Nirsevimab

Table of contents

• What is nirsevimab?
• How does nirsevimab work?
• Which conditions is nirsevimab used or studied for?
• How is nirsevimab given?
• What do the clinical trials show?
• What side effects are known?

What is nirsevimab?

Nirsevimab is a protein-based substance, specifically a monoclonal antibody. Monoclonal antibodies are laboratory-made proteins that mimic the antibodies the immune system produces naturally. Nirsevimab is already authorised as a medicine in the European Union, where it is sold under the brand name Beyfortus in two strengths: 50 mg and 100 mg solution for injection in a pre-filled syringe.^[1] The marketing authorisation is held by Sanofi Winthrop Industrie.^[1] The substance is classified under ATC code J06BD08, a group of immune sera and immunoglobulins.^[1]

How does nirsevimab work?

Nirsevimab is a monoclonal antibody that prevents severe RSV disease in infants. RSV, or respiratory syncytial virus, usually causes mild, cold-like symptoms in most people, but in young infants it can lead to serious lower respiratory tract infections such as bronchiolitis.^[4] Instead of training the infant's immune system to make its own antibodies, as a vaccine does, nirsevimab provides ready-made antibodies that neutralise the virus directly. This means protection starts quickly after the injection and does not depend on the infant's immature immune system mounting a response. Nirsevimab is recommended for infants and for some young children at increased risk of severe RSV.^[4]

Which conditions is nirsevimab used or studied for?

Nirsevimab is used to prevent disease caused by respiratory syncytial virus (RSV). The conditions it targets include:

• Respiratory syncytial virus infection — the viral infection itself, which nirsevimab is designed to prevent from causing illness.^[1]
• Bronchiolitis — a common and potentially serious lung infection in infants that RSV frequently causes.^[1]
• Respiratory tract infection — a broader term covering infections of the airways, including the lungs.^[1]

How is nirsevimab given?

Nirsevimab is given as a solution for injection by the intramuscular route, meaning it is injected into a muscle.^[1] It comes in a pre-filled syringe in two strengths: 50 mg and 100 mg.^[1] In the clinical trials described on this page, nirsevimab is administered as a single intramuscular injection.^[1][3][2]

What do the clinical trials show?

Three Phase 3 trials are listed in the registry, all authorised and all investigating nirsevimab for the prevention of RSV infection in infants. Phase 3 is the final stage of testing before or alongside wider use, involving large numbers of participants to confirm effectiveness and safety.

Which conditions are studied in the trials?

The trials focus on RSV infection and its consequences in infants. One trial, PIPELINE-RSV-France, lists respiratory syncytial virus infection, bronchiolitis, and respiratory tract infection as the conditions under study.^[1] The PIPELINE-RSV adaptive platform trial studies respiratory syncytial virus infection more broadly.^[3] The IRIS trial includes healthy infants receiving an RSV immunisation to prevent serious disease due to RSV infection.^[2]

What are the trials measuring?

Two of the trials measure symptomatic RSV respiratory tract infection as their primary outcome. In PIPELINE-RSV-France, the main measure is symptomatic RSV respiratory tract infection by 12 months of age, confirmed by a positive RSV test such as RT-PCR or an antigen test on nasal swabs.^[1] The PIPELINE-RSV trial uses the same primary outcome: symptomatic RSV respiratory tract infection in an infant by 12 months of age, confirmed by an approved positive RSV test.^[3] The IRIS trial takes a different approach, measuring RSV-specific antibody levels at 12 months after nirsevimab immunisation, to understand how the immunisation affects the infant's own immune response.^[2]

Who can take part in these trials?

The trials enrol infants in their first year of life. PIPELINE-RSV-France plans to enrol 2,000 participants and compares giving nirsevimab at 4 months of age plus a maternal RSV vaccine during pregnancy with giving nirsevimab alone to infants entering their first RSV season.^[1] The PIPELINE-RSV trial plans to enrol 1,500 participants and compares three approaches: maternal vaccine plus infant nirsevimab at 4 months, infant nirsevimab alone in the first RSV season, and maternal vaccine alone.^[3] The IRIS trial is smaller, with 250 participants, and studies healthy infants receiving nirsevimab immunisation.^[2]

What side effects are known?

The supplementary sources provided do not include a detailed list of side effects for nirsevimab. The CDC guidance focuses on recommendations for use and does not describe specific adverse events.^[4] The Lancet study abstract describes effectiveness outcomes and does not report side effect data in the text provided.^[5]