MSD’s Clesrovimab Shows Promise for Extended RSV Protection in High-Risk Infants
Rome, Italy – March 6, 2026 – MSD (known as Merck & Co., Inc. In the United States and Canada) has announced positive findings from the Phase 3 SMART trial (MK-1654-007) evaluating the safety and efficacy of clesrovimab, marketed as ENFLONSIA™, in infants and children under two years of age at increased risk for severe respiratory syncytial virus (RSV) disease over two RSV seasons. The data were presented at the 9th Respiratory Syncytial Virus Foundation (ReSViNET) conference, RSVVW’26, in Rome, Italy.
The trial focused on children who remained at increased risk for severe RSV disease through their second RSV season and received clesrovimab at the start of that season. Results indicated a safety profile consistent with that observed in infants who received the monoclonal antibody during their first RSV season in the MK-1654-007 trial.
serum monoclonal antibody (mAb) concentrations achieved in these children were similar to those observed in healthy infants participating in the pivotal Phase 2b/3 CLEVER trial (MK-1654-004).
MSD stated that these findings support the potential to extend the efficacy of clesrovimab to children under two years of age who remain at increased risk for severe RSV disease through their second RSV season. According to Dr. Paolo Manzoni, a researcher involved in the clinical trial, all children who received the monoclonal antibody in their second RSV season had a higher risk of severe RSV disease, with the majority having chronic lung disease or congenital heart disease.
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