Summary:
Beyfortus (nirsevimab-alip) is a preventive antibody (monoclonal antibody) that provides reliable respiratory syncytial virus (RSV) protection from day 1.1 It is not a vaccine. One dose of Beyfortus works right away to provide antibodies that help protect babies throughout the RSV season, typically 5 months.2 Some children may need another dose if they’re still at higher risk in their second RSV season, or if they’ve had heart surgery.
Beyfortus, a preventive antibody
Babies and newborns under a year old might not have all the antibodies they need to fight RSV. Beyfortus steps in with an extra layer of preventive monoclonal antibodies that attach to the virus to help stop it from infecting the lungs.
Beyfortus is like a bodyguard for your baby's immune system. Unlike vaccines, which train the body to respond over time, Beyfortus offers ready made protection against serious RSV lung infection—no training required. That means that your baby's immune system doesn't need to do anything itself for protection—the antibodies take care of it all. This is why immunity given by antibodies is sometimes called "passive immunity."3
Is Beyfortus a vaccine or preventive monoclonal antibody?
Beyfortus is a preventive monoclonal antibody, not a vaccine. While all protect against disease, they work differently.
Beyfortus provides an extra layer of antibodies that a baby's developing immune system lacks to help them fight RSV.3
How does Beyfortus work?
Beyfortus is a preventive monoclonal antibody that can help protect babies by physically blocking RSV from entering their lungs.1
Think of RSV as an intruder trying to unlock your baby's lung cells using a special key called the F protein. Beyfortus works like a protective cap placed right over that key.1
Once Beyfortus binds to the virus, the key can no longer turn the lock and therefore cannot enter the cells of your baby’s lungs and make them sick.1
One dose of Beyfortus can help protect your baby for an entire RSV season. A second dose may be needed for babies who are at high risk of serious RSV disease entering their second RSV season.
How long does it take Beyfortus to work?
Beyfortus begins working shortly after it is administered.4 Because it delivers preventive monoclonal antibodies directly into the bloodstream, it can help strengthen your baby’s immune system right away.4
Preventive antibodies peak in a baby within a few days. This is important because RSV is unpredictable and your baby can get seriously sick at any time in the RSV season.5
Find out more information on how RSV spreads in babies.
How long does protection last with Beyfortus?
Beyfortus® is created with "extended half-life" technology, which means the preventive antibodies take longer to leave the body than naturally created ones.1
A dose of Beyfortus® helps provide protection for the full RSV season which is typically 5 months.1,6
Discover resources for parents
Explore cost and coverage
FAQs
Is Beyfortus a live vaccine?
No, Beyfortus is not a live vaccine. Beyfortus is a preventive monoclonal antibody. A live vaccine introduces a weakened form of a virus to teach the body how to defend itself. Beyfortus provides "ready-to-use" antibodies directly to your baby.
Can Beyfortus be taken by people of all ages?
Beyfortus is an immunization used to help prevent a serious lung disease caused by respiratory syncytial virus (RSV) and recommended in:
- Newborns and babies under 1 year of age born during or entering their first RSV season.
- Children up to 24 months of age who remain at risk of severe RSV disease through their second RSV season.1,4
Is Beyfortus an mRNA vaccine?
No, Beyfortus is not an mRNA vaccine. Unlike vaccines, preventive antibodies act like a bodyguard to help protect your baby's immune system, delivering a fast-acting defense against serious RSV lung infection.
How long does it take for Beyfortus to be effective?
Beyfortus begins providing RSV protection shortly after the shot.4 While it begins working right away, it typically takes a few days for the medicine to reach peak levels in the body, providing the most robust defense against serious RSV lung infection.1
How is Beyfortus made?
Beyfortus is a preventive monoclonal antibody that’s active against RSV. Preventive monoclonal antibodies are laboratory-made proteins that fight off infection similarly to how the body’s immune system does. Beyfortus also includes a specific modification that slows down how quickly it is broken down in your baby's body, so RSV protection can be present throughout the entire season.1
How long does Beyfortus last?
Beyfortus can help give your baby long-lasting protection throughout their first RSV season, which is usually around five months.
References
- Barbas Del Buey , JF et al. The effectiveness of nirsevimab in reducing the burden of disease due to respiratory syncytial virus (RSV) infection over time in the Madrid region (Spain): a prospective population-based cohort study. Front. Public Health. 2024; 12, 1441786. doi: 10.3389/fpubh.2024.1441786
- Beyfortus Prescribing Information. https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/761328s000lbl.pdf, 2023.
- CDC. Respiratory Syncytial Virus (RSV) Preventive Antibody IIS. Vaccines & Immunizations https://www.cdc.gov/vaccines/hcp/current-vis/rsv-iis.html, 2026.
- Nirsevimab for prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants. Aust. Prescr. 2024; 47, 102–103.
- Law AW, Lin J, Judy J, Pugh SJ, Cane A. Respiratory syncytial virus (RSV) hospitalization seasonal patterns and economic burden in the US: implications for further optimizing the use of RSV preventives. Vaccines (Basel). 2025;13(4):366. Accessed May 29, 2026. https://doi.org/10.3390/vaccines13040366
- Jones, JM et al. Use of nirsevimab for the prevention of respiratory syncytial virus disease among infants and young children: Recommendations of the Advisory Committee on Immunization Practices - United States, 2023. MMWR Morb. Mortal. Wkly. 2023; Rep. 72, 920–925.
- Hammitt, LL et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. N. Engl. J. Med. 2022; 386, 837–846.
Your child should not take Beyfortus if your child has a history of serious allergic reactions to nirsevimab-alip or any of the ingredients in Beyfortus.
Before your child receives Beyfortus, tell your healthcare provider about all of your child’s medical conditions, including if your child:
- has ever had a reaction to Beyfortus.
- has bleeding or bruising problems. If your child has a problem with bleeding or bruises easily, an injection could cause a problem.
Tell your healthcare provider about all the medicines your child takes, including prescription and over-the-counter medicines, vitamins, and herbal supplements. Your infant should not receive a medicine called palivizumab if they have already received Beyfortus in the same RSV season.
Serious allergic reactions have happened with Beyfortus. 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
- bluish color of skin, lips, or under fingernails
- muscle weakness
- severe rash, hives, or itching
The most common side effects of Beyfortus include rash and pain, swelling, or hardness at the site of your child’s injection. These are not all the possible side effects of Beyfortus. Call your healthcare provider if you have questions about side effects.
Please see full Prescribing Information, including Patient Information, for more details.
Click here to learn more about Sanofi’s commitment to fighting counterfeit drugs.
Beyfortus is a prescription medicine used to help prevent a serious lung disease caused by Respiratory Syncytial Virus (RSV) in:
- Newborns and babies under 1 year of age born during or entering their first RSV season.
- Children up to 24 months of age who remain at risk of severe RSV disease through their second RSV season.