A long-acting antibody to help protect infants from RSV bronchiolitis
Nirsevimab
A long-acting antibody given to infants to help prevent serious lung infection (bronchiolitis) caused by respiratory syncytial virus (RSV).
What is Nirsevimab?
Nirsevimab is a long-acting antibody given as a single injection to help protect babies and young infants from serious lung infection caused by respiratory syncytial virus (RSV), the most common cause of bronchiolitis in young children. Rather than training the immune system like a classic vaccine, it provides ready-made protective antibodies straight away, which last through an RSV season; this is called passive protection. It is given to prevent illness, not to treat an infection that has already started. The most common side effects are mild reactions where the injection is given, such as redness or a rash, and serious allergic reactions are rare.
Education and reference only. This is a plain-language guide to Nirsevimab — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Nirsevimab is a long-acting monoclonal antibody used to help protect infants against respiratory syncytial virus (RSV). RSV is a very common virus that usually causes mild cold-like symptoms but can cause bronchiolitis, a serious chest infection, in babies and young children, sometimes needing hospital care. Nirsevimab is given as a single injection, usually before or during the RSV season. It is not a vaccine in the classic sense: instead of prompting the baby's own immune system to make antibodies, it gives ready-made antibodies directly, providing protection that lasts for several months. It is given under UK health service or specialist arrangements to babies who would benefit.
How it works
Nirsevimab is a ready-made antibody that sticks to RSV and stops it from infecting the cells lining the airways, so the virus is far less able to cause serious lung infection. Because the protective antibodies are given directly rather than being made by the baby's own immune system, protection starts almost straight away; this is known as passive protection, in contrast to a vaccine, which teaches the body to make its own antibodies over time. The antibody is long-acting, so a single injection can help protect a baby through an RSV season. Like all such protection it is given to prevent infection, not to treat one that is already underway.
Company & origin
Originated / developed by: Specialist manufacturer.
A medicine used in the UK to help protect babies and young infants against serious lung infection from respiratory syncytial virus (RSV).
Practical use
How to take Nirsevimab
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given by a healthcare professional as a single injection, usually before or during the RSV season.
- Have it at the timing advised by the healthcare team, so protection covers the RSV season.
- Understand it helps prevent RSV illness and does not treat a baby who is already ill.
- Tell the team about any previous serious allergic reaction before it is given.
- Seek urgent care if the baby has breathing difficulty or signs of a serious chest infection, even after the injection.
Weighing it up
Advantages & disadvantages of Nirsevimab
Advantages
- Helps protect babies against serious RSV lung infection (bronchiolitis) with a single injection.
- Gives ready-made protection that starts quickly and lasts through an RSV season.
- Useful for protecting infants who are too young to make their own strong immune response.
Disadvantages
- Provides passive protection only and is not a vaccine that trains the baby's own immune system.
- Can cause mild reactions where the injection is given, such as redness or a rash.
- Prevents RSV illness but does not treat a baby who is already infected.
Practical use
Good to know
An important point is that nirsevimab gives passive protection: it supplies ready-made antibodies directly, so protection begins quickly and lasts through the RSV season, but it is not a vaccine in the classic sense and does not train the baby's own immune system. It is given to prevent RSV illness, not to treat a baby who is already ill, so a baby with breathing difficulty or signs of a serious chest infection still needs urgent medical care regardless of having had it. The most common side effects are mild reactions where the injection is given, such as redness, swelling or a rash, which usually settle quickly. Serious allergic reactions are rare, and it is given where any reaction can be managed. The healthcare team will advise on the best timing in relation to the RSV season.
Who should not take it / use with caution
- Babies who have had a serious allergic reaction to nirsevimab or one of its ingredients should not have it.
- Timing or use may be adjusted in certain situations, on the advice of the healthcare team.
- It should be given by, or under the guidance of, a healthcare professional who can manage any reaction.
Monitoring
- A short period of observation after the injection in case of a rare immediate allergic reaction.
- No routine blood tests are needed for healthy babies having it.
- Watching for any signs of RSV illness, which still need medical attention if they appear.
Side effects
- Redness, swelling or a rash where the injection is given.
- A rash elsewhere or a mild raised temperature in some babies.
- Rarely, a serious allergic reaction, which is why it is given where it can be managed.
Key interactions
- There are few well-established medicine interactions, but tell the team about everything the baby is given.
- The team will advise on how it fits alongside the baby's routine vaccinations.
- Let the team know about any other treatments the baby is receiving.
Available as: A solution for injection, usually into the muscle of the thigh.
Answers
Nirsevimab: frequently asked questions
What is nirsevimab for?
It is given to babies and young infants to help prevent serious lung infection (bronchiolitis) caused by respiratory syncytial virus (RSV).
Is nirsevimab a vaccine?
Not in the classic sense. It gives ready-made antibodies directly, called passive protection, rather than training the baby's own immune system as a vaccine does.
Does it treat RSV?
No. It is given to prevent RSV illness; it does not treat a baby who is already ill, so a baby with breathing problems still needs urgent care.
How long does protection last?
It is long-acting, so a single injection can help protect a baby through an RSV season; the team advises on the best timing.
What are the common side effects?
The most common are mild reactions where the injection is given, such as redness, swelling or a rash; serious allergic reactions are rare.
Authoritative sources
- BNF
- NICE CKS
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