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Know the Facts

This hub is intended for the UK general public and has been created and funded by Sanofi.

Respiratory syncytial virus (RSV) is a common virus that can cause infection of the lower part of the respiratory system in infants. These are known as lower respiratory tract infections (LRTIs).1

A lower respiratory tract infection is an infection of the airways and lungs. One or both lungs can be affected.2 Bronchiolitis and pneumonia are types of LRTIs that are often caused by RSV.3

Facts

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Symptoms

A close-up photograph of a sleeping infant wrapped in a soft, white textured blanket.

What are the symptoms of RSV-related illness in infants?

According to NHS resources, symptoms of a lower respiratory tract infection (LRTI) caused by a virus such as RSV, are usually:11

  • A runny or blocked nose
  • A cough
  • Sneezing
  • Tiredness
  • A high temperature – signs include your baby's back or chest feeling hotter than usual, sweatiness and shivering (chills)
  • Babies with RSV may also be irritable and feed less than usual.
  • All infants are at risk of more severe RSV illnesses such as croup, brochiolitis, and pneumonia.3,13 See below for links to NHS resources on these illnesses

While RSV is one cause of these three illnesses, bronchiolitis, croup and pneumonia can also be caused by other viruses.3,12

The NHS recommends that you should contact your GP if you are worried about your child's symptoms, or notice any of the following:12

  • Your child is unwell and it's getting worse
  • Your child is feeding or eating much less than normal
  • Your child has had a dry nappy for 12 hours or more, or shows other signs of dehydration
  • Your baby feels hotter than usual when you touch their back or chest, feels sweaty or is shivering
  • Your baby is under 3 months old and has a temperature of 38C, or is older than 3 months and has a temperature of 39C or higher
  • You or a child aged 5 years or older has a temperature below 36C
  • Your child is very tired or irritable

Although not common, sometimes symptoms can become severe quickly and more urgent help is needed. The NHS recommends that you call 999 if you notice that:11

  • Your child is having difficulty breathing – you may notice grunting noises, long pauses in their breathing or their tummy sucking under their ribs
  • You have severe difficulty breathing – you're gasping, choking or not able to get words out
  • You or your child is floppy and will not wake up or stay awake
  • You or your child's lips or skin are turning very pale, blue or grey – on brown or black skin, this may be easier to see on the palms of the hands
  • Your child is under 5 years old and has a temperature below 36C

If you're concerned about your baby, you should always seek immediate medical advice.

RSV Illnesses

What is Bronchiolitis?

Bronchiolitis is one of the illnesses that can be caused by RSV. It's a common lower respiratory tract infection (LRTI) that affects infants and young children under two years of age. Most cases are mild and clear up within 2 to 3 weeks without the need for treatment.

Early symptoms of bronchiolitis are similar to those seen with a common cold, for example runny nose and a cough. More symptoms may develop and can include a fever, dry persistent cough, wheezing and difficulty feeding.

Bronchiolitis can sometimes worsen, meaning that infants may struggle to breathe and need to be admitted to hospital.11 You can find out more about bronchiolitis here: https://www.nhs.uk/conditions/bronchiolitis/

What is Croup?

Croup is another illness that can be caused by RSV. It's a common childhood condition that mainly affects the airways of infants and young children.

Symptoms of croup are characterised by a barking cough which sounds similar to a seal, a hoarse voice, breathing difficulty, and a rasping sound when drawing breath in.

Croup often begins with cold-like symptoms which can worsen at night.13 You can find out more about croup here: https://www.nhs.uk/conditions/croup/

What is Pneumonia?

RSV can also cause pneumonia. It's an infection characterised by swelling (inflammation) of the tissue in one or both lungs.

Symptoms in infants include a high temperature (fever) coughing, shortness of breath, noisy breathing or wheezing and making grunting sounds. You can find out more about pneumonia here: https://www.asthmaandlung.org.uk/conditions/pneumonia-in-children

References

  1. Verwey C. & Nunes M. RSV lower respiratory tract infection and lung health in the first 2 years of life. Lancet Glob Health. 2020. 8(10): e1247-1248. DOI: 10.1016/S2214-109X(20)30303-X
  2. Asthma + Lung UK. How to spot respiratory tract infections in children. Available at: https://www.asthmaandlung.org.uk/conditions/signs-breathing-problems-children/how-spot-respiratory-tract-infections-children. Last Accessed: May 2026
  3. Piedimonte G. & Perez M.K. Respiratory syncytial virus infection and bronchiolitis. Pediatr Rev. 2014. 35(12): pp519-530. DOI: 10.1542/pir.35-12-519
  4. Mayo Clinic. Respiratory syncytial virus (RSV) Overview. Available at: https://www.mayoclinic.org/diseases-conditions/respiratory-syncytial-virus/symptoms-causes/syc-20353098 Last Accessed: May 2026
  5. House of Lords Library. Protecting infants from respiratory syncytial virus. Available at: https://lordslibrary.parliament.uk/protecting-infants-from-respiratory-syncytial-virus/. Last Accessed: May 2026
  6. NHS England. NHS to roll out long-lasting ‘suit of armour’ jab to protect thousands of premature babies from RSV. Available at: https://www.england.nhs.uk/2025/07/nhs-to-roll-out-long-lasting-suit-of-armour-jab-to-protect-thousands-of-premature-babies-from-rsv/. Last Accessed: May 2026
  7. Murray. J., Bottle. A, Sharland, M. et al. Risk Factors for Hospital Admission with RSV Bronchiolitis in England: A Population-Based Birth Cohort Study. PLoS One. 2014; 9(2)
  8. Cleveland Clinic. Respiratory Syncytial Virus in Children and Adults. Available at: https://my.clevelandclinic.org/health/diseases/rsv-respiratory-syncytial-virus. Last Accessed: May 2026
  9. KidsHealth. Respiratory Syncytial Virus. Available at: https://kidshealth.org/en/parents/rsv.html. Last Accessed: May 2026
  10. UKHSA. Respiratory Syncytial Virus (RSV): symptoms, transmission, prevention, treatment. (n.d.). Available at: https://www.gov.uk/government/publications/respiratory-syncytial-virus-rsv-symptoms-transmission-prevention-treatment/respiratory-syncytial-virus-rsv-symptoms-transmission-prevention-treatment. Last Accessed: May 2026
  11. NHS. Respiratory syncytial virus (RSV). Available at: https://www.nhs.uk/conditions/respiratory-syncytial-virus-rsv/. Last Accessed: May 2026
  12. NHS. Bronchilitis. Available at: https://www.nhs.uk/conditions/bronchiolitis/. Last Accessed: May 2026
  13. NHS. Croup. Available at: https://www.nhs.uk/conditions/croup/. Last Accessed: May 2026
  14. NHS. Pneumonia. Available at: https://www.nhs.uk/conditions/pneumonia/. Last Accessed: May 2026
  15. Asthma and Lung UK. Pneumonia in children and babies. Available at: https://www.asthmaandlung.org.uk/conditions/pneumonia-in-children/. Last Accessed: May 2026