Merck’s ENFLONSIA Approved as Groundbreaking Preventive Treatment for RSV in Infants

RAHWAY, N.J., June 09, 2025–The U.S. Food and Drug Administration (FDA) has officially approved ENFLONSIA™ (clesrovimab-cfor), a pioneering monoclonal antibody treatment designed to prevent respiratory syncytial virus (RSV) in newborns and infants. This innovative treatment aims to protect young children from the severe complications of RSV, a serious respiratory infection that is a leading cause of hospitalization in infants across the United States.

Understanding RSV and Its Impact

Respiratory syncytial virus (RSV) is a common virus that causes infections in the lungs and respiratory tract. While it typically results in mild symptoms, RSV can lead to severe respiratory conditions such as bronchiolitis and pneumonia, particularly in infants. According to Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude Children’s Research Hospital, RSV is the leading cause of infant hospitalization in the U.S., making preventative measures crucial.

How ENFLONSIA Works

ENFLONSIA is a long-acting monoclonal antibody, which means it is engineered to mimic the immune system’s ability to fight off harmful pathogens like RSV. This preventive treatment is unique in that it is administered in a uniform dose of 105 mg, regardless of the infant’s weight. This simplifies the dosing process, ensuring that all infants receive the same level of protection without the need to calculate dosages based on weight.

The treatment offers rapid and durable protection, lasting through a typical RSV season, which usually runs from autumn to spring. With a single dose, ENFLONSIA provides significant defense against RSV, helping to protect infants during their vulnerable first RSV season.

Clinical Trials Show Promising Results

The approval of ENFLONSIA was supported by data from two pivotal clinical trials, the CLEVER trial and the SMART trial. The CLEVER trial demonstrated a remarkable reduction in RSV-related lower respiratory infections requiring medical attention by 60.5% and a significant decrease in hospitalizations by 84.3% compared to placebo. These findings highlight the treatment’s potential to alleviate the burden of RSV and safeguard infant health.

  • Primary Endpoint: 60.5% reduction in RSV-associated medically attended lower respiratory infections.
  • Key Secondary Endpoint: 84.3% reduction in RSV-associated hospitalizations.

Safety Considerations

As with any new medical treatment, safety is paramount. ENFLONSIA should not be given to infants with a known history of severe allergic reactions to any of its components. In clinical trials, the most common side effects were mild, including redness or swelling at the injection site.

Future Availability and Recommendations

Merck is committed to making ENFLONSIA available in the United States before the upcoming RSV season. The Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is set to convene later this month to discuss the usage recommendations for ENFLONSIA. Anticipated ordering is expected to begin in July, ensuring that infants can receive this important preventive treatment in time for RSV season.

The Broader Impact of ENFLONSIA

ENFLONSIA represents a significant advancement in the fight against RSV, particularly for at-risk populations, including premature infants and those with underlying health conditions. By providing a simple and effective preventive measure, Merck aims to reduce the substantial healthcare burden caused by RSV infections, ultimately improving outcomes for infants and families across the nation.