The U.S. Food and Drug Administration (FDA) has approved Merck’s Enflonsia (clesrovimab-cfor) to prevent respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants during their first RSV season. The approval applies regardless of infant weight, according to a recent Merck news release.
RSV is a common respiratory virus that affects the nose, throat, and lungs, posing serious risks to infants. In the U.S., two to three out of every 100 infants under six months old are hospitalized annually due to RSV, according to the Centers for Disease Control and Prevention (CDC). Worldwide, RSV causes over 100,000 child deaths annually.
“RSV is the leading cause of infant hospitalization in the U.S. and can cause serious respiratory issues like bronchiolitis and pneumonia,” said Dr. Octavio Ramilo, chair of Infectious Diseases at St. Jude Children’s Research Hospital.
The RSV season usually runs from autumn through spring, peaking in December or January. Symptoms such as runny nose and cough can be confused with the common cold or other viruses like COVID-19, the CDC notes.
Enflonsia is a long-acting monoclonal antibody designed to protect infants for five months with a single 105 mg intramuscular injection, regardless of age.
The FDA approval is based on the phase 2b/3 CLEVER trial (MK-1654-004), which tested a single dose of Enflonsia in over 3,600 preterm and full-term infants up to one year old. Participants were randomized 2:1 to receive either Enflonsia or a saline placebo.
Safety and efficacy were further supported by the phase 3 SMART trial (MK-1654-007). The most common side effects included injection-site redness (4%), swelling (3%), and rash (2%). Infants with a history of severe allergic reactions like anaphylaxis should not receive Enflonsia.
The main goal was to prevent RSV-related lower respiratory infections within 150 days post-dose. Symptoms monitored included cough or difficulty breathing with signs such as wheezing or dehydration. About 61% of infants receiving Enflonsia reached this endpoint compared to placebo. Enflonsia also reduced RSV-related hospitalizations by 84.3% during this period.
“Enflonsia offers convenient dosing and strong evidence of reduced RSV disease and hospitalizations, making it a promising new option to protect infants,” said Dr. Ramilo, an investigator in both trials.
Merck plans to launch Enflonsia before the 2025-2026 RSV season, with ordering beginning in July. The CDC’s Advisory Committee on Immunization Practices will meet later this month to review Enflonsia and make further recommendations for infant use.
Related topics:
