behavior and do not establish safety or appropriate dosing for adults. 1
Bleeding Risk
Bleeding complications could theoretically occur in patients with bleeding disorders, and special precautions are recommended for intramuscular injections. 2
This risk would apply to adults with coagulopathies or those on anticoagulation therapy.
Overdose Considerations
There is limited experience with overdose of Beyfortus even in the approved pediatric population. 1
In the event of overdose (which administering to an adult could constitute given lack of approved dosing), the individual should be monitored for adverse reactions and provided symptomatic treatment. 1
No specific antidote exists for nirsevimab overdose. 1
Reporting Requirements
Any adverse events resulting from off-label use in adults must be reported to FDA MedWatch, as all clinically significant adverse events require reporting even without certainty of causation. 6
Any adverse event resulting in hospitalization, disability, or death requires VAERS reporting. 6
Clinical Bottom Line
Administering Beyfortus to adults represents off-label use without supporting safety or efficacy data, carries unknown risks, and should be avoided. The drug was specifically developed and tested for RSV prevention in infants and young children, with dosing, pharmacokinetics, and safety profiles established only for this population. 2, 1, 3