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What is RSV?

Welcome to

Together Against RSV

Parenthood is full of big decisions, and the safety 
and health of your little one play a big role in making them.
 
That’s why it’s important that you know about respiratory syncytial virus (RSV).
 
RSV is a respiratory virus, which means it affects the airways and lungs.1
 
RSV infections occur throughout the year in Hong Kong and are highly transmissible.2 Studies show that RSV is up to four times more contagious than influenza.3,4

While most cases of RSV disease are mild, all babies are at risk of lung infections such as bronchiolitis or pneumonia
if they catch RSV.5,6

Rodzice trzymający dziecko

Understanding how
RSV can affect a baby

Here, you’ll discover:

  • Why all babies are at risk of RSV infection
  • How RSV affects babies’ lungs
  • Severe respiratory illness caused by RSV
  • The severity and consequences of RSV in Infants
  • How to prevent RSV in babies

So, whether it’s your first time hearing about RSV or you 
just want to learn more about the virus, find out more here.

RSV in babies

For most babies, an RSV infection will cause mild symptoms such as a runny nose, coughing, sneezing, and congestion.*,5,8 But for some babies, it can get worse and lead to severe respiratory or lung infections.

RSV Severity in Infants

  • Around 2 in 3 babies will catch RSV before their first birthday.7
  • Among every 10 babies infected with RSV, 4 may develop severe lung infections such as bronchiolitis or pneumonia.8 These infections can cause significant breathing difficulties5,9-10 and often require hospitalization.30


RSV is a Leading Cause of Hospitalization for Infants Under 1 Year Old.11-13

  • Younger infants face a higher risk of needing emergency care or hospitalization.14

RSV Hospitalization Burden in Hong Kong

  • RSV places a substantial burden on Hong Kong’s healthcare system:
  • Infants under 6 months: about 55 per 1,000 were hospitalized in 202315
  • Infants under 1 year: about 34 per 1,000 hospitalized during 2023–202415
  • Children under 5 years: about 9-13 per 1,000 hospitalized between 2016-202416 (excluding 2020-2022#)

Among RSV-Related Pediatric Hospitalizations in Hong Kong17-18:

  • About 1.5-2.4% of cases required Pediatric Intensive Care Unit (PICU) admission
  • The average PICU stay was approximately 3 days


RSV vs. Influenza: Risks for Infants Under 1 Year

  • Mortality rate: 10 times higher than influenza19
  • Hospitalization rate: 16 times higher than influenza20

* A baby may not experience all of the symptoms described here. The symptoms shown can also occur with other infections and this is not a complete list, others can occur. Whenever you are concerned about a child’s health, always seek medical advice.
#excluding COVID-19 period from 2020 to 2022.

Who can get RSV?

People of all ages can get RSV.21
However, certain groups including babies, the elderly, and people with health conditions are more at risk of getting a severe illness if they get infected.21
 
While RSV often causes a mild illness, in some cases it can become more severe.5,9
Even a baby born healthy and at full term is still at risk of being hospitalized in their first year, 
if they catch RSV.6,11,22-23

RSV is most common in infants under 6 months old, with those aged 2 to 4 months at the highest risk.5,23,25

Why are all babies at risk of RSV?

During their first year of life, babies have very small, fragile lungs, and their immune 
systems continue to mature and develop after their first year.1,26-29 This can make them more vulnerable to serious illness due to an RSV infection, compared to older children and adults.1,26-28

RSV is a Leading Cause of Hospitalization in Infants Under 1 Year Old 6,7,29-31:

  • 1 in 50 infants will be hospitalized within the first year of life.30
  • Over 70% of hospitalized RSV cases are in healthy, full-term infants.30
  • Infants under 6 months are at the highest risk of developing severe illness.6,29-31
  • 2 out of every 5 infants with RSV may develop lung infections such as bronchiolitis or pneumonia.7
  • Inflammation and blockage in the small airways can lead to wheezing and breathing difficulties.

Younger infants have the higher the risk of admission to the Pediatric Intensive Care Unit (PICU), especially for those under 6 months.22 In North America, the proportion of hospitalized infants admitted to ICU due to RSV22:

  • 39% for newborns aged 0–2 months
  • 38% for infants aged 3–5 months
  • 18% for infants aged 6–11 months

Why should you care about RSV?

Even though most babies will recover from RSV disease at home, you should be aware of RSV 
because it can progress from a mild illness to a severe one within a matter of days.33

In fact, almost 1 in 10 cases in children under 2 years progress to lung infections, such as bronchiolitis or pneumonia.34

RSV infects up to 70% of babies under 1 year old7 and 90% of children under 2 years old.29

It’s also difficult to predict which babies will get seriously ill and require hospital care after becoming infected.12,23

The Together Against RSV Movement

Together Against RSV is a disease awareness initiative 
from Sanofi, to educate and inform parents, parents-to-be, and caregivers about infant illnesses caused by RSV.

Download the RSV fact sheet

Learn more today. It might make a world of difference. 

Download (PDF)

Understanding RSV

Click here to learn more about how RSV poses a risk for babies.

Visit Understanding RSV

Footnotes & references

RSV, respiratory syncytial virus.
1. Pickles RJ and De Vincenzo JP. J Pathol 2015; 235(2): 266–276.
2. Centre for Health Protection (DoH HK). Respiratory Syncytial Virus Infection 2025. Available at
https://www.chp.gov.hk/en/healthtopics/content/24/36.html (Accessed: July 2025).
3. Reis J and Shaman J. Infect Dis Model 2018; 3: 23–34.
4. Liu L et al. Front Public Health 2021; 9(587425): 1–10.
5. Piedimonte G and Perez MK. Pediatr Rev 2014; 35(12): 519–530.
6. Hall CB et al. Pediatrics 2013; 132(2): e341–e348.
7. Esposito S, et al. Front Immunol. 2022;13:880368.
8. Piedi-monte G. Curr Opin Pediatr 2013; 25(3): 344–349.
9. Meissner HC. N Engl J Med 2016; 374(1): 62–72.
10. Centers for Disease Control and Prevention (CDC). RSV in infants and young children. Available at
https://www.cdc.gov/rsv/infants-young-children/?CDC_AAref_Val=https://www.cdc.gov/rsv/high-risk/infants-young-children.html (Accessed: July 2025).
11. Leader S and Kohlhase K. Pediatr Infect Dis J 2002; 21(7): 629–632.
12. Demont C et al. BMC Infect Dis 2021; 21(1): 730.
13. Suh M et al. J Infect Dis 2022; 226(Supp 2): S154–Sl63.
14. Rainisch G et al. Vaccine 2020; 38(2):251-57.
15. Centre for Heath Protection. Communicable Diseases Watch. 2025; 21(8): 58-62.
16. Centre for Health Protection. RSV disease and Interim Consensus on the use of RSV vaccines in Hong Kong. Communicable Diseases Watch. 2025; 21(1):1-5.
17. Hon KL et al. Journal of Critical Care 2012 ; 27 :464-468.
18. Leung TF et al. Infection 2014.
19. Thompson WW, et al. JAMA. 2003; 289(2):179-186.
20. Zhou H, et al. Clin Infect Dis. 2012; 54(10):1427-1436.
21. Du Yet al. Int J Infect Dis 2023; 135: 70–76.
22. Arriola C et al. J Pediatric Infect Dis Soc 2020; 9(5): 587–595 (Suppl Appendix).
23. Bianchini S et al. Microorganisms 2020; 8(12): 2048.
24. Bont L, et al. Infect Dis Ther. 2016;5(3):271-298.
25. Hussain M et al. Biochim Biophys Acta Mol Basis Dis 2017; 1863(12): 3226–3242.
26. Lambert L et al. Front lmmunol 2014; 5: 466.
27. Di Cicco M et al. Pediatr Pulmonol 2021; 56(1): 240–251.
28. Simon AK et al. Proc Royal Soc 2015; 282: 20143085.
29. Glezen, WP, et al. Am J Dis Child 1986;140(6):543-546.
30. McLaughlin, et al. JID, 2020;jiaa752.
31. Doachowske J, et al. Pediatr Ann 2018;47(9):e371–e376.
32. Simoes EAF. Lancet 1999; 354: 847–852.
33. Smyth RL and Breary SP. Encyclopedia of Respiratory Medicine. Bronchiolitis, Elsevier Ltd. 2006.
34. Langley J et al. J Infect Dis 2022; 226: 374–385.