European Commission Approves MSD’s ENFLONSIA™ for Prevention of RSV Lower Respiratory Tract Disease in Infants
MSD has announced that the European Commission has approved ENFLONSIA™ (clesrovimab) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants during their first RSV season.
Clesrovimab is a preventive, long-acting monoclonal antibody designed to provide direct, rapid and durable protection for five months, which typically covers one RSV season. It is administered using non-weight-based dosing.
Clesrovimab is contraindicated for infants with hypersensitivity to the active substance or any of its excipients. Availability in Ireland will depend on several factors, including the completion of reimbursement procedures.
RSV is the main cause of bronchiolitis and pneumonia in very young children and can be particularly serious for infants in their first year of life. While most RSV infections are mild and resolve within a few weeks, newborns and young infants, including those entering their first RSV season or those with certain underlying medical conditions, are at higher risk of developing more severe lower respiratory tract disease that may require medical attention or hospitalisation.
In Ireland, national focus on RSV prevention has increased significantly in recent seasons. An evaluation published earlier this year by public health authorities found that infant RSV immunisation achieved a high uptake rate of 83%, with more than 22,000 newborns immunised during the 2024/25 winter season.
The evaluation estimated that the programme averted approximately 1,030 laboratory-confirmed RSV infections in infants, including 440 non-hospitalised emergency department presentations, 433 hospitalisations and 79 intensive care admissions. Overall, RSV cases among infants were reduced by an estimated 65% compared with the previous season, helping to ease pressure on paediatric services during peak winter months.
Commenting on the approval, Samantha Humpreys, Managing Director of MSD Ireland Human Health, said:
“The European Commission approval marks a significant milestone, with the potential to help reduce the burden of RSV disease on infants in Ireland. RSV is one of the leading causes of infant hospitalisation globally given it can progress to severe conditions like bronchiolitis and pneumonia in both healthy and at-risk infants.
“This milestone also reflects our continued commitment to advancing science-led innovation and bringing new preventive and treatment options to patients, as we build a strong pipeline of future medicines across areas of unmet need today and into the future.”
The European Commission approval is supported by results from the pivotal Phase 2b/3 CLEVER trial, which evaluated the safety and efficacy of a single dose of clesrovimab administered to preterm and full-term infants from birth to one year of age.
The approval is also supported by interim data from RSV season one of the Phase 3 SMART trial, which evaluated the safety, efficacy and pharmacokinetics of clesrovimab compared with palivizumab in infants at increased risk of severe RSV disease.
Clinical data from the CLEVER and SMART trials were published in the New England Journal of Medicine in September 2025.