FDA Approves Merck’s ENFLONSIA To Protect Infants From Respiratory Syncytial Virus (RSV)

Merck announced today that the U.S. FDA has approved ENFLONSIA™ (clesrovimab-cfor), a long-acting monoclonal antibody (mAb), for the prevention of lower respiratory tract disease caused by respiratory syncytial virus (RSV) in newborns and infants entering their first RSV season. The therapy offers season-long protection with a single 105 mg dose, regardless of infant weight, and is designed to last five months, the typical duration of RSV season.

“ENFLONSIA provides an important new preventive option to help protect healthy and at-risk infants born during or entering their first RSV season with the same dose regardless of weight. We are committed to ensuring availability of ENFLONSIA in the U.S. before the start of the upcoming RSV season to help reduce the significant burden of this widespread seasonal infection on families and health care systems,” said Dr. Dean Y. Li, president, Merck Research Laboratories. 

The approval was based on positive results from the Phase 2b/3 CLEVER trial, which showed that ENFLONSIA reduced the incidence of medically attended RSV lower respiratory infections by 60.5% and RSV-related hospitalizations by 84.3% compared to placebo. Additional support came from the Phase 3 SMART trial, where ENFLONSIA demonstrated safety and efficacy against palivizumab in high-risk infants. 

“RSV disease is the leading cause of infant hospitalization in the U.S. and can lead to serious respiratory conditions like bronchiolitis and pneumonia. ENFLONSIA combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV,” said Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude Children’s Research Hospital and investigator for the CLEVER (MK-1654-004) and SMART (MK-1654-007) trials. 

ENFLONSIA should not be given to infants with a history of serious hypersensitivity to any of its components. The CDC’s Advisory Committee on Immunization Practices (ACIP) is expected to issue recommendations later this month. Ordering will begin in July, with shipments planned ahead of the 2025–2026 RSV season.