Merck ENFLONSIA Approved in EU for Infant RSV Protection

RAHWAY, N.J., April 17, 2026

Merck & Co., Inc. announced that the European Commission (EC) has approved ENFLONSIA™ (clesrovimab), a long-acting monoclonal antibody, for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in infants during their first RSV season. This approval grants marketing authorization across all EU member states, marking a major milestone in expanding access to innovative preventive therapies for one of the leading causes of infant hospitalization globally.

Strong Clinical Data Demonstrates Significant Protection

The EC approval is supported by robust results from the Phase 2b/3 CLEVER trial and interim data from the Phase 3 SMART trial, which demonstrated substantial reductions in RSV-related disease and hospitalizations. ENFLONSIA achieved a 60.4% reduction in medically attended RSV lower respiratory infections (MALRI) and an impressive 84.2% reduction in RSV-associated hospitalizations within five months after a single dose. Additionally, the therapy showed up to 91.7% reduction in severe RSV disease, highlighting its strong protective efficacy in both healthy and at-risk infants.

The trials included a broad population of preterm and full-term infants, confirming consistent performance across diverse patient groups. Importantly, ENFLONSIA demonstrated a safety profile comparable to placebo, with most adverse events being mild to moderate, such as injection-site reactions and rash, reinforcing its suitability for widespread pediatric use.

Innovative Monoclonal Antibody with Seasonal Protection

ENFLONSIA is a preventive monoclonal antibody designed for passive immunization, offering rapid, direct, and durable protection for up to five months, covering a typical RSV season. Unlike traditional approaches, it features non-weight-based dosing, simplifying administration and improving accessibility in clinical practice.

The therapy works by providing immediate immune protection without requiring activation of the infant’s immune system, making it particularly valuable for newborns and high-risk populations. Its extended half-life allows a single-dose regimen, reducing the burden on healthcare systems and caregivers compared to repeated dosing strategies.

Furthermore, ENFLONSIA has demonstrated compatibility with routine childhood vaccinations, enabling seamless integration into existing immunization schedules without compromising safety or efficacy.

Addressing a Major Global Pediatric Health Burden

RSV remains a significant global health challenge, responsible for hundreds of thousands of infant hospitalizations annually and severe complications such as bronchiolitis and pneumonia. The burden is particularly high among infants in their first RSV season, where immune systems are still developing and vulnerability to infection is greatest.

With this approval, ENFLONSIA becomes the first and only RSV preventive option in the European Union that does not require weight-based dosing, representing a major advancement in pediatric infectious disease prevention. The availability of such a therapy is expected to reduce healthcare strain, improve patient outcomes, and support public health strategies aimed at preventing severe respiratory infections in infants.

Merck also confirmed that ENFLONSIA is already approved in several global markets, including the United States and Canada, and continues to pursue additional regulatory filings worldwide.

The European Commission approval of ENFLONSIA (clesrovimab) underscores the growing importance of monoclonal antibody-based prevention strategies in addressing infectious diseases. With strong clinical efficacy, simplified dosing, and broad regulatory support, ENFLONSIA is poised to become a key innovation in protecting infants from RSV, reinforcing Merck’s leadership in biopharmaceutical research and global health advancement.

Source: Merck press release