Effectiveness Timeline of Beyfortus (Nirsevimab) Against RSV
Beyfortus (nirsevimab) begins providing protection against RSV immediately after administration, with full protection established within 24 hours of receiving the injection.
Mechanism and Onset of Protection
Nirsevimab is a long-acting monoclonal antibody that provides immediate passive immunity against RSV. Unlike vaccines that require the body to develop an immune response over time, monoclonal antibodies like nirsevimab:
- Provide immediate protection through direct binding to the RSV fusion protein
- Reach protective levels in the bloodstream within hours of administration
- Maintain protective levels for at least 5 months (covering a typical RSV season)
Evidence Supporting Rapid Effectiveness
The Advisory Committee on Immunization Practices (ACIP) guidelines clearly establish that nirsevimab provides immediate protection, which is why it's recommended for:
- Infants born during RSV season who need immediate protection 1
- Infants whose mothers received maternal RSV vaccine less than 14 days before delivery 1
This recommendation specifically highlights that maternal RSV vaccination requires at least 14 days to develop and transfer protective antibodies to the infant, while nirsevimab provides protection immediately upon administration.
Clinical Application
For optimal protection, nirsevimab should be administered:
- Before or at the beginning of RSV season for infants born outside RSV season
- As soon as possible after birth for infants born during RSV season
- For infants whose mothers received maternal RSV vaccine less than 14 days before delivery 1
Dosing Considerations
A single intramuscular injection provides protection throughout the entire RSV season:
- 50 mg for infants weighing <5 kg
- 100 mg for infants weighing ≥5 kg 2
Effectiveness Duration
The protection from a single dose of nirsevimab lasts approximately 5 months (150 days), which covers a typical RSV season 1. This extended protection is a significant advantage over previous monoclonal antibodies like palivizumab that required monthly injections.
Clinical Efficacy
Clinical trials have demonstrated that nirsevimab is highly effective in preventing:
- Medically attended RSV-associated lower respiratory tract infections (74.5% efficacy) 3
- Hospitalizations for RSV-associated lower respiratory tract infections (83.2% efficacy) 4
- Very severe RSV-associated lower respiratory tract infections (75.7% efficacy) 4
Important Considerations
- Unlike maternal RSV vaccination which requires at least 14 days to develop and transfer protective antibodies, nirsevimab provides immediate protection 1
- The protection is effective across different geographic locations and against different RSV subtypes 5
- Nirsevimab is recommended for all infants aged <8 months born during or entering their first RSV season 1
- For infants and children aged 8-19 months who are at increased risk for severe RSV disease and entering their second RSV season, nirsevimab is also recommended 1
Nirsevimab represents a significant advancement in RSV prevention by providing immediate and season-long protection with a single dose, eliminating the need for monthly injections and offering broader protection for all infants regardless of risk factors.