Summary:
While most cases are mild, RSV can get worse in just a few days, and when hospitalization happens, it typically lasts 3-4 days. Some babies, like those born premature, are at an even higher risk of serious RSV lung infection, but all babies are at some risk. RSV protection like Beyfortus is an effective way to help protect from medically attended serious RSV lung infection, including hospitalization.
What is RSV?
Respiratory syncytial virus (RSV) is a common and highly contagious virus that can lead to respiratory infection in babies.1 RSV is unpredictable, and though most of the time it will cause a mild, cold-like illness, babies can develop serious RSV lung infection.1 Thankfully, it's possible to help prevent serious RSV infection in babies.
What severe RSV means for your baby
For some babies, RSV gets worse very quickly, becoming serious a few days after infection.2 It can lead to hospitalization, with the average stay being 3 to 4 days, often involving breathing support and oxygen therapy to help babies recover.3
RSV lung infection can become serious when it spreads to the lungs, often leading to bronchiolitis (inflammation of the airway passages entering the lungs) or pneumonia (infection of the lungs leading to buildup of fluid).1,2 In the United States, RSV lung infection is a leading cause of hospitalization in babies under one year old.4
How to tell if your child has severe RSV
The signs and symptoms of serious RSV in babies can be different than in adults.2
Signs and symptoms of serious RSV in babies include1,2:
- Short, shallow, abnormally fast breathing
- A wheezing cough
- Nasal flaring
- Poor feeding
- Unusual tiredness
- A blue tint to their lips or fingernails
- Fever
If your baby is experiencing difficulty breathing, high fever, or blue skin, seek medical attention immediately.
When RSV can become an emergency
RSV symptoms can worsen quickly and lead to serious illness. Get medical help right away if your baby shows signs of dehydration, such as persistent vomiting or not having a wet diaper for more than 12 hours. Call 911 or go to the emergency room if your baby is struggling to breathe, making grunting noises, or showing signs of severe distress. Trust your instincts—if something feels wrong, get help.
Who is at risk for severe RSV?
RSV is unpredictable. Babies, particularly premature babies and babies younger than 6 months old, are most at risk of serious RSV lung infection.2 However, all babies can get serious RSV lung infections.6
When is RSV season?
The typical RSV season starts in the fall and lasts through spring, but exact timing can vary depending on where you live. In most areas, Beyfortus is available starting on October 1, right before the season gets underway.
Children at greatest risk of serious illness from RSV2:
- Infants and young children (the younger the age, the higher the risk)
- American Indian and Alaska Native children
- Children born prematurely
- Children with chronic lung disease or congenital (present from birth) heart disease
- Children with weakened immune systems
- Children with severe cystic fibrosis
- Children who have neuromuscular disorders, especially those who have difficulty swallowing or clearing mucus secretions (e.g., phlegm, snot, saliva)
Protecting babies from the risk of severe RSV
Beyfortus is a preventive antibody designed to help protect babies from serious RSV lung infection. It's not a vaccine—it works by providing the antibodies that a baby's developing immune system lacks to help them fight RSV.7
RSV protection is highly effective in preventing serious lung infection and is a key part of protecting your baby. This is even more important for babies who are at increased risk.1,3
Start the conversation around RSV protection with your doctor and ask about Beyfortus.
Frequently asked questions
Who is most at risk for RSV complications?
Babies, particularly premature babies and babies younger than 6 months old, are most at risk of serious RSV lung infection.1 Children at greatest risk of serious illness from RSV include2:
- Infants and young children (the younger the age, the higher the risk)
- American Indian and Alaska Native children
- Children born prematurely
- Children with chronic lung disease or congenital (present from birth) heart disease
- Children with weakened immune systems
- Children with severe cystic fibrosis
- Children who have neuromuscular disorders, especially those who have difficulty swallowing or clearing mucus secretions
When is RSV season?
The typical RSV season usually occurs from fall through spring, but can vary by local area.
How do you test for RSV in babies?
Aside from a physical examination from a doctor, two main types of tests are used to test for RSV in babies9:
- PCR testing—a highly accurate nasal swab test that detects the virus's genetic material
- Antigen testing—a nasal swab test with faster results that work well in young children but is less reliable in adults
My baby has RSV—is it too late to give them Beyfortus?
Beyfortus is designed to help prevent serious RSV lung infection before it occurs and is not indicated to treat an active RSV infection once a baby is already sick.7
References
- Mayo Clinic Staff. Respiratory syncytial virus (RSV): symptoms & causes. Mayo Clinic. Published October 4, 2023. Accessed May 7, 2026. https://www.mayoclinic.org/diseases-conditions/respiratory-syncytial-virus/symptoms-causes/syc-20353098
- Centers for Disease Control and Prevention. RSV in Infants and Young Children. CDC; 2026. Accessed May 26, 2026. https://www.cdc.gov/rsv/infants-young-children/index.html
- Gantenberg JR, Thompson KD, van Aalst R, et al. Inpatient service utilization amongst infants diagnosed with Respiratory Syncytial Virus infection (RSV) in the United States. PLOS ONE. 2025; 20(1):e0317367. doi:10.1371/journal.pone.0317367
- Suh M, Movva N, Jiang X, et al. Respiratory syncytial virus is the leading cause of United States infant hospitalizations, 2009-2019: a study of the National (Nationwide) Inpatient Sample. J Infect Dis. 2022; 226(suppl 2):S154-S163. doi:10.1093/infdis/jiac120
- Hall CB, Weinberg GA, Iwane MK, et al. The burden of respiratory syncytial virus infection in young children. N Engl J Med. 2009; 360(6):588-598. doi:10.1056/NEJMoa0804877
- 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
- Beyfortus (nirsevimab-alip) [prescribing information]. Swiftwater, PA: Sanofi Pasteur, Inc.; 2023. Accessed May 26, 2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/761328s000lbl.pdf
- Centers for Disease Control and Prevention. Surveillance of RSV. CDC; 2026. Accessed May 26, 2026. https://www.cdc.gov/rsv/php/surveillance/index.html
- Chamarthi S, Chamarthi VS, Daley SF. Respiratory syncytial virus infection in children. In: StatPearls [Internet]. StatPearls Publishing; 2026. Updated December 14, 2025. Accessed May 29, 2026. https://www.ncbi.nlm.nih.gov/books/NBK459215/
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.
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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.