European Commission Approves Merck’s ENFLONSIA™ (clesrovimab) for the Prevention of Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease in Infants During Their First RSV Season
Merck & Co.—known as MSD outside the United States and Canada—has announced a major regulatory milestone with the approval of ENFLONSIA by the European Commission for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants during their first RSV season. This decision marks a significant advancement in pediatric infectious disease prevention across Europe, where RSV continues to represent a major cause of infant morbidity and hospitalization.
The approval enables the commercialization of ENFLONSIA across all 27 European Union member states, as well as in Iceland, Liechtenstein, and Norway. However, the timing of product availability will vary by country, as national reimbursement processes and healthcare system factors will determine the pace of rollout. The therapy is contraindicated in infants with known hypersensitivity to its active substance or any of its excipients.
Addressing a Major Global Pediatric Health Burden
RSV is a highly contagious respiratory virus and one of the leading causes of lower respiratory tract infections in infants worldwide. It can progress to serious conditions such as bronchiolitis and pneumonia, even in otherwise healthy infants, and is a frequent cause of hospital admissions during seasonal outbreaks. The burden is particularly significant during the first months of life, when infants are most vulnerable and immune protection is still developing.
ENFLONSIA has been designed as a preventive, long-acting monoclonal antibody that provides immediate and sustained protection against RSV. Unlike vaccines, which rely on the body’s immune response to generate protection, monoclonal antibodies deliver ready-made immunity. ENFLONSIA is administered as a single intramuscular injection and is engineered to provide protection for approximately five months, covering the duration of a typical RSV season. Importantly, the dosing is not weight-based, simplifying administration in clinical practice and supporting broader implementation.
Clinical experts have emphasized the importance of expanding preventive options for RSV. Paolo Manzoni, Head of Maternal–Infant Medicine at the University of Torino Hospital Degli Infermi, highlighted that RSV remains a leading cause of infant hospitalization globally. He noted that the approval of ENFLONSIA represents an important public health achievement, supported by strong clinical data demonstrating meaningful reductions in RSV disease incidence and related hospitalizations.
Robust Clinical Evidence from CLEVER and SMART Trials
The European Commission’s decision is based on a comprehensive body of clinical evidence, including results from the pivotal Phase 2b/3 CLEVER trial and interim findings from the Phase 3 SMART trial. These studies collectively evaluated the safety, efficacy, and pharmacokinetics of ENFLONSIA across a broad population of infants, including those at standard and elevated risk of severe RSV disease.
The CLEVER trial (MK-1654-004; NCT04767373) was a randomized, double-blind, placebo-controlled, multicenter study involving more than 3,600 infants. Participants included both preterm and full-term infants entering their first RSV season. They were randomized in a 2:1 ratio to receive either a single 105 mg dose of ENFLONSIA or a saline placebo via intramuscular injection.
The primary endpoint of the study was the incidence of RSV-associated medically attended lower respiratory infection (MALRI) within 150 days (approximately five months) following dosing. This endpoint encompassed cases requiring healthcare intervention, including outpatient visits, emergency care, or hospitalization, and was defined by symptoms such as cough or difficulty breathing accompanied by clinical indicators of lower respiratory involvement or disease severity.
Results from the CLEVER trial demonstrated that ENFLONSIA reduced the incidence of RSV-associated MALRI by 60.4% compared to placebo, a statistically significant outcome that underscores the therapy’s effectiveness in preventing clinically meaningful disease. The incidence rates were 0.026 in the ENFLONSIA group versus 0.065 in the placebo group.
Equally notable were the results for the key secondary endpoint—RSV-associated hospitalizations. ENFLONSIA achieved an 84.2% reduction in hospitalization rates through five months, highlighting its potential to significantly alleviate the burden on healthcare systems. The protective effect appeared to increase with disease severity, suggesting that the therapy is particularly effective in preventing the most serious outcomes.
Additional exploratory analyses further reinforced these findings. ENFLONSIA reduced the incidence of severe MALRI by 91.7% and decreased RSV-related lower respiratory infection hospitalizations by 90.9% over the same time period. The therapy also maintained a reduction in MALRI incidence through six months, demonstrating sustained efficacy beyond the primary evaluation window.
Safety Profile and Tolerability
Safety data from the CLEVER trial indicate that ENFLONSIA has a favorable tolerability profile in infants. The overall safety outcomes were comparable to placebo, with the most commonly reported adverse events being mild to moderate injection-site reactions, including pain, erythema, and swelling. Rash was also observed in a small proportion of participants. Importantly, more than 96% of adverse events were classified as mild or moderate in severity.
The therapy was also evaluated for co-administration with routine childhood vaccines, an important consideration for real-world clinical use. Results showed that the safety profile remained consistent when ENFLONSIA was administered alongside standard immunizations, supporting its integration into existing pediatric care schedules.
Insights from the SMART Trial in High-Risk Populations
The Phase 3 SMART trial (MK-1654-007; NCT04938830) provides additional evidence supporting the use of ENFLONSIA in infants and children at increased risk for severe RSV disease. This includes early and moderate preterm infants, as well as those with chronic lung disease of prematurity or congenital heart disease.
In this study, participants were randomized to receive either a single dose of ENFLONSIA or monthly injections of palivizumab, an established monoclonal antibody used for RSV prevention in high-risk populations. Interim results from the first RSV season demonstrated that ENFLONSIA has a safety profile comparable to palivizumab, with consistent findings across both the SMART and CLEVER trials.
Efficacy in the high-risk population was established through pharmacokinetic bridging, extrapolating results from the CLEVER study. Incidence rates of RSV-associated MALRI and hospitalizations were similar between ENFLONSIA and palivizumab, suggesting comparable protective effects with the added advantage of a single-dose regimen.
Expanding Global Access and Future Outlook
ENFLONSIA has already received regulatory approval in multiple regions, including the United States, Canada, and Switzerland, with additional filings underway globally. The European Commission’s approval represents a critical step in expanding access to this preventive therapy, particularly in regions where RSV imposes a significant seasonal burden on healthcare systems.
From a strategic perspective, Merck views this approval as part of a broader effort to address unmet needs in infectious disease prevention. Macaya Douoguih, Vice President and Therapeutic Area Head for Global Clinical Development at Merck Research Laboratories, described the approval as a milestone in the company’s mission to reduce the global impact of RSV. She emphasized the importance of providing families and healthcare providers with new tools to protect infants from this widespread and potentially severe disease.
A Transformative Step in RSV Prevention
The introduction of ENFLONSIA into the European market has the potential to reshape the landscape of RSV prevention. Its long-acting, single-dose design addresses key limitations of existing preventive strategies, such as the need for repeated dosing and restricted eligibility criteria. By offering broad protection to both healthy and at-risk infants during their first RSV season, the therapy could significantly reduce the incidence of severe disease and associated hospitalizations.
Moreover, the strong clinical evidence supporting its efficacy and safety provides confidence in its role as a new standard of care in RSV prevention. As healthcare systems continue to prioritize early intervention and preventive strategies, innovations like ENFLONSIA are likely to play an increasingly important role in improving pediatric health outcomes.
In conclusion, the European Commission’s approval of ENFLONSIA marks a major advancement in the fight against RSV. Backed by robust clinical data and supported by a favorable safety profile, the therapy offers a practical and effective solution to a longstanding public health challenge. As it becomes available across Europe, it holds the promise of reducing the burden of RSV on infants, families, and healthcare systems alike, while setting a new benchmark for preventive care in pediatric infectious diseases.
About RSV Globally
Respiratory syncytial virus (RSV) is a contagious virus that causes widespread seasonal infections and can lead to serious respiratory conditions such as bronchiolitis and pneumonia. As a leading cause of hospitalization among infants globally, there is persisting unmet need for RSV preventive options for both healthy and high-risk infants during their first RSV season. RSV season is the time of year when RSV infections are most common, usually occurring autumn through spring of the next year in temperate climates. Timing and severity in a given community or region can vary year to year.
About ENFLONSIA™ (clesrovimab-cfor) in the U.S.
ENFLONSIA is Merck’s FDA-approved extended half-life monoclonal antibody (mAb) indicated for passive immunization for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants who are born during or entering their first RSV season. ENFLONSIA is administered using the same dose regardless of weight (105 mg/0.7 mL in a prefilled syringe) and is designed to provide direct, rapid and durable protection through 5 months, a typical RSV season.
For infants born during the RSV season, ENFLONSIA is to be administered within the first week of life. For infants born outside of the RSV season, ENFLONSIA should be administered shortly before the RSV season begins. For infants undergoing cardiac surgery with cardiopulmonary bypass during or entering their first RSV season, an additional 105 mg dose is recommended as soon as the infant is stable after surgery. ENFLONSIA has a 30-month shelf life.
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