FDA Approves Enflonsia To Protect Infants During Their First RSV Season

The FDA has approved Merck’s Enflonsia (clesrovimab-cfor) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants during their first RSV season, regardless of weight, according to the results of a Merck news release published yesterday.

RSV is a common respiratory virus that affects the nose, throat and lungs and can be especially dangerous for infants. In the United States, two to three out of every 100 infants under 6 months of age are hospitalized with RSV every year, according to the Centers for Disease Control and Prevention. Worldwide, it’s estimated RSV causes the death of over 100,000 children annually.

“RSV disease is the leading cause of infant hospitalization in the U.S. and can lead to serious respiratory conditions like bronchiolitis and pneumonia,” Octavio Ramilo, M.D., chair of the Department of Infectious Diseases at St. Jude Children’s Research Hospital, said in the news release.

The RSV season usually spans from autumn to spring of the next year, with a peak in December or January. Symptoms include a runny nose and cough, which can make it difficult to distinguish it from the common cold or other viruses such as COVID-19, according to the CDC.

Enflonsia is an extended half-life monoclonal antibody designed to provide protection for 5 months. The approved dosage is a single 105 mg dose, regardless of age, given as an intramuscular injection.

This approval is based on the results of the phase 2b/3 CLEVER trial (MK-1654-004), which evaluated the effectiveness of a single Enflonsia dose in preterm and full-term infants up to one year old. More than 3,600 participants were randomized 2:1 to receive either a saline placebo or a 105 mg injection of Enflonsia.

The primary endpoint was met if infants exhibited no RSV-associated lower respiratory infection symptoms from Day 1 through Day 150 after dosing. Symptoms researchers watched for included cough or difficulty breathing paired with at least one symptom of a lower respiratory tract infection, like wheezing or dehydration. Approximately 61% of participants met this endpoint when compared with placebo.

The results also showed an 84.3% reduction in RSV-related hospitalizations in infants who received Enflonsia during the five-month observation period.

“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,” said Ramilo, who also served as an investigator in the CLEVER (MK-1654-004) and SMART (MK-1654-007) trials.

The safety and efficacy of Enflonsia was determined during the phase 3 SMART trial (MK-1654-007) The most common adverse reactions were injection-site erythema (4%), injection-site swelling (3%) and rash (2%). Infants with a history of hypersensitivity reactions such as anaphylaxis should not take Enflonsia.

The commercial launch of Enflonsia is expected before the start of the 2025-2026 RSV season, with ordering beginning in July, the news release says. 

The CDC’s Advisory Committee on Immunization Practices will meet later this month to discuss and make further recommendations for infants using Enflonsia.