Merck’s Enflonsia (clesrovimab) gets positive EMA CHMP opinion for infant RSV prevention

Merck reported that the European Medicines Agency’s Committee for Medicinal Products for Human Use has issued a positive opinion recommending approval of clesrovimab (ENFLONSIA) to prevent respiratory syncytial virus lower respiratory tract disease in neonates and infants during their first RSV season. The opinion now goes to the European Commission for a marketing authorization decision covering the European Union, Iceland, Liechtenstein, and Norway, and Merck expects a ruling before year end.¹

If authorized, clesrovimab would be the first RSV preventive option in Europe that uses the same dose regardless of infant weight and is designed to protect for a typical five-month season with a single administration. In June 2025, the US Food and Drug Administration approved clesrovimab for the prevention of RSV lower respiratory tract disease in newborns and infants entering their first season, positioning the antibody for availability in the 2025 to 2026 season.¹

Merck said the CHMP opinion rests on data from the pivotal Phase 2b/3 CLEVER trial and the Phase 3 SMART trial, which enrolled preterm and full-term infants and included a head-to-head comparison with palivizumab in higher-risk populations. In CLEVER, clesrovimab reduced medically attended RSV lower respiratory tract infection by 60.5 percent and RSV-related hospitalizations by 84.3 percent through Day 150 compared with placebo. Results from both programs were recently published in the New England Journal of Medicine.¹

“We are very proud that the FDA has approved clesrovimab, now called enflonsia, for the prevention of RSV for newborns entering their first RSV season,” said Macaya Douoguih, MD, MPH, therapeutic area head for global vaccine clinical development at Merck. Douoguih added that the strong clinical data, combined with weight-independent dosing, make the product a practical option for infant protection.²

Listen to our full interview from the approval with Donoguih: Merck’s Clesrovimab (Enflonsia) Receives FDA Approval to Protect Infants From RSV During First Season

Clesrovimab is a long acting monoclonal antibody intended to provide direct, rapid, and durable protection for about five months. In the US, the approved presentation is a single dose of 105 mg per .7 mL in a prefilled syringe, and guidance includes an additional 105 mg dose after cardiopulmonary bypass once the infant is clinically stable, with a labeled shelf life of 30 months.²

Merck advises against use in infants with a history of serious hypersensitivity to any component of the product, and notes that serious hypersensitivity reactions, including anaphylaxis, have been observed with other IgG1 monoclonal antibodies. The most common adverse reactions reported in studies were injection site erythema at 3.8 percent, injection site swelling at 2.7 percent, and rash at 2.3 percent.²

Clesrovimab is already approved in the US and the United Arab Emirates and remains under review in several additional markets. RSV continues to be a leading cause of infant hospitalization worldwide. In the Northern Hemisphere, the season typically spans autumn through spring, and timing and severity vary by region and year.¹

References

1.Merck. Merck Receives Positive EU CHMP Opinion for ENFLONSIA™ (clesrovimab) for the Prevention of Respiratory Syncytial Virus (RSV) in Infants During Their First RSV Season. BioSpace. September 22, 2025. Accessed September 22, 2025. https://www.biospace.com/press-releases/merck-receives-positive-eu-chmp-opinion-for-enflonsia-clesrovimab-for-the-prevention-of-respiratory-syncytial-virus-rsv-in-infants-during-their-first-rsv-season?utm_source=chatgpt.com

2.US FDA Approves Merck’s ENFLONSIA™ (clesrovimab-cfor) for Prevention of Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease in Infants Born During or Entering Their First RSV Season. Merck. Press release. June 9, 2025. Accessed June 9, 2025. https://www.merck.com/news/u-s-fda-approves-mercks-enflonsia-clesrovimab-cfor-for-prevention-of-respiratory-syncytial-virus-rsv-lower-respiratory-tract-disease-in-infants-born-during-or-entering-their-fir/