Merck Seeks FDA Approval to Expand Enflonsia Use for High-Risk Children During Their Second RSV Season

FDA Accepts Supplemental Application for Enflonsia

Merck, known as MSD outside the United States and Canada, has announced that the US Food and Drug Administration (FDA) has accepted a supplemental Biologics License Application (sBLA) for Enflonsia (clesrovimab-cfor).

The company is seeking approval to expand the use of Enflonsia to help prevent respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD) in children under two years of age who remain at high risk for severe RSV disease during their second RSV season.

The FDA has assigned a Prescription Drug User Fee Act (PDUFA) target action date of March 22, 2027.

European Review Is Also Underway

Merck is also moving forward with regulatory review in Europe.

In July 2026, the European Medicines Agency (EMA) accepted the company's application to expand the marketing authorization of Enflonsia for the same group of high-risk children.

If approved, the expanded indication would allow more vulnerable children to receive protection during their second RSV season.

Enflonsia Is Already Approved for Infants

Enflonsia received FDA approval in June 2025 and European Commission approval in April 2026.

It is currently approved for preventing RSV lower respiratory tract disease in infants who are born during or entering their first RSV season.

According to Merck, Enflonsia is currently approved in more than 40 countries, including:

• United States
• Canada
• European Union
• China
• Japan

The company is also pursuing approvals in additional countries while working on expanding its approved use where available.

Why Some Children Need Protection Beyond Their First RSV Season

Although many infants receive protection during their first RSV season, some children continue to face a higher risk of severe illness when they enter their second season.

These include children with conditions such as:

• Chronic lung disease
• Congenital heart disease
• Early or moderate premature birth with certain additional risk factors

According to Merck, these children remain vulnerable even after receiving protection during infancy.

Company Says Expanded Use Could Reduce RSV Burden

Dr. Macaya Douoguih, Vice President and Therapeutic Area Head of Global Clinical Development at Merck Research Laboratories, said RSV can continue to pose a serious threat to certain children after their first season.

She explained that expanding the use of Enflonsia could provide additional protection for vulnerable children while helping reduce the burden of RSV on families and healthcare systems.

Phase 3 SMART Trial Supports the Filing

The FDA and EMA submissions are supported by findings from the Phase 3 SMART clinical trial.

The study evaluated the safety, effectiveness, and pharmacokinetics of Enflonsia over two RSV seasons in infants and young children who were at increased risk for severe RSV disease.

The results were presented at the RSVVW'26 conference in February 2026.

The study findings have also been published in:

• The New England Journal of Medicine for interim first-season results.
• JAMA Pediatrics for the complete study results.

How the SMART Study Was Conducted

SMART was a Phase 3, randomized, partially blinded, multicenter clinical trial.

The study included infants and children with:

• Early premature birth
• Moderate premature birth
• Chronic lung disease of prematurity
• Congenital heart disease

During the first RSV season, participants received either:

• A single 105 mg dose of Enflonsia.
• Monthly injections of palivizumab.

Children who remained eligible entered the second RSV season and received a single open-label 210 mg dose of Enflonsia.

Nearly all children who received Enflonsia during the second season had either chronic lung disease or congenital heart disease.

Understanding RSV

Respiratory syncytial virus, commonly called RSV, is a contagious virus that causes seasonal respiratory infections.

The virus can lead to serious illnesses such as:

• Bronchiolitis
• Pneumonia

RSV is one of the leading causes of hospitalization among infants worldwide.

Although healthy infants can become infected, children with certain medical conditions are at much greater risk of severe disease.

RSV season usually occurs from autumn through spring, although the exact timing and severity can vary depending on the region.

About Enflonsia

Enflonsia is an extended half-life monoclonal antibody developed to provide passive protection against RSV lower respiratory tract disease.

Unlike some preventive treatments, Enflonsia is given using the same dose regardless of an infant's weight.

The approved dose is a 105 mg prefilled syringe administered by intramuscular injection.

The treatment is designed to provide protection for approximately five months, covering a typical RSV season.

For infants born during RSV season, Enflonsia is recommended within the first week of life.

For infants born outside RSV season, it is recommended shortly before the RSV season begins.

Infants who undergo heart surgery involving cardiopulmonary bypass during their first RSV season may receive an additional 105 mg dose after they recover from surgery.

According to Merck, Enflonsia has a shelf life of 30 months.