Pronunciation: Bay-for-Tus
Generic name: nirsevimab-alip
Dosage form: single-dose prefilled syringe for intramuscular injection (50 mg/0.5 mL, 100 mg/mL)
Drug class: Antiviral monoclonal antibodies

Medically reviewed by Carmen Pope, BPharm. Last updated on Jun 18, 2025.

What is Beyfortus?

Beyfortus is used to prevent respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants under 1 year born during or entering their first RSV season. It may also be given to children up to 24 months of age who remain at risk of severe RSV disease through their second RSV season. Beyfortus 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). 

Beyfortus contains a long-acting monoclonal antibody, called nirsevimab-alip, that targets the RSV F protein, stopping the virus from entering cells and multiplying, and causing infections. 

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

Beyfortus gained FDA approval on July 17, 2023, following positive results of three clinical trials (MELODY phase IIb trial [Trial 03], NCT02878330; MELODY phase III trial [Trial 04], NCT03979313; and MEDLEY phase II/III [Trial 05], NCT03959488).

• In healthy late preterm and term infants, Beyfortus reduced medically attended RSV lower respiratory tract infections by 74.5%, hospitalizations by 76.8%, and very severe infections by 78.6%.
• Beyfortus demonstrated a favorable safety and tolerability profile in high-risk infants. 

Side effects

The most common side effects of Beyfortus are:

• a rash
• pain, swelling, or a hard area at the site of your child's injection. 

Serious side effects and warnings

Beyfortus may cause the following serious side effects:

• Serious allergic reactions. Beyfortus may cause serious allergic (hypersensitivity) reactions, including anaphylaxis. Your child should not receive Beyfortus if they have had serious allergic reactions in the past to nirsevimab-alip, Beyfortus, or any inactive ingredients in the Beyfortus injection. 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.

It is not known if Beyfortus is safe and effective in children older than 24 months of age.

These are not all of the possible side effects of Beyfortus. Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

Related/similar drugs

Before taking

Your child should not receive the Beyfortus vaccine if they have had serious allergic reactions in the past to nirsevimab-alip, Beyfortus, or any inactive ingredients in the Beyfortus injection.

Before your child receives Beyfortus, tell your healthcare provider about all of your child's medical conditions, including if your child:

• has ever reacted Beyfortus or nirsevimab-alip
• has bleeding or bruising problems. If your child has a problem with bleeding or bruises easily, an injection could cause a problem.

How is Beyfortus administered?

Beyfortus is given by a healthcare provider as an injection into the upper thigh.

• Usually, only 1 dose is needed.
• A second dose may be given if your child remains at increased risk for RSV the following year.

Your child should be given Beyfortus before or during the RSV season. 

• RSV season is usually late fall (November) through spring (April), with a peak in January and February.
• Your child may still get RSV disease after receiving Beyfortus. Talk to your child’s healthcare provider about what symptoms to look for.

If your child has heart surgery, your child’s healthcare provider may need to give your child an additional Beyfortus injection soon after surgery.

Beyfortus can be administered at the same time as childhood vaccines, at a different injection site. Do not mix Beyfortus with any vaccines or medications in the same syringe or vial.

Dosing information

Dose of Beyfortus for infants aged under 1 year born during or entering their first RSV season:

• Weight less than 5 kg (11 lb): 50 mg as a single injection.
• Weight more than or equal to 5 kg (11 lb): 100 mg as a single injection.

Dose of Beyfortus for children who remain vulnerable through their second RSV season:

• Any weight: 200 mg administered as two IM injections (2 x 100 mg injections).

Is Beyfortus a vaccine?

Technically, Beyfortus is not a vaccine, because it provides infants with the antibodies needed to protect them against RSV, unlike vaccines that teach your immune system to make its own antibodies.

• This is called passive immunity.
• Beyfortus protects your baby immediately from getting sick with RSV.
• The protection lasts around 5 months.

Beyfortus cost

Beyfortus is included in the approved immunization schedule and may be available at no cost for parents through private health insurance companies and the Vaccines for Children (VFC) program. See here for more details.

What other drugs will affect Beyfortus?

Other drugs may interact with Beyfortus, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell your doctor about all the current medicines your baby takes. Especially tell your healthcare provider if your baby:

• has received palivizumab (another monoclonal antibody for RSV) recently
• receives anticoagulants (such as heparin or warfarin).

This list is not complete. See the Beyfortus prescribing information for a full list of interactions.

Ingredients

Active ingredients: nirsevimab-alip.
Inactive ingredients: L-histidine, L-histidine hydrochloride, L-arginine hydrochloride, sucrose, polysorbate 80, water for injections.

Available as:

• Beyfortus 50 mg/0.5 mL in a single-dose pre-filled syringe
• Beyfortus 100 mg/1 mL in a single-dose pre-filled syringe.

Company

Beyfortus is manufactured by Astra Zeneca AB, Södertälje, Sweden, and distributed by Sanofi Pasteur, Inc., Swiftwater, PA 18370 USA.

Beyfortus 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 Beyfortus.

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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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