Enflonsia

Generic name: clesrovimab-cfor
Dosage form: intramuscular injection (105 mg)
Drug class: Antiviral monoclonal antibodies

Medically reviewed by Melisa Puckey, BPharm. Last updated on Jun 10, 2025.

What is Enflonsia?

Enflonsia is an FDA-approved medication used to prevent respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants. Enflonsia helps to protect healthy and at-risk infants from RSV complications and hospitalizations by providing rapid, long-lasting passive immunity against the respiratory syncytial virus (RSV).

Enflonsia is a long-acting monoclonal antibody (mAb) that works by preventing the virus from entering cells, multiplying, and causing infections. 

Enflonsia is used in newborns and infants in their first RSV season from autumn to spring. Enflonsia is administered as a single intramuscular injection (into the muscle) for protection through a typical RSV season of 5 months.

Enflonsia FDA approval was granted on June 9, 2025, after positive results from the CLEVER clinical trial, MK-1654-004 (NCT04767373), which demonstrated Enflonsia patients had:

• 84.3% reduction in hospitalizations due to RSV
• 60.5% reduction in medically attended lower respiratory infections (MALRI).

What is Enflonsia used for?

Enflonsia FDA approval is the prevention of respiratory syncytial virus lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.  Enflonsia active ingredient is clesrovimab-cfor.

What is RSV?

RSV (respiratory syncytial virus) is a common virus that affects the lungs and respiratory passages. While most people experience mild cold-like symptoms, RSV can cause severe illness in infants and older adults, who are more likely to develop severe RSV and need medical attention and hospitalization.

How does Enflonsia work? 

Enflonsia provides long-lasting passive immunity by preventing the virus from entering cells and causing infections.

Enflonsia mechanism of action is as a monoclonal antibody with anti-RSV activity; it targets the RSV F protein, stopping the virus from entering cells and multiplying.  This helps to reduce the risk of severe lower respiratory tract infections and complications caused by RSV.

Enflonsia side effects

The most common Enflonsia side effects were injection-site redness (3.7%), injection-site swelling (2.7%), and rash (2.3%). These Enflonsia side effects occurred in clinical trial 004. The swelling and redness occurred 5 days after treatment, and the rash side effect may have occurred up to 14 days after treatment.

Other human immunoglobulin G1 (IgG1) monoclonal antibodies have caused serious hypersensitivity reactions, including anaphylaxis. Get medical help right away if your child has any of the following signs or symptoms of a serious allergic reaction:

• swelling of the face, mouth, or tongue
• difficulty swallowing or breathing
• unresponsiveness or loss of muscle tone (goes floppy)
• bluish color of skin, lips, or under the fingernails
• muscle weakness
• severe rash, hives (urticaria), or itching.

Related/similar drugs

Warnings

Serious hypersensitivity reactions, including anaphylaxis, have occurred with other human immunoglobulin G1 (IgG1) monoclonal antibodies. Initiate appropriate medications and/or supportive care if such reactions occur.

Contraindications

This medicine is contraindicated in infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of Enflonsia.

Enflonsia dose and administration

Enflonsia recommended dose

Enflonsia is given as a single intramuscular injection (105 mg) for neonates and infants born during or entering their first RSV season.

Babies born during the RSV season: Enflonsia is given once starting from birth. 

Babies born outside the RSV season: Enflonsia is given once before the start of their first RSV season, considering the duration of protection provided

Infants Undergoing Cardiac Surgery with Cardiopulmonary Bypass 

For infants undergoing cardiac surgery with cardiopulmonary bypass during or entering their first RSV season, an additional 105 mg dose administered as an IM injection is recommended as soon as the infant is stable after surgery to ensure adequate clesrovimab-cfor serum levels.

What other drugs will affect this medicine

Other drugs may interact with this medicine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

Clesrovimab-cfor may interfere with immunologically-based RSV diagnostic assays (rapid antigen tests). Confirmation of RSV infection is recommended using a reverse transcriptase polymerase chain reaction (RT-PCR) assay when rapid antigen assay results are negative and clinical observations are consistent with RSV infection.

Storage

• Store prefilled syringes under refrigeration at 36°F to 46°F (2°C to 8°C).
• Keep the prefilled syringe in the original carton to protect from light until the time of use.
• After removal from the refrigerator, this medicine must be used within 48 hours or discarded
• Enflonsia may be kept at room temperature between 68°F to 77°F (20°C to 25°C) for a maximum of 48 hours.
• Do not freeze. Do not shake.

Ingredients

Active ingredient: clesrovimab-cfor (105 mg)

Inactive ingredients: arginine hydrochloride, histidine, L-histidine monohydrochloride monohydrate, polysorbate 80, sucrose, water for injection (USP). 

Manufacturer

Enflonsia manufacturer Merck Sharp & Dohme LLC Rahway, NJ 07065, USA

Enflonsia Biosimilars

Biosimilar and interchangeable products are biological products that are highly similar to and have no clinically meaningful differences from the reference product.

Reference products

These are biological products that have already been approved by the FDA, against which biosimilar products are compared. There is 1 for Enflonsia.

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Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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