Also known as: Enflonsia, ENFLONSIA, ENFLONSIA™
Clesrovimab-cfor is a long-acting preventive monoclonal antibody marketed as ENFLONSIA for RSV prevention in newborns and infants entering their first RSV season. It is designed for a single non-weight-based dose to provide protection through a typical five-month RSV season. The FDA approved it in June 2025.
A real-world study in Galicia showed nirsevimab reduced RSV-related hospitalizations by 85.9% in the first season and 55.3% in the second. The RSV prevention market is expanding to cover high-risk children, with Beyfortus the only antibody approved for second-season use. The FDA has cited Sanofi for misleading promotional claims about the product.
Merck's Enflonsia (clesrovimab) receives EU approval for RSV prevention in infants. Separately, researchers identify a monoclonal antibody cocktail from pediatricians' immune cells that shows potent protection against RSV and hMPV in animal studies.
The European Commission approved Enflonsia for prevention of RSV lower respiratory tract disease in infants during their first RSV season. Merck also reported new Phase 3 SMART data showing generally consistent second-season safety in high-risk children under 2 years old.
Merck announced positive second RSV season findings from the Phase 3 SMART trial evaluating ENFLONSIA (clesrovimab) in infants and children under 2 years at increased risk for severe RSV disease, showing consistent safety and antibody levels similar to healthy infants.
Phase III SMART trial data show clesrovimab maintained consistent safety and efficacy in high-risk children across two RSV seasons, with no drug-related serious adverse events reported in either season.
Merck stock reached a 52-week high at $122.69, rising 46.25% over 12 months and 7.4% in the past month following strong Q4 results and improved long-term pipeline outlook beyond Keytruda's 2028 patent expiration.
| NCT ID | Title | Status | Phase |
|---|---|---|---|
| NCT04938830 |
Clesrovimab (MK-1654) in Infants and Children at Increased Risk for Severe Respiratory Syncytial Virus (RSV) Disease (MK-1654-007) |
COMPLETED | PHASE3 |
| NCT04767373 |
Efficacy and Safety of Clesrovimab (MK-1654) in Infants (MK-1654-004) |
COMPLETED | PHASE2/PHASE3 |
| NCT04086472 |
Phase 2a Respiratory Syncytial Virus (RSV) Human Challenge Study of Clesrovimab (MK-1654) in Healthy Participants (MK-1654-005) |
COMPLETED | PHASE2 |
| NCT03524118 |
Safety, Tolerability, and Pharmacokinetics of Clesrovimab (MK-1654) in Infants (MK-1654-002) |
COMPLETED | PHASE1/PHASE2 |
