The data were presented during an oral session (Abstract #P455) at RSVVW’26, the 9th conference of the Respiratory Syncytial Virus Foundation (ReSViNET) in Rome, Italy.
New data from the SMART trial in children under 2 years of age who remained at increased risk for severe RSV disease through their second RSV season and received ENFLONSIA at the start of RSV season 2 showed that safety was generally consistent with safety observed in MK-1654-007 infants who received ENFLONSIA during RSV season.
Additional safety data can be found below. Additionally, the monoclonal antibody (mAb) serum concentrations achieved in children under 2 years of age at increased risk for severe RSV disease through their second RSV season (secondary endpoint) were similar to those in healthy infants in the pivotal Phase 2b/3 CLEVER trial (MK-1654-004) (NCT04767373). Results from the SMART study support extrapolation of efficacy to children under 2 years of age at increased risk for severe RSV disease through RSV season 2.
“All children who received ENFLONSIA in their second RSV season were at increased risk for severe RSV disease, and nearly all had chronic lung disease or congenital heart disease,” said Dr. Paolo Manzoni, Head of Maternal–Infant Medicine, University of Torino Hospital Degli Infermi, Ponderano, Italy, and an investigator for the SMART clinical trial. “These new findings from SMART demonstrate the potential of ENFLONSIA to help protect these vulnerable children, who may require an additional dose for their second RSV season.”
Interim data from RSV season 1 of the Phase 3 SMART trial — alongside data from the pivotal Phase 2b/3 CLEVER trial—supported the FDA approval of ENFLONSIA in June 2025 and subsequent global regulatory approvals. Interim data from RSV season 1 of the SMART trial were also presented at IDWeek 2024 and published in the New England Journal of Medicine. The SMART trial enrolled infants at increased risk of severe RSV disease due to prematurity (<29 weeks through ≤35 weeks gestational age), chronic lung disease of prematurity or hemodynamically significant congenital heart disease entering their first RSV season.
“RSV is among the leading causes of infant hospitalization globally and is especially serious for children under 2 years of age at high risk for severe disease,” said Dr. Macaya Douoguih, vice president, Therapeutic Area Head, Global Clinical Development, Merck Research Laboratories. “These new data from the SMART study further position ENFLONSIA as an important potential new option to help protect young children who remain at high risk entering their second RSV season. We aspire to bring ENFLONSIA to all eligible infants and high-risk children under 2 years of age around the world and look forward to sharing these encouraging data with global regulatory authorities to help achieve this goal.”
The second season results will be shared with the FDA and global regulatory authorities for evaluation for an expanded indication in children at increased risk for severe RSV disease through their second RSV season. ENFLONSIA is currently approved in the United States, Canada and several other countries for use in infants during their first RSV season, and regulatory filings are underway in additional markets globally.
ENFLONSIA is a preventive, long-acting mAb designed to provide direct, rapid and durable protection through 5 months, a typical RSV season, with the same dose regardless of infant weight.
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.
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 as well as for children at increased risk for severe RSV disease through their second 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 Merck
At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than 130 years, we have brought hope to humanity through the development of important medicines and vaccines. We aspire to be the premier research-intensive biopharmaceutical company in the world – and today, we are at the forefront of research to deliver innovative health solutions that advance the prevention and treatment of diseases in people and animals. We foster a diverse and inclusive global workforce and operate responsibly every day to enable a safe, sustainable and healthy future for all people and communities.
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