Enflonsia: Uses, Side Effects and Dosage

What ENFLONSIA® is used for:

ENFLONSIA® is a prescription medicine to help prevent lung disease caused by Respiratory Syncytial Virus (RSV) in newborns and babies who are born during or entering their first RSV season.

RSV season is the time of year when RSV infections are most common, usually occurring fall through spring of the next year. RSV infection can lead to serious lung disease.

RSV is a common respiratory virus that usually causes symptoms similar to the common cold but can also affect the lungs. Symptoms of RSV infection may include a runny nose, trouble feeding, difficulty breathing, coughing, sneezing, wheezing or fever. Anyone can become infected by RSV and almost all children get an RSV infection by the time they are 2 years old. While most recover quickly, RSV can cause severe illness including bronchiolitis (inflammation of the small airways in the lung) and pneumonia (infection of the lungs) that may lead to hospitalization and even death. Children at greatest risk include newborns and babies up to 12 months of age, especially those 6 months and younger, or those with heart or lung problems.

How ENFLONSIA® works:

ENFLONSIA® contains antibodies (proteins the body uses to fight harmful germs) to help prevent RSV disease.

The ingredients in ENFLONSIA® are:

Medicinal ingredient: clesrovimab

Non-medicinal ingredients: L-arginine hydrochloride, L-histidine, L-histidine monohydrochloride monohydrate, polysorbate 80, sucrose and water for injection.

ENFLONSIA® comes in the following dosage form:

Solution for Injection.

One single-use, prefilled syringe of 0.7 mL solution contains 105 mg clesrovimab.

Do not use ENFLONSIA® if:

• Your child has had a serious allergic reaction to any of the ingredients in ENFLONSIA®.

To help avoid side effects and ensure proper use, talk to your child’s healthcare professional before your child is given ENFLONSIA®. Talk about any health conditions or problems your child may have.

Tell your child’s healthcare provider about any medical conditions or allergies your child has or had.

Serious allergic reactions have happened with other medicines like ENFLONSIA®. Tell your child’s healthcare provider and seek medical care right away if your child has any of the following signs or symptoms of a serious allergic reaction, which may include:

• swelling of the face, mouth, or tongue
• bluish color of skin, lips or under fingernails
• difficulty swallowing or breathing
• muscle weakness
• unresponsiveness
• severe rash, hives, or itching

Tell your child's healthcare provider about any issues with blood clotting, such as a low number of platelets, a bleeding problem or bruise easily, or if your child is taking anticoagulant medication (to prevent blood clots).

You may obtain further information from your child’s healthcare provider, who has more detailed information.

Tell your child’s healthcare professional about all the medicines your child takes, including any drugs, vitamins, minerals, natural supplements or alternative medicines.

Your child can get ENFLONSIA® at the same time as routine childhood vaccines.

How is ENFLONSIA® given:

ENFLONSIA® is given as an injection, usually in the thigh muscle, by your child’s healthcare provider.

Your child should get ENFLONSIA® before the start of or during the RSV season.

Your child’s healthcare provider can tell you when the RSV season starts in your area.

If your child has surgery for certain types of heart disease, your child’s healthcare provider may need to give your child an additional ENFLONSIA® injection after surgery.

Usual dose:

ENFLONSIA® is given as one dose of 105 mg.

Overdose:

If you think you, or a person you are caring for, have taken too much ENFLONSIA®, contact a healthcare professional, hospital emergency department, regional poison control centre or Health Canada’s toll-free number, 1-844 POISON-X (1-844-764-7669) immediately, even if there are no signs or symptoms.

Possible side effects from using ENFLONSIA®:

Any medicine may have unintended or undesirable effects, so-called side effects, although not everybody gets them.

These are not all the possible side effects your child may have when taking ENFLONSIA®. If you experience any side effects not listed here, tell your healthcare professional.

The most common side effects of ENFLONSIA® are:

• redness and swelling where your child got the injection
• rash

If your child has a troublesome symptom or side effect that is not listed here or becomes bad enough to interfere with their daily activities, tell your child’s healthcare professional.

Reporting side effects

You can report any suspected side effects associated with the use of health products to Health Canada by:

• Visiting the Web page on Adverse Reaction Reporting (canada.ca/drug-device-reporting) for information on how to report online, by mail or by fax; or
• Calling toll-free at 1-866-234-2345.

NOTE: Contact your health professional if you need information about how to manage your side effects. The Canada Vigilance Program does not provide medical advice.

Storage:

Keep out of reach and sight of children.

Store refrigerated at 2°C to 8°C. Do not freeze. Do not shake.

Keep the prefilled syringe in the original carton to protect from light until time of use.

ENFLONSIA® may be kept at room temperature between 20°C to 25°C for a maximum of 48 hours. After removal from the refrigerator, ENFLONSIA® must be used within 48 hours or discarded.

If you want more information about ENFLONSIA®:

• Talk to your child’s healthcare professional.
• Your child may still get RSV disease after receiving ENFLONSIA®. Talk to your child’s healthcare provider about what symptoms to look for.
• Find the full product monograph that is prepared for healthcare professionals and includes the Patient Medication Information by visiting the Health Canada website (https://www.canada.ca/en/health-canada/services/drugs-health-products/drug-products/drug-product-database.html); the manufacturer’s website www.merck.ca; or by calling 1-800-567-2594.

Date of Authorization: 2026-03-06

® Merck Sharp & Dohme B.V. Used under license.

© 2026 Merck & Co., Inc., Rahway, NJ, USA and its affiliates. All rights reserved.