FDA Approves New RSV Prevention for Infants: Merck's Enflonsia

In a significant advancement for infant health, the U.S. Food and Drug Administration (FDA) approved Merck's Enflonsia (clesrovimab-cfor) on June 9, 2025, for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants entering their first RSV season.

RSV is a highly contagious virus that leads to respiratory infections such as bronchiolitis and pneumonia in infants, making it the leading cause of infant hospitalizations in the United States. Enflonsia is a long-acting monoclonal antibody administered as a single 105 mg intramuscular dose, regardless of the infant's weight, providing protection throughout the typical five-month RSV season.

The FDA's approval was based on data from the Phase IIb/III CLEVER trial, which demonstrated a 60.5% reduction in medically attended lower respiratory infections and an 84.3% reduction in RSV-related hospitalizations compared to placebo.

Following the FDA's approval, the Centers for Disease Control and Prevention's (CDC) Advisory Committee on Immunization Practices (ACIP) voted on June 26, 2025, to recommend Enflonsia for infants younger than eight months who are born during or entering their first RSV season. The committee also voted to include Enflonsia in the Vaccines for Children Program, ensuring broader access to this preventive option.

Prior to Enflonsia's approval, the primary preventive option for RSV was Beyfortus (nirsevimab), developed by Sanofi and AstraZeneca. Beyfortus faced supply limitations in 2023, highlighting the need for additional preventive measures. Enflonsia offers a simplified dosing regimen and an alternative for healthcare providers and parents.

Merck plans to make Enflonsia available for the 2025–2026 RSV season. The manufacturing process involves production at Merck Sharp & Dohme LLC facilities, ensuring quality and consistency in the product's availability.

The approval and recommendation of Enflonsia have significant implications for public health, particularly in reducing the burden of RSV-related hospitalizations among infants. The inclusion of Enflonsia in the Vaccines for Children Program ensures that uninsured families have access to this preventive treatment, promoting health equity.

Enflonsia's approval marks a pivotal development in the prevention of RSV lower respiratory tract disease in infants. This advancement is poised to reduce the incidence of severe RSV infections and associated hospitalizations, contributing to improved infant health outcomes nationwide.