Merck (NYSE: MRK), known as MSD outside the United States and Canada, has announced a significant advancement in infant healthcare with the U.S. Food and Drug Administration (FDA) approval of ENFLONSIA™ (clesrovimab-cfor). This innovative monoclonal antibody (mAb) is designed for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants born during their first RSV season.
ENFLONSIA is a long-acting monoclonal antibody aimed at providing rapid and durable protection against RSV for up to five months—covering the typical RSV season that generally runs from autumn to spring. Administered in a single dose of 105 mg, ENFLONSIA’s non-weight-based dosage simplifies treatment for healthcare providers and families alike.
“RSV disease is the leading cause of infant hospitalization in the U.S. and can lead to serious respiratory conditions like bronchiolitis and pneumonia,” stated Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude Children’s Research Hospital. He emphasized the significance of ENFLONSIA, noting the strong clinical data indicating a substantial reduction in RSV disease incidences and hospitalizations.
The FDA’s approval of ENFLONSIA is bolstered by results from the pivotal Phase 2b/3 CLEVER trial (MK-1654-004). This trial evaluated the efficacy of a single dose of ENFLONSIA administered to both preterm and full-term infants (aged birth to 1 year). The trial met its primary and secondary endpoints, demonstrating a remarkable 60.5% reduction in RSV-associated medically attended lower respiratory infections (MALRI) compared to placebo, and an 84.3% reduction in RSV-associated hospitalizations through five months.
The approval was further supported by data from the Phase 3 SMART trial (MK-1654-007), which compared the safety and efficacy of ENFLONSIA against palivizumab in infants at higher risk for severe RSV disease. “ENFLONSIA provides an important new preventive option to help protect both healthy and at-risk infants during their first RSV season,” remarked Dr. Dean Y. Li, president of Merck Research Laboratories.
While ENFLONSIA presents a promising solution for RSV prevention, it is crucial to note that it should not be administered to infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of the product. Common adverse reactions observed include injection-site erythema (3.8%), swelling (2.7%), and rash (2.3%).
For infants born during the RSV season, ENFLONSIA should be given at birth, while those born outside of the RSV season should receive it just before the onset of their first RSV season. An additional dose is recommended for infants undergoing cardiac surgery during this critical period.
The U.S. Centers for Disease Control and Prevention’s (CDC) Advisory Committee on Immunization Practices is set to meet later this month to discuss recommendations for the use of ENFLONSIA in infants. Ordering is expected to commence in July, with shipments slated for delivery before the start of the 2025-2026 RSV season.
At Merck, known as MSD in regions outside the U.S. and Canada, we strive to harness the power of advanced science to improve lives globally. With over 130 years of experience, we are dedicated to developing medicines and vaccines that offer hope to humanity. Our goal is to be the leading research-intensive biopharmaceutical company, continually advancing health solutions to combat diseases affecting people and animals alike.
For more information about ENFLONSIA, including prescribing information and patient guides, please visit Merck's official website.
