Beyfortus is a long-acting monoclonal antibody designed to help protect infants from serious respiratory syncytial virus (RSV) lower respiratory tract disease by providing direct, rapid protection that extends through 5 months, the length of a typical RSV season.1,2
Beyfortus and RSV disease
A monoclonal antibody (mAb) injection for the prevention of RSV disease1
Beyfortus was approved to prevent RSV lower respiratory tract disease by the US Food and Drug Administration in July 2023 in1,3:
- Neonates and infants born during or entering their first RSV season
- Children up to 24 months who remain vulnerable to severe RSV disease through their second RSV season
Unlike an infection or vaccination, RSV antibody therapy does not stimulate the immune system. Instead, mAbs provide direct protection against the disease, representing a form of passive immunization against RSV disease. Protection is typically at its peak in the weeks following administration of Beyfortus and diminishes gradually over time.4
Half-life
Beyfortus is the mAb with the longest half-life approved for the prevention of RSV disease in infants1,5,6
With an extended half-life of 71 days, Beyfortus is present in the body for longer than any other mAb approved for the prevention of RSV disease in infants.1,5,6
Half-life of mAbs approved for the prevention of RSV disease in infants
This chart does not represent a comparison of safety or efficacy between Beyfortus, Enflonsia, and Synagis, as no head-to-head studies have been conducted. The clinical significance of this information has not been established. Please refer to individual product Prescribing Information for additional and the most up-to-date information. Synagis is not available in the US.
Enflonsia is an RSV F protein-directed fusion inhibitor indicated for the prevention of RSV lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.5
Synagis is an RSV F protein inhibitor monoclonal antibody indicated for the prevention of serious lower respiratory tract disease caused by RSV in pediatric patients6:
- with a history of premature birth (less than or equal to 35 weeks gestational age) and who are 6 months of age or younger at the beginning of RSV season,
- with bronchopulmonary dysplasia (BPD) that required medical treatment within the previous 6 months and who are 24 months of age or younger at the beginning of RSV season,
- with hemodynamically significant congenital heart disease (CHD) and who are 24 months of age or younger at the beginning of RSV season.
How Beyfortus protects against RSV-LRTI
Beyfortus is a protein-directed fusion inhibitor that protects against RSV disease by preventing membrane fusion.1
How Beyfortus Works1
Binds to the RSV prefusion F protein
Interferes with membrane fusion
Prevents viral entry into respiratory cells
The mechanism of action of Beyfortus is explained in the video below:
Role of glycoproteins in RSV
RSV is coated with two types of glycoproteins: the attachment glycoprotein (G protein) and the fusion glycoprotein (F protein). Only the F protein is essential for RSV entry into the cells lining the respiratory tract.7
The RSV F protein exists as a type I viral fusion glycoprotein that forms a trimeric complex and mediates viral entry.8,9
Role of RSV F protein
Upon infection, the RSV F protein—typically responsible for facilitating pH-independent viral membrane fusion with the host cell plasma membrane—triggers the merging of adjacent cells when expressed on the cell surface.7
This membrane fusion process results in the formation of syncytia, which are large, multinucleated structures that can induce epithelial lesions in the upper respiratory tract. The formation of syncytia (cell-cell fusion) is what gives RSV its characteristic name.7,10
Prolonged half-life of Beyfortus
Beyfortus RSV antibody therapy is long-acting due to a specially designed triple amino acid substitution—M252Y/S254T/T256E (YTE)—in its fragment crystallizable (Fc) region, which increases its affinity to the neonatal Fc receptor.11
This YTE substitution in the Fc segment of Beyfortus prolongs its serum half-life and is designed to provide season-long protection that extends through 5 months after injection with a single intramuscular dose.1,11
Specifically, the YTE modification results in a long anti-RSV antibody half-life extension from 21–28 days to 71 days.1,12
Frequently asked questions
LRTD, lower respiratory tract disease; LRTI, lower respiratory tract infection.
Important Safety Information
References: 1. Beyfortus (nirsevimab-alip). Prescribing Information. Sanofi. 2. Obando-Pacheco P, Justicia-Grande AJ, Rivero-Calle I, et al. Respiratory syncytial virus seasonality: a global overview. J Infect Dis. 2018;217(9):1356-1364. 3. U.S. CDC Advisory Committee unanimously recommends routine use of Beyfortus™ (nirsevimab-alip) to protect infants against RSV disease. Press Release. Sanofi. August 3, 2023. Accessed August 10, 2026. https://www.sanofi.com/assets/dotcom/pressreleases/2023/2023-08-03-19-21-36-2718475-en.pdf 4. RSV immunization guidance for infants and young children. Centers for Disease Control and Prevention. Updated August 18, 2025. Accessed August 10, 2026. https://www.cdc.gov/rsv/hcp/vaccine-clinical-guidance/infants-young-children.html 5. Enflonsia (clesrovimab-cfor). Prescribing Information. Merck & Co Inc. 6. Synagis (palivizumab). Prescribing Information. Sobi. 7. Battles MB, Langedijk JP, Furmanova-Hollenstein P, et al. Molecular mechanism of respiratory syncytial virus fusion inhibitors. Nat Chem Biol. 2016;12(2):87-93. 8. Hu M, Bogoyevitch MA, Jans DA. Impact of respiratory syncytial virus infection on host functions: implications for antiviral strategies. Physiol Rev. 2020;100(4):1527-1594. 9. Gilman MSA, Furmanova-Hollenstein P, Pascual G, et al. Transient opening of trimeric prefusion RSV F proteins. Nat Commun. 2019;10(1):2105. 10. Fraire AE, Woda BA. Respiratory syncytial virus. In: Fraire AE, Woda BA, Welsh R, Kradin R, eds. Viruses and the Lung. Springer Berlin Heidelberg; 2014:95-99. 11. Brady T, Cayatte C, Roe TL, et al. Fc-mediated functions of nirsevimab complement direct respiratory syncytial virus neutralization but are not required for optimal prophylactic protection. Front Immunol. 2023;14:1283120. 12. Domachowske J, Khan AA, Esser MT, et al. Safety, tolerability and pharmacokinetics of MEDI8897, an extended half-life single-dose respiratory syncytial virus prefusion F-targeting monoclonal antibody administered as a single dose to healthy preterm infants. Pediatr Infect Dis J. 2018;37(9):886-892.
