Yes, a 3-week-old infant can receive Beyfortus (nirsevimab).
ACIP recommends nirsevimab for all infants aged <8 months who are born during or entering their first RSV season 1. A 3-week-old infant clearly falls within this age range and is eligible for administration.
Timing and Administration
The key consideration is RSV seasonality in your region:
- If born during RSV season (typically October through March in most of the continental United States): Administer shortly after birth or as soon as feasible 1
- If entering RSV season: Administer shortly before or during the season onset 1
For a 3-week-old infant, you should:
- Verify the current RSV season status in your area
- Administer nirsevimab promptly if within or approaching RSV season
- Use chronologic age (not corrected age) to determine eligibility, even for preterm infants 1
Dosing
Weight-based dosing 2:
- <5 kg: 50 mg IM (single dose)
- ≥5 kg: 100 mg IM (single dose)
Most 3-week-old infants will weigh <5 kg and receive the 50 mg dose.
Safety Profile
Clinical trials demonstrated excellent safety in infants as young as newborns 2:
- Adverse reactions occurred in only 1.2% of recipients
- Most common: mild rash (0.9%) and injection site reactions (0.3%)
- 97% of adverse reactions were mild to moderate
- Median age at administration in trials was 2 months, with 65% of infants ≤3 months old
Important Clinical Points
Coadministration with vaccines: Nirsevimab can be given simultaneously with routine childhood immunizations without interference 1. This is particularly relevant for a 3-week-old who may be due for hepatitis B vaccine.
Contraindications: Only severe allergic reaction to nirsevimab or its components 1
Monitoring: Observe for at least 60 minutes post-dose for hypersensitivity reactions 2
Common Pitfall to Avoid
Do not delay administration waiting for the infant to be older. The goal is protection throughout the RSV season, and RSV is the leading cause of hospitalization in infants, with 79% of hospitalized children <2 years having no underlying medical conditions 1. Early administration maximizes protection during the vulnerable first months of life when RSV disease burden is highest.