Merck (NYSE: MRK), known as MSD outside of the United States and Canada, announced the presentation of positive results from the Phase 2b/3 clinical trial (MK-1654-004) evaluating clesrovimab, the company’s investigational prophylactic monoclonal antibody designed to protect infants from respiratory syncytial virus (RSV) disease during their first RSV season. The results, along with interim findings from the ongoing Phase 3 trial (MK-1654-007) of clesrovimab, were presented during IDWeek 2024, held October 16-19 in Los Angeles, California.

Results from MK-1654-004, a placebo-controlled Phase 2b/3 pivotal trial evaluating a single dose of clesrovimab administered to healthy preterm and full-term infants (birth to 1 year of age) met all prespecified endpoints, with consistent results through both the 5-month and 6-month time points. The incidence of adverse events (AEs) and serious AEs were comparable between the clesrovimab and placebo groups, and there were no treatment or RSV-related deaths during the study.

“RSV continues to be a widespread seasonal infection that can affect both healthy and at-risk infants and is the leading cause of hospitalization for infants,” said Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude’s Children’s Research Hospital and investigator for the MK-1654-004 and MK-1654-007 trials. “The MK-1654-004 study evaluated a broad spectrum of RSV disease ranging from mild outpatient illness to severe disease requiring hospitalization. These promising results demonstrating decreased incidence of RSV disease, including hospitalizations, highlight the potential for clesrovimab to play an important role in helping to alleviate the continued burden of RSV on infants and their families.”

The primary efficacy endpoint of the trial, the reduction in incidence of RSV-associated medically attended lower respiratory infections (MALRI) requiring ≥ 1 indicator of lower respiratory infection (LRI) or severity compared to placebo through Day 150 (5 months) postdose, was 60.4% (95% CI: 44.1, 71.9, p<0.001). Clesrovimab also reduced RSV-associated hospitalizations (secondary endpoint) and RSV-associated LRI hospitalizations (tertiary endpoint) through Day 150 (5 months) compared to placebo by 84.2% (95% CI: 66.6, 92.6, p<0.001) and 90.9% (95% CI: 76.2, 96.5), respectively. Clesrovimab reduced the incidence of severe MALRI (tertiary endpoint) by 91.7% (95% CI: 62.9, 98.1).