Mechanism of Action Behind Infant RSV Immunisation
Passive immunisation against RSV relies on directly delivering a monoclonal antibody that neutralises the virus before it can establish infection in the respiratory tract. Unlike vaccines, which stimulate active antibody production, this approach provides immediate circulating antibody levels regardless of the infant's immune maturity. The antibody is engineered for an extended half-life, supporting protection across a single RSV season from one dose. This page explains the scientific basis for this mechanism and its clinical relevance.
Beyfortus™ is designed for protection against respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD) throughout an infants first RSV season, via a single administration of antibodies.1
Beyfortus® is a direct, long-acting antibody1
Direct administration of antibodies via intramuscular injection doesn’t rely on an infant’s developing immune system1,2
Beyfortus™ is a long-acting antibody – it has been modified to have an extended half-life, so that a single dose affords at least 5 months of protection**,1
The antibody targets the prefusion conformation of the RSV F protein to inhibit membrane fusion, an essential step in viral entry – this neutralises the virus and blocks cell-to-cell fusion1
Watch the video to find out more about how Beyfortus™ works.
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Take look at the clinical data
Beyfortus® significantly reduces the risk of medically-attended RSV-LRTI in a broad infant population vs. placebo1,3,4
*Beyfortus™ is a human monoclonal antibody. 1
** Based on clinical and pharmacokinetic data.1
LRTD, lower respiratory tract disease; LRTI, lower respiratory tract infection; RSV, respiratory syncytial virus.
- BEYFORTUS™ Professional Information. September 2025
- Heinonen S, Rodriguez-Fernandez R, Diaz A, Oliva Rodriguez- Pastor S, Ramilo O, Mejias A. Infant Immune Response to Respiratory Viral Infections. Immunol Allergy Clin North Am. 2019;39(3):361-376.
- Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants. N Engl J Med. 2022;386(9):837-846.
- Griffin MP, Yuan Y, Takas T, et al. Single-Dose Nirsevimab for Prevention of RSV in Preterm Infants [published correction appears in N Engl J Med. 2020 Aug 13;383(7):698.
S4 BEYFORTUS 50 mg™; BEYFORTUS 100 mg™ (solution for injection).QUALITATIVE AND QUANTITATIVE COMPOSITION: One mL solution for injection contains 100 mg nirsevimab. Each BEYFORTUS 50 mg pre-filled syringe contains 50 mg of nirsevimab in 0,5 mL (100 mg/mL).Each BEYFORTUS 100 mg pre-filled syringe contains 100 mg of nirsevimab in 1 mL (100 mg/mL). REGISTRATION NUMBER: BEYFORTUS 50 mg: A59/30.1/0957; BEYFORTUS 100 mg: A59/30.1/0958.HOLDER OF CERTIFICATE OF REGISTRATION: sanofi-aventis south africa (pty) ltd., Reg. no.: 1996/010381/07, Floor 5, Building I, Hertford Office Park, 90 Bekker Road, Midrand, 2196. Tel: (011) 256 3700.
For Medical Information Enquiries kindly contact ZA.Medinfo@sanofi.com.
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The monoclonal antibody binds to a specific site on the RSV fusion protein, blocking the virus's ability to fuse with and enter respiratory epithelial cells. By neutralising the virus at this stage, the antibody prevents the establishment of infection rather than treating disease after onset. This mechanism provides protection independent of the infant's own immune response. The approach targets a conserved viral structure, supporting activity against circulating RSV strains.
An extended antibody half-life allows a single dose to maintain protective antibody levels across an infant's entire RSV season, reducing the need for repeat dosing. This is particularly important for infant immunisation programmes, where dosing simplicity supports higher uptake and completion. Extended half-life engineering distinguishes this approach from earlier, shorter-acting RSV antibody products. Protection duration should still be considered relative to the timing of RSV seasonal circulation.
Yes, this approach delivers pre-formed antibody directly, providing immediate passive protection, whereas vaccines stimulate the recipient's own immune system to generate antibodies over time. This distinction makes passive immunisation particularly suited to infants, who have limited capacity to mount a robust vaccine response in early life. Active RSV vaccines are being developed and used in other populations, such as older adults and pregnant women. Each approach has a distinct role depending on the target population's immune status.
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