Merck’s sBLA for ENFLONSIA™ Accepted by FDA for Expanded RSV Prophylaxis in High-Risk Infants Under Two for Second Season

August 17, 2026

Merck.com (News releases) USA

Overview

Merck announced that the U.S. FDA has accepted its supplemental Biologics License Application (sBLA) for ENFLONSIA™ (clesrovimab-cfor) to expand its Respiratory Syncytial Virus (RSV) lower respiratory tract disease indication. The expansion would include children under two years of age at increased risk for severe RSV during their second RSV season. This acceptance represents a critical step in broadening RSV prevention options for vulnerable pediatric populations.

In Depth

Key Findings

On August 6, 2026, Merck announced the U.S. Food and Drug Administration (FDA) has accepted a supplemental Biologics License Application (sBLA) for ENFLONSIA™ (clesrovimab-cfor), its anti-respiratory syncytial virus (RSV) monoclonal antibody. This sBLA seeks to expand the indication for RSV lower respiratory tract disease prophylaxis to include infants under two years of age at increased risk for severe RSV during their second RSV season.

Technical / Clinical Details

ENFLONSIA™ (clesrovimab-cfor) is a long-acting monoclonal antibody designed to bind specifically to RSV, thereby preventing viral entry into cells and subsequent infection. It is anticipated to offer sustained protective effects throughout the RSV season due to its extended half-life compared to earlier RSV prophylactics. The current sBLA aims to extend protection specifically for high-risk infants under two, such as premature infants or those with chronic lung disease or congenital heart disease, through their second RSV season. This expansion recognizes the vulnerability of these patients to multiple severe RSV infections and the need for prolonged protection.

Background & Context

Respiratory Syncytial Virus (RSV) is a leading cause of severe lower respiratory tract infections, particularly in infants and the elderly. Severe RSV can lead to hospitalization, intensive care admission, and, in some cases, fatality, posing a significant public health burden. There exists a substantial unmet medical need for effective prophylactic strategies, especially in infants with underdeveloped immune systems. The proposed expansion of ENFLONSIA™’s indication directly addresses this critical need.

Strategic Significance & Outlook

The FDA’s review of the ENFLONSIA™ sBLA will proceed over the coming months. If approved, this would significantly extend the period of protection for high-risk infants, benefiting a wider population and potentially reducing the burden of RSV-related hospitalizations and severe outcomes, thus making a substantial contribution to public health. Merck is poised to strengthen its leadership in the RSV prophylaxis market and reinforce its commitment to protecting vulnerable patient populations through this expanded indication.

Source: https://www.merck.com/media/news/

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