The US Food and Drug Administration (FDA) has approved Merck’s clesrovimab (MK-1654), a long-acting monoclonal antibody designed to provide passive immunization against respiratory syncytial virus (RSV) for infants during their first RSV season. This single-dose, weight-independent treatment demonstrated strong efficacy in pivotal Phase IIb/III trials, reducing medically attended RSV lower respiratory infections by 60.4% and RSV-related hospitalizations by 84.2% compared to placebo. Merck plans to make clesrovimab available for the 2025-26 RSV season, expanding preventative options beyond currently approved therapies.1
RSV remains a leading cause of hospitalization and death in infants worldwide, highlighting the critical need for effective prevention. Clesrovimab’s approval positions Merck alongside Sanofi and AstraZeneca’s Beyfortus, the first FDA-approved long-acting RSV antibody, which has shown 90% effectiveness against RSV hospitalizations.
FDA Accepts Merck’s Biologics License Application for Infant RSV Protection
On December 17, 2024, the FDA accepted Merck’s Biologics License Application (BLA) for clesrovimab (MK-1654), a long-acting monoclonal antibody designed to protect infants from RSV during their first RSV season.2
The BLA was supported by data from the Phase 2b/3 CLEVER trial, a randomized placebo-controlled study evaluating a single dose of clesrovimab in healthy preterm and full-term infants (birth to 1 year). The trial demonstrated a 60.4% reduction in medically attended RSV lower respiratory infections (MALRI) and an 84.2% reduction in RSV-related hospitalizations through Day 150, regardless of infant weight. Interim data from the ongoing Phase 3 SMART trial, comparing clesrovimab to palivizumab in infants at increased risk for severe RSV disease, further support the safety and efficacy of clesrovimab.2
Merck’s Long-Acting RSV Antibody Shows High Efficacy in Infant Prevention Studies
In an October 2024 interview at IDWeek, Anushua Sinha, MD, infectious disease physician and product development lead at Merck, presented results from the Phase 2b/3 CLEVER trial, which enrolled 3,632 healthy preterm and full-term infants. The double-blind, placebo-controlled study showed clesrovimab reduced medically attended lower respiratory infections by 60.4%, RSV-related hospitalizations by 84.2%, and hospitalizations due to RSV-associated lower respiratory infections by 90.9% over 150 days. Additionally, severe cases of MALRI declined by 91.7%. Safety data indicated a comparable profile to placebo, with no treatment-related deaths.3,4,5
Interim findings from the ongoing Phase 3 SMART trial, which compares clesrovimab with the current standard of care, palivizumab, in high-risk infants, including those with chronic lung or congenital heart disease, showed comparable safety and efficacy, supporting clesrovimab’s use across both healthy and high-risk infant populations.
