Mechanism of action for Beyfortus® (nirsevimab-alip) 50mg and 100mg Injection

A monoclonal antibody injection for prevention of RSV disease

Beyfortus was approved to prevent RSV lower respiratory tract disease by the FDA in July of 2023¹˒²:
  • 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

Beyfortus® (nirsevimab) is a recombinant human immunoglobulin G1 kappa (IgG1κ) monoclonal antibody (mAb) designed to prevent respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants.¹

Unlike infection or vaccination, RSV antibody therapy does not stimulate the immune system. Instead, mAbs provide direct protection against disease, representing a form of passive immunization against RSV disease.² Protection is typically at its peak in the weeks following administration of Beyfortus® but diminishes gradually over time.²

How Beyfortus® protects against RSV disease by preventing membrane fusion

Beyfortus® is a protein-directed fusion inhibitor that provides long-acting passive immunity to RSV disease by binding to a conserved epitope on the RSV prefusion F protein, interfering with cellular membrane fusion, thus preventing viral entry into cells.¹

The mechanism of action of RSV prophylaxis and antibody therapy Beyfortus® is explained in the video below.

The role of glycoproteins in RSV

RSV is coated with two types of glycoproteins, the attachment glycoprotein (G protein) and the fusion glycoprotein (F protein).³ Only the F protein is essential for RSV entry into cells lining the respiratory tract.³

The RSV F protein structure exists as a type I viral fusion glycoprotein that forms a trimeric complex and mediates viral entry. ⁴,

Blocking viral attachment and entry into bronchial epithelial cells

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.³

This membrane fusion process results in the formation of syncytia, large multinucleated structures that can induce epithelial lesions in the upper respiratory tract.³,⁶ Formation of syncytia (cell-cell fusion) is what gives RSV its characteristic name.³,

YTE substitution prolongs the half-life of Beyfortus® RSV antibody therapy

Beyfortus® RSV antibody therapy is long-acting due to a specially designed triple amino acid substitution (YTE) in its fragment crystallizable (Fc) region, increasing its affinity to the neonatal Fc receptor.⁸

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,

Specifically, the YTE modification results in a long anti-RSV antibody half-life extension from 21–28 days to 87–117 days.¹

Discover more about Beyfortus®

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Dosing and Administration

beyfortus pacifier

Demonstrated Safety and Efficacy Profile¹


Beyfortus is indicated for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in:

  • Neonates and infants born during or entering their first RSV season.
  • Children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season.

Important safety information


Beyfortus is contraindicated in infants and children with a history of serious hypersensitivity reactions, including anaphylaxis, to nirsevimab-alip or to any of the excipients.

Warnings and Precautions

  • Hypersensitivity Reactions Including Anaphylaxis: Serious hypersensitivity reactions have been reported following Beyfortus administration. These reactions included urticaria, dyspnea, cyanosis, and/or hypotonia. Anaphylaxis has been observed with human immunoglobulin G1 (IgG1) monoclonal antibodies. If signs and symptoms of anaphylaxis or other clinically significant hypersensitivity reactions occur, initiate appropriate treatment.
  • Use in Individuals with Clinically Significant Bleeding Disorders: As with other IM injections, Beyfortus should be given with caution to infants and children with thrombocytopenia, any coagulation disorder or to individuals on anticoagulation therapy.

Most common adverse reactions with Beyfortus were rash (0.9%) and injection site reactions (0.3%).

Please see full Prescribing Information.

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