Merck’s ENFLONSIA Receives ACIP Endorsement for RSV Prevention in Infants, Marking a Major Milestone in Pediatric Infectious Disease Prevention
U.S. CDC panel backs long-acting monoclonal antibody to protect infants during their first RSV season; drug also included in federal Vaccines for Children program
Merck, operating as MSD outside the United States and Canada, has announced a key regulatory milestone for its respiratory syncytial virus (RSV) prevention program. The U.S. Centers for Disease Control and Prevention’s (CDC) Advisory Committee on Immunization Practices (ACIP) has voted to recommend ENFLONSIA™ (clesrovimab-cfor) as a preventive option for infants younger than 8 months who are either born during or about to enter their first RSV season. In a parallel vote, the ACIP also approved ENFLONSIA’s inclusion in the Vaccines for Children (VFC) Program, an initiative aimed at ensuring equitable access to essential immunizations across the U.S. pediatric population.
These dual decisions mark a significant advancement in efforts to combat RSV-related lower respiratory tract disease (LRTD) in infants—a leading cause of hospitalization in this vulnerable age group.
Understanding the Burden of RSV in Infants
RSV is a common respiratory virus that typically circulates during the fall and winter months in the U.S., often peaking between October and March. While RSV infections usually result in cold-like symptoms in older children and adults, infants and young children—especially those under 6 months of age—are at high risk of developing severe lower respiratory tract complications, such as bronchiolitis and pneumonia.
According to CDC estimates, RSV leads to 58,000–80,000 hospitalizations and 100–300 deaths annually among children under five in the United States. The disease burden is disproportionately high among infants under 1 year of age, and even more so among premature infants or those with underlying heart or lung conditions. There are currently no approved antiviral treatments specifically for RSV in this age group, placing greater emphasis on preventive strategies such as immunization and passive protection through monoclonal antibodies.
What Is ENFLONSIA™?
ENFLONSIA is a long-acting monoclonal antibody (mAb) developed by Merck to provide direct and durable protection against RSV. Unlike traditional vaccines that stimulate the immune system to produce antibodies over time, ENFLONSIA is a form of passive immunization—it delivers RSV-targeted antibodies directly into the bloodstream, offering immediate protection.
The drug has been specifically designed to cover the typical 5-month RSV season with just one dose, regardless of the infant’s weight. This simplicity in dosing may help streamline clinical workflows and facilitate broader use in outpatient, hospital, and community health settings.
Dr. Richard M. Haupt, Vice President and Head of Global Medical & Scientific Affairs for Vaccines and Infectious Diseases at Merck Research Laboratories, emphasized the significance of this new option:
“Ahead of the 2025–2026 RSV season, we are proud to offer ENFLONSIA as a new preventive option designed to protect healthy and at-risk infants from RSV disease across a spectrum of severity, including worsening disease requiring hospitalization. The Committee’s recommendation is an important step forward in efforts to help reduce the significant burden RSV continues to place on infants, families, and health care systems.”
Clinical Evidence Supporting ENFLONSIA
The U.S. Food and Drug Administration (FDA) granted approval to ENFLONSIA in early June 2025 based on compelling results from two pivotal clinical trials: the Phase 2b/3 CLEVER trial and the Phase 3 SMART trial.
These studies demonstrated that ENFLONSIA significantly reduces the incidence of medically attended RSV lower respiratory tract infections in infants. Both trials enrolled diverse populations across multiple geographies and health backgrounds, ensuring the data’s relevance to real-world conditions. In these trials, ENFLONSIA not only met its primary efficacy endpoints but also showed a favorable safety profile.
Safety Profile and Contraindications
As with any biologic therapy, safety remains a key consideration. ENFLONSIA is contraindicated in infants with a known history of serious hypersensitivity reactions, including anaphylaxis, to any of its components. Merck has emphasized that the product label includes guidance on identifying and managing potential hypersensitivity reactions, although such incidents have been rare in clinical trials.
Further Selected Safety Information and full prescribing information will be made available to health care providers as the product is launched.
A Public Health Win: Inclusion in the Vaccines for Children Program
The ACIP’s vote to include ENFLONSIA in the Vaccines for Children (VFC) program is particularly noteworthy. The VFC program, which provides vaccines at no cost to children who might not otherwise be vaccinated due to inability to pay, covers over 50% of all children in the U.S. under the age of 19.
By including ENFLONSIA in this federally funded program, Merck and public health officials aim to reduce barriers to access and expand protection across socioeconomically vulnerable populations. This move could have meaningful implications for health equity and reduce RSV-related hospitalizations in communities with limited access to pediatric care.
Next Steps: Finalization and Product Availability
While the ACIP’s recommendation is currently provisional, it is expected to become official following a final review and approval by either the CDC Director or, in the current absence of a permanent director, the Secretary of the U.S. Department of Health and Human Services.
In parallel, Merck is moving quickly to operationalize the product’s launch. ENFLONSIA is expected to become available for ordering by physicians and health care systems in July 2025, ensuring sufficient time for distribution ahead of the upcoming RSV season.
The Broader Landscape of RSV Prevention
ENFLONSIA’s arrival joins a growing armamentarium of tools to prevent RSV, a space that has seen substantial innovation in recent years. The recent approval of maternal vaccines and other monoclonal antibodies, such as Sanofi and AstraZeneca’s nirsevimab (Beyfortus™), has reshaped the landscape for infant RSV protection.
ENFLONSIA differentiates itself with its weight-independent dosing, rapid onset of action, and broad efficacy demonstrated across diverse trial populations. Its inclusion in the VFC program adds another layer of competitive advantage and social impact.
The ACIP’s endorsement of ENFLONSIA marks a significant public health achievement in the ongoing fight against RSV in infants. By expanding options for early-life protection and ensuring equitable access through the Vaccines for Children program, Merck is helping pave the way toward reduced RSV hospitalizations, fewer emergency room visits, and more peace of mind for parents and pediatricians alike.
As the U.S. prepares for the 2025–2026 RSV season, the deployment of ENFLONSIA represents a proactive step toward mitigating a virus that has long been a source of anxiety for families and strain on pediatric health care systems. With its innovative formulation and robust clinical backing, ENFLONSIA has the potential to change the trajectory of RSV disease burden for the most vulnerable among us—our infants.
