A virus that almost all children are infected with before the age of two can cause severe respiratory illness in infants and young children, even requiring hospitalisation. This virus is the Respiratory Syncytial Virus (RSV). However, did you know that its peak season in Hong Kong differs from Western countries? This article will explain the impact of RSV, its unique local epidemic patterns, and prevention methods, to help parents protect their children's health.
What is Respiratory Syncytial Virus (RSV)?
Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections in children worldwide, and a major cause of hospitalisation among infants and young children.The virus is transmitted via droplets; respiratory secretions from an infected person are released when they cough or sneeze and enter another person’s eyes, nose or mouth. The incubation period for RSV is 2 to 8 days, and approximately 90% of children will be infected at least once before the age of 2.
The virus primarily affects the epithelial cells of the respiratory tract, and in severe cases, can cause bronchiolitis and pneumonia. Due to the immature immune systems of infants and young children and their narrower airways, they are more susceptible to lower respiratory tract infections caused by the virus. In high-risk infants, including premature babies, infants under six months old, and children with congenital heart disease or chronic lung disease, infection can develop into severe illness requiring hospitalisation.
Seasonal Trends and Hong Kong's Regional Characteristics
In temperate regions of the Northern Hemisphere, RSV typically circulates from November to March, with peak activity between December and January. However, as a subtropical region, Hong Kong's RSV seasonal pattern differs from that of temperate areas. According to surveillance data from the Hong Kong Centre for Health Protection, RSV is active year-round in Hong Kong, but infection numbers usually peak between March and April and again between July and August. Unlike Western countries, Hong Kong does not exhibit a clear single winter peak.
Research shows that RSV activity lasts from approximately late February to late September, a variation linked to Hong Kong's warm and humid climate. Therefore, parents should remain vigilant all year round, paying particular attention to respiratory symptoms in infants and young children during the spring and summer months.
Typical symptoms of RSV infection in infants
RSV infection symptoms tend to appear in stages rather than all at once. The severity of symptoms varies greatly, ranging from mild symptoms to life-threatening breathing difficulties. Initial symptoms are similar to a cold, with patients experiencing a low-grade fever, cough, runny nose, vomiting, or diarrhoea.
However, when the condition worsens, there is a chance of shortness of breath, wheezing with a high-pitched sound during breathing, or bluish skin. For infants, especially those under six months old, symptoms can be more subtle and worrying. In addition to signs of breathing difficulties such as nostril flaring and chest wall retractions, feeding, sleep, and activity levels may be affected and decline. There is also a possibility of apnoea (cessation of breathing). Parents need to be particularly vigilant, as over half of RSV hospital admissions are for infants under one year old.
When concerned about infection, parents can initially use a rapid antigen test kit for nasopharyngeal swabs themselves. If uncertain, it is recommended to consult a healthcare professional to undergo a more sensitive and accurate PCR test to screen for RSV genomes in nasopharyngeal secretions. Currently, there are no specific antiviral medications to treat RSV; only symptomatic and supportive care can be provided. If an infant develops severe respiratory failure, it can be life-threatening or lead to long-term sequelae such as recurrent wheezing, making various preventive measures particularly important.
While personal and environmental hygiene are important, frequent hand washing, avoiding crowded places, keeping indoor spaces well-ventilated, and wearing masks outdoors can also reduce the chance of infection. As infants and young children are a high-risk group, there have been revolutionary advancements in recent years in preventing RSV infections in this demographic. Pregnant women can now receive an RSV vaccine between 32 and 36 weeks of gestation, stimulating their body to produce antibodies. These antibodies are then transferred from the mother to the fetus via the placenta, providing protection from birth, with the best protection typically seen in the first 3 to 6 months after birth.
Clinical trial data show that this vaccine reduces the risk of hospitalisation due to RSV in infants (within three months of birth) by 68%, and reduces the risk of severe outcomes (including low oxygen saturation, the need for mechanical ventilation or admission to an ICU) by 82%.
Another preventative option is the direct injection of long-acting monoclonal antibodies into full-term or premature infants, which can provide immediate protection. Previously, hospitals only had short-acting monoclonal antibodies to prevent RSV, but antibody levels could only be maintained for about a month after injection, requiring monthly injections for a total of five to six doses. This was also more costly and therefore only administered to extremely premature infants at the highest risk.
Currently, long-acting monoclonal antibodies only require a single dose. Studies show that antibody levels can be maintained for up to a year, and with lower costs, this preventative method is becoming increasingly common. Clinical trial data demonstrates that this antibody can effectively prevent lower respiratory tract infections caused by RSV, reduce hospitalisation rates, and prevent very severe RSV cases (requiring oxygen or intravenous fluids).
Source: Heart of Wellness, Spring 2026
