ENFLONSIA RSV data from the Phase 3 SMART trial show positive results for infants and children under 2 years at increased risk for severe respiratory syncytial virus (RSV) disease. Merck presented the findings at RSVVW’26, hosted by the Respiratory Syncytial Virus Foundation in Rome, Italy.
ENFLONSIA™ (clesrovimab) is a long-acting monoclonal antibody. It delivers rapid and durable protection across a typical five-month RSV season. Moreover, it uses a fixed dose regardless of infant weight.
Phase 3 SMART Trial Second Season Results
The Phase 3 SMART trial evaluated safety, efficacy, and pharmacokinetics over two RSV seasons. In the second season, researchers focused on children who remained at high risk for severe RSV disease.
Children who received ENFLONSIA at the start of RSV season 2 showed a safety profile consistent with season 1. In addition, serum monoclonal antibody concentrations were similar to those seen in healthy infants from the Phase 2b/3 CLEVER trial.
Therefore, the results support efficacy extrapolation to children under 2 who remain at risk during their second RSV season.
Focus on Vulnerable Pediatric Patients
All children in the second-season cohort faced elevated risk. Nearly all had chronic lung disease or congenital heart disease.
Because RSV is a leading cause of infant hospitalization worldwide, prevention remains critical. High-risk children often experience more severe outcomes. As a result, additional protection strategies are essential.
Dr. Paolo Manzoni, a SMART trial investigator, stated that the findings highlight ENFLONSIA’s potential to protect vulnerable children who may need coverage in a second season.
Regulatory Progress and Next Steps
Previously, interim data from RSV season 1 supported approval by the U.S. Food and Drug Administration in June 2025. Canada and several other countries also approved ENFLONSIA for infants entering their first RSV season.
Now, Merck plans to submit the second-season data to the FDA and other global regulators. If approved, the expanded indication would cover high-risk children through their second RSV season.
Strengthening RSV Prevention
Overall, the new ENFLONSIA RSV data strengthen Merck’s RSV prevention strategy. The therapy offers season-long protection with one fixed dose. Furthermore, it simplifies administration for healthcare providers.
If regulators grant expanded approval, ENFLONSIA could become a key preventive option for high-risk children under 2 years worldwide.
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