Key Takeaways
- Advisory Committee on Immunization Practices (ACIP) recommendation expands respiratory syncytial virus (RSV) prevention options: Merck’s Enflonsia is now recommended for infants under eight months of age to help prevent RSV lower respiratory tract disease.
- Strong clinical data supports approval: Enflonsia showed a 60.5% reduction in MALRI and an 84.3% reduction in RSV-related hospitalizations in the CLEVER trial.
- Availability ahead of 2025–2026 season: Merck plans to launch Enflonsia in July 2025, with ACIP inclusion in the Vaccines for Children Program supporting broader access.
The Centers for Disease Control and Prevention’s (CDC) Advisory Committee on Immunization Practices (ACIP) has voted to recommend Merck’s Enflonsia (clesrovimab-cfor) as a preventive option against respiratory syncytial virus (RSV) lower respiratory tract disease in infants under eight months of age who are born during or entering their first RSV season. ACIP also voted to include Enflonsia in the Vaccines for Children Program, potentially expanding access for vulnerable populations.1
How Could Enflonsia Help Protect Infants Against RSV?
“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,” said Richard M. Haupt, VP, head, global medical & scientific affairs, vaccines and infectious diseases, Merck Research Laboratories, in a press release. “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.”
FDA Approval Based on CLEVER and SMART Trial Data
The recommendation comes weeks after the FDA approved Enflonsia in this patient population. Approval was based on results from the Phase IIb/III CLEVER trial, which demonstrated a notable reduction in RSV-related hospitalizations as well as medically attended lower respiratory infections (MALRI).
- The randomized, double-blind, placebo-controlled CLEVER trial evaluated the safety and efficacy of Enflonsia in 3,632 early and moderate preterm infants.
- Patients were randomly assigned to receive either a single 105 mg dose of Enflonsia or saline placebo.
- The primary endpoint of the trial was incidence of patients with RSV-associated MALRI characterized as cough or difficulty breathing and requiring ≥1 indicator of lower respiratory infection or severity from day one to day 150, while a notable secondary endpoint was RSV-associated hospitalization through five months.
- Full results showed that Enflonsia lead to a 60.5% reduction in MALRI and an 84.3% reduction in RSV-related hospitalizations.
- Common adverse events included injection-site erythema, injection-site swelling, and rash.
- Additionally, approval was supported by the Phase III SMART trial, which found that Enflonsia demonstrated a safety profile comparable to palivizumab.2
Global and US Impact of RSV
According to the World Health Organization, RSV results in more than 3.6 million hospitalizations as well as an estimated 100,000 deaths in children under five years of age annually. Approximately 97% of pediatric RSV-related deaths occur in low- and middle-income countries, in which access to proper medical care is limited. In people aged 65 years and older in the United States, it is estimated that the hospitalization rate is around 160,000 per year, with approximately 10,000 deaths. The hospitalization rate is higher in adults with underlying conditions such as asthma, chronic obstructive pulmonary disease, or congestive heart failure.3
Commercial Rollout and Final ACIP Endorsement
Merck expects Enflonsia to be available for ordering by July 2025 ahead of the 2025–2026 RSV season. The ACIP recommendation will become official upon endorsement by the director of the CDC or HHS.1
Investigator Perspective on Clinical Impact
“RSV disease is the leading cause of infant hospitalization in the US and can lead to serious respiratory conditions like bronchiolitis and pneumonia,” said Octavio Ramilo, chair, department of infectious diseases, St. Jude Children’s Research Hospital, investigator for the CLEVER (MK-1654-004) and SMART (MK-1654-007) trials, in a press release. “Enflonsia combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV.”2
References
- ACIP Recommends Use of Merck’s ENFLONSIA™ (clesrovimab-cfor) for Prevention of Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease in Infants Younger than 8 Months of Age Born During or Entering Their First RSV Season. Merck. June 26, 2025. Accessed June 27, 2025. https://www.merck.com/news/acip-recommends-use-of-mercks-enflonsia-clesrovimab-cfor-for-prevention-of-respiratory-syncytial-virus-rsv-lower-respiratory-tract-disease-in-infants-younger-than-8-months-of-age/
- FDA Approves Merck’s Enflonsia to Prevent Respiratory Syncytial Virus. PharmExec. June 10, 2025. Accessed June 27, 2025. https://www.pharmexec.com/view/fda-approves-merck-enflonsia-prevent-respiratory-syncytial-virus
- Respiratory syncytial virus (RSV). WHO. Accessed June 27, 2025. https://www.who.int/news-room/fact-sheets/detail/respiratory-syncytial-virus-(rsv)
