MSD (Merck & Co) is advancing its respiratory syncytial virus (RSV) vaccine, clesrovimab, towards potential approval and launch, targeting the 2025-2026 RSV season. This follows the presentation of positive results from a Phase IIb/III trial evaluating the vaccine's efficacy in infants.
The Phase IIb/III MK-1654-004 trial (NCT04767373) assessed clesrovimab as a preventative measure against RSV in infants. Data presented at IDWeek 2024 in Los Angeles highlighted that clesrovimab met its primary endpoint by significantly reducing the incidence of RSV-associated medically attended lower respiratory infections (MALRI), requiring at least one indicator of lower respiratory infection, by 60% over the initial five months post-dosing, when compared to placebo.
Clinical Efficacy and Trial Results
The placebo-controlled trial enrolled 3,632 infants under one year old. Key findings from the study include:
- A reduction of 84.2% in RSV-associated hospitalizations over five months compared to placebo, meeting the secondary endpoint.
- A 90.9% reduction in RSV-associated lower respiratory tract disease (LRTD) hospitalizations versus placebo through five months, fulfilling the tertiary endpoint.
Comparison with Existing Therapies
Clesrovimab, a single-dose monoclonal antibody with an extended half-life, is designed to provide passive immunization against RSV. If approved, it will compete with AstraZeneca and Sanofi’s Beyfortus (nirsevimab), a currently approved option for preventing RSV lower respiratory tract disease (LRTD) in newborns and infants in the US. While GlobalData projects Beyfortus to generate $2.5 billion in sales by 2030 due to its first-to-market advantage, clesrovimab is expected to reach $408 million in the same period.
Interim Analysis of Phase III Trial
MSD also presented interim data from the Phase III MK-1654-007 trial (NCT04938830), which compared clesrovimab to Sobi’s Synagis (palivizumab), an existing RSV monoclonal antibody vaccine. This study evaluated both vaccines in infants and children at increased risk for severe RSV disease. The interim analysis, involving 901 participants, indicated that clesrovimab has a safety profile comparable to Synagis. The incidence rates of RSV-associated MALRI and hospitalizations were similar between the two groups: 3.6% and 1.3% in the clesrovimab group versus 3% and 1.5% in the Synagis group, respectively.
Regulatory Pathway and Future Prospects
MSD is actively engaging with health authorities globally to facilitate the availability of clesrovimab for infants as early as the 2025-2026 RSV season. The company's ongoing discussions aim to address the unmet need for effective RSV prevention strategies in vulnerable infant populations.
