Merck, known as MSD outside the U.S. and Canada has announced that the European Commission has approved ENFLONSIA™ (clesrovimab). This long-acting monoclonal antibody is indicated for preventing Respiratory Syncytial Virus lower respiratory tract disease in newborns and infants during their first RSV season. The approval allows marketing across all EU member states, as well as Iceland, Liechtenstein, and Norway. However, availability will vary depending on national reimbursement processes.
A Long-Acting Preventive Option
ENFLONSIA delivers rapid and durable protection for up to five months covering a typical RSV season with a single, non-weight-based dose. Importantly, it is contraindicated in infants with hypersensitivity to the active substance or its components.
Experts highlight its clinical relevance. Paolo Manzoni noted that RSV remains a leading cause of infant hospitalisation globally and can progress to severe conditions such as bronchiolitis and pneumonia. Therefore, this approval marks a significant public health milestone.
Strong Clinical Trial Evidence
As reported by Merck Press release, the approval is supported by data from the Phase 2b/3 CLEVER trial and interim results from the Phase 3 SMART trial. Findings, published in the New England Journal of Medicine, demonstrated robust efficacy and safety.
In the CLEVER trial, ENFLONSIA reduced RSV-associated medically attended lower respiratory infections by 60.4% and hospitalisations by 84.2% over five months. Additionally, it reduced severe infections by over 90%, underscoring its effectiveness as disease severity increased.
Meanwhile, the SMART trial showed comparable safety and efficacy between ENFLONSIA and palivizumab in high-risk infants, reinforcing its clinical utility.
Safety and Global Outlook
The therapy demonstrated a safety profile similar to placebo and existing treatments, with mostly mild to moderate injection-site reactions. Furthermore, co-administration with routine childhood vaccines showed no additional safety concerns.
ENFLONSIA is already approved in multiple countries, including the U.S. and Canada, with further global filings underway. Consequently, this development represents a major step toward reducing RSV burden and improving infant health outcomes worldwide.