FDA Approves New RSV Preventive Treatment for Infants

June 9, 2025 3:11 pm ET

ENFLONSIA is a preventive monoclonal antibody designed to protect infants against varying severity of RSV disease that can lead to hospitalization.

This is the first RSV preventive treatment for infants that uses a single dose regardless of their weight.

RAHWAY, N.J. – Merck announced today that the U.S. Food and Drug Administration (FDA) has approved ENFLONSIA™ (clesrovimab-cfor) for preventing lower respiratory tract disease caused by respiratory syncytial virus (RSV) in newborns and infants during their first RSV season. This long-acting monoclonal antibody aims to provide efficient protection throughout a typical RSV season, which lasts about five months, using the same dose for all infants.

“RSV disease is a primary cause of infant hospitalizations in the U.S. and can result in severe conditions like bronchiolitis and pneumonia,” commented Dr. Octavio Ramilo from St. Jude Children’s Research Hospital. “With the convenience of dosing and robust clinical data supporting a significant drop in RSV incidence and hospitalizations, ENFLONSIA appears to be a valuable tool for safeguarding infants.”

However, it’s critical to note that ENFLONSIA should not be given to infants who have had severe allergic reactions, including anaphylaxis, to any of its components.

The approval is backed by findings from the pivotal Phase 2b/3 CLEVER trial which assessed ENFLONSIA in both preterm and full-term infants up to one year old. It successfully met its primary and secondary endpoints, demonstrating a 60.5% reduction in RSV-related medically attended lower respiratory infections and an 84.3% decrease in RSV-related hospitalizations through the five-month period.

• ENFLONSIA reduced the incidence of RSV-associated medically attended lower respiratory infections compared to placebo by 60.5% (primary endpoint).
• It also showed an 84.3% reduction in hospitalizations due to RSV (secondary endpoint).

The approval is further reinforced by data from the Phase 3 SMART trial, which evaluated ENFLONSIA against palivizumab in high-risk infants.

“ENFLONSIA is an essential new preventive measure for both healthy and vulnerable infants during their first RSV season, using a uniform dose irrespective of weight,” said Dr. Dean Y. Li, president of Merck Research Laboratories. “We are dedicated to making ENFLONSIA available in the U.S. in time for the upcoming RSV season to lessen the impact of this common seasonal virus on families and healthcare systems.”

The CDC’s Advisory Committee on Immunization Practices is set to convene later this month to discuss and recommend the use of ENFLONSIA in infants, with ordering expected to start in July.

About ENFLONSIA™ (clesrovimab-cfor)

ENFLONSIA (clesrovimab-cfor) is a monoclonal antibody by Merck for the passive immunization against RSV in newborns during their first RSV season. Administered without regard to weight, it provides immediate, effective protection through a typical RSV season. For infants born during this season, it should be given from birth, while those born outside of it should receive it before their first RSV season begins.

Selected Safety Information

ENFLONSIA should not be used in infants with a history of severe hypersensitivity reactions, including anaphylaxis. Serious reactions have been noted with other monoclonal antibodies, so if symptoms arise, appropriate medical intervention is necessary.

Common side effects include mild injection-site reactions like erythema (3.8%) and swelling (2.7%).

Clinical Support for FDA Approval

The CLEVER trial (MK-1654-004) was a randomized, double-blind study assessing ENFLONSIA’s safety and efficacy in preterm infants. The primary goal was focused on the frequency of RSV-related medically attended lower respiratory infections over five months after dosing.

The safety profile of ENFLONSIA was found to be similar to that of placebo, with most adverse reactions being mild or moderate.

Further evaluations in the SMART trial showed comparable safety and efficacy results between ENFLONSIA and palivizumab among infants at higher risk of severe RSV disease.

About Merck

Merck, known as MSD outside the U.S. and Canada, focuses on leveraging science to improve lives globally. With over 130 years in the field, the company aims to be at the forefront of biopharmaceutical innovation, working diligently to provide effective solutions for disease prevention and treatment.