On Jun. 9, 2025, Merck announced the U.S. Food and Drug Administration (FDA) has approved ENFLONSIA™ (clesrovimab-cfor) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates (newborns) and infants who are born during or entering their first RSV season.

ENFLONSIA is a preventive, long-acting monoclonal antibody (mAb) designed to provide direct, rapid and durable protection through 5 months, a typical RSV season, with the same 105 mg dose regardless of weight. A typical RSV season usually spans autumn to spring of the next year.

“RSV disease is the leading cause of infant hospitalization in the U.S. and can lead to serious respiratory conditions like bronchiolitis and pneumonia,” 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 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.”

ENFLONSIA should not be administered to infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA. See additional Selected Safety Information below.

The approval is based on results from the pivotal Phase 2b/3 CLEVER trial (MK-1654-004) evaluating a single dose of ENFLONSIA administered to preterm and full-term infants (birth to 1 year of age). The trial met its primary and key secondary endpoints. The approval is also supported by results from the Phase 3 SMART trial (MK-1654-007) evaluating the safety and efficacy of ENFLONSIA versus palivizumab in infants at increased risk for severe RSV disease.