Merck’s Enflonsia Approved in EU for RSV Prevention in Infants Without Weight-Based Dosing

Merck reported that the European Commission has approved clesrovimab (brand name Enflonsia) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants during their first RSV season. The approval covers all 27 European Union member states, as well as Iceland, Liechtenstein, and Norway.

The decision establishes Enflonsia as a preventive, long-acting monoclonal antibody designed to provide protection for approximately five months, corresponding to a typical RSV season. The product is administered as a single

fixed 105 mg dose regardless of body weight, a feature the company said differentiates it from other available RSV preventive options.

Merck described Enflonsia as the first and only RSV preventive option in the European Union for infants that does not require weight-based dosing. The therapy is contraindicated in infants with known hypersensitivity to the active substance or excipients.

Clinical efficacy supported by CLEVER and SMART trials

EU approval was supported by data from the Phase 2b/3 CLEVER trial¹ and interim results from the Phase 3 SMART trial².

CLEVER evaluated a single intramuscular dose of Enflonsia in more than 3,600 preterm and full-term infants entering their first RSV season. The trial met its primary and key secondary endpoints, demonstrating a 60.4% reduction in RSV-associated medically attended lower respiratory infection through 150 days compared with placebo. RSV-associated hospitalizations were reduced by 84.2% over the same period.

Additional exploratory analyses showed reductions of 91.7% in severe lower respiratory tract infection and 90.9% in RSV-associated hospitalizations. Protection was sustained through six months, with a 59.5% reduction in medically attended disease.

In SMART, which evaluated infants at increased risk for severe RSV disease, safety and pharmacokinetic data from an interim analysis showed a safety profile comparable to palivizumab. Efficacy outcomes were generally similar between treatment arms, supported by exposure-based extrapolation from CLEVER.

Across trials, the safety profile was generally comparable to placebo, with the most common adverse reactions including injection-site pain, erythema, swelling, and rash. Most events were mild or moderate.

Mechanism and dosing convenience highlighted

Enflonsia is a long-acting monoclonal antibody designed to provide passive immunization through rapid and durable neutralization of RSV. Unlike weight-based dosing regimens used for some prophylactic agents, Enflonsia is administered as a fixed dose intended to simplify administration in newborns and infants.

Merck said the approach is intended to support broad use across both healthy infants and those at increased risk for severe RSV disease, including preterm infants and those with underlying cardiopulmonary conditions.

Regulatory scope and availability

The European Commission approval enables marketing across the EU as well as Iceland, Liechtenstein, and Norway. Timing of commercial availability will vary by country and depend on reimbursement decisions.

Enflonsia is already approved in the United States, Canada, Switzerland, and several other markets. Regulatory filings are ongoing in additional geographies.

Broader RSV burden context

RSV is a leading cause of lower respiratory tract infection and infant hospitalization globally, often resulting in bronchiolitis or pneumonia during seasonal outbreaks. Merck and clinical investigators highlighted persistent unmet need for preventive options that can reduce disease burden across both healthy and high-risk infant populations during the first year of life.

References

1. Zar, H.J.; Simoes, E.; Madhi, S. et al. (2025, January 29). A Phase 2b/3 Study to Evaluate the Efficacy and Safety of an Investigational Respiratory Syncytial Virus (RSV) Antibody, Clesrovimab, in Healthy Preterm and Full-Term Infants. National Library of Medicine. PubMed Central. https://pmc.ncbi.nlm.nih.gov/articles/PMC11777356/

2. Jennings, S. (2026, February 19). Phase 3 SMART Trial Supports Clesrovimab for High-Risk Children in Second RSV Season. Patient Care. https://www.patientcareonline.com/view/phase-3-smart-trial-supports-clesrovimab-for-high-risk-children-in-second-rsv-season