Respiratory Syncytial Virus (RSV)

Respiratory syncytial virus (RSV) is a highly contagious virus that infects the airways and lungs. It is one of the most common causes of respiratory illness worldwide, and by around age two, nearly all children will have been infected at least once. For most healthy children and adults, RSV causes a mild, cold-like illness. But at both ends of life — in very young infants and in older adults — and in people with certain underlying health conditions, RSV can cause serious lower respiratory tract infection, including bronchiolitis and pneumonia, and is a leading cause of hospitalisation in these groups.

RSV does not confer lasting immunity, which is why reinfection throughout life is the rule rather than the exception — most adults will be infected with RSV every few years, usually experiencing symptoms similar to a common cold. What changes with age and health status is not whether someone will encounter RSV again, but how well their body handles it.

How RSV Affects the Body

RSV spreads through respiratory droplets released when an infected person coughs or sneezes, through direct contact such as kissing or shaking hands, and through contaminated surfaces — the virus can survive for several hours on hard surfaces such as doorknobs, toys and countertops. It typically takes 4 to 6 days from exposure for symptoms to appear, and infected individuals are usually contagious for 3 to 8 days, though infants and people with weakened immune systems can shed the virus for several weeks even after symptoms improve.

In most people, RSV infection stays confined to the upper airway, causing symptoms similar to a common cold. In infants, older adults, and people with weakened immune systems or underlying lung or heart disease, the virus can spread to the lower respiratory tract — the bronchioles (the smallest airways in the lungs) and the lungs themselves — causing bronchiolitis or pneumonia. This progression, and how quickly it can happen, is why RSV is taken far more seriously in these groups than a typical cold.

Bronchiolitis — inflammation and mucus build-up in the smallest airways of the lungs — is the most common serious complication of RSV in infants, and RSV is its leading cause worldwide. Because an infant's airways are so small to begin with, even modest inflammation and mucus can significantly narrow them, leading to rapid, laboured breathing, wheezing, and difficulty feeding due to breathlessness. RSV bronchiolitis is the leading cause of hospitalisation in infants globally, with the highest risk in babies under six months of age, especially those born prematurely.

In older adults, natural immune function gradually declines (a process called immunosenescence), and pre-existing heart and lung conditions become more common — both of which make it harder for the body to contain RSV to the upper airway. RSV in older adults is associated with a substantial burden of hospitalisation each year, comparable in scale to influenza in some seasons, and can trigger flare-ups of underlying chronic obstructive pulmonary disease (COPD), asthma or heart failure, sometimes leading to serious cardiac events in the weeks following infection.

Symptoms of RSV

Symptoms typically appear gradually, often starting with mild upper respiratory symptoms before potentially progressing over several days. Symptoms and their severity vary considerably by age and underlying health.

In Healthy Older Children and Adults

• Runny or congested nose
• Sore throat and dry cough
• Headache and mild fever
• Sneezing
• General tiredness, usually lasting one to two weeks

Warning Signs of Lower Respiratory Involvement (Any Age)

• Rapid or laboured breathing, or visible effort to breathe (using neck or rib muscles)
• Wheezing or a whistling sound when breathing
• Persistent, severe cough
• Chest pain or tightness
• A bluish tinge to the lips, face or fingertips (cyanosis) — a medical emergency
• Lethargy or unusual sleepiness

Specific Signs in Infants

• Short, shallow, rapid breathing, sometimes with grunting
• Poor feeding or difficulty feeding due to breathlessness
• Irritability alternating with lethargy
• Brief pauses in breathing (apnoea) — which, notably, can be the presenting sign of RSV in very young infants even before typical cold symptoms appear
• Reduced wet nappies, a sign of dehydration

Who Is at Greatest Risk of Severe RSV

• Infants under 6 months, especially under 3 months: Infants have small airways that are easily narrowed by inflammation, and immune systems that have not yet developed full protective memory against the virus.
• Premature infants: Premature birth means less time for lung development and for maternal antibodies (which normally cross the placenta late in pregnancy) to be transferred to the baby, leaving these infants more vulnerable.
• Infants and children with underlying health conditions: Congenital heart disease, chronic lung disease of prematurity, neuromuscular disorders, and conditions affecting the immune system all increase the risk of severe RSV in infants and young children.
• Adults aged 60 and above: Immune function naturally declines with age, and adults aged 60 and above — particularly those 75 and older — face substantially higher rates of RSV hospitalisation and death than younger adults.
• Adults with chronic heart, lung, kidney or metabolic disease: Asthma, COPD, congestive heart failure, coronary artery disease, diabetes, chronic kidney disease and obesity all increase the risk of severe outcomes from RSV, regardless of age.
• People who are immunocompromised: People undergoing chemotherapy, organ transplant recipients, and those with conditions or medications that suppress the immune system are at increased risk of severe and prolonged RSV infection.
• Residents of long-term care facilities: Residents of nursing homes and long-term care facilities face higher exposure risk and often have multiple risk factors for severe disease simultaneously.

Possible Complications of RSV

• Bronchiolitis, or the inflammation of the small airways, the leading cause of RSV hospitalisation in infants
• Pneumonia, from RSV itself or from a secondary bacterial infection developing on top of it
• Middle ear infection (otitis media), particularly common in infants and toddlers
• Worsening of underlying asthma or COPD, sometimes severe enough to require hospitalisation
• Heart failure exacerbation and, less commonly, other cardiac events in older adults or those with existing heart disease, which can occur even in the weeks following RSV infection
• Respiratory failure requiring oxygen support or, in severe cases, mechanical ventilation
• Dehydration, particularly in infants who feed poorly due to breathing difficulty
• An increased long-term risk of developing asthma in children who had severe RSV bronchiolitis in infancy
• Repeated infection throughout life, as RSV does not confer long-lasting immunity

How RSV is Diagnosed

• Clinical Assessment: A doctor will assess breathing rate and effort, listen to the chest for wheezing or crackles, check oxygen levels, and consider the pattern of symptoms and any risk factors present.
• RSV Antigen or PCR Test: A swab taken from the nose or throat can rapidly detect RSV antigen or genetic material, confirming the diagnosis and helping to distinguish RSV from influenza, COVID-19 and other respiratory viruses that can look similar clinically.
• Blood Tests: May be used to check white blood cell counts, assess for signs of bacterial co-infection, or evaluate overall severity in more unwell patients.
• Chest X-Ray: Used to check for pneumonia or other lung changes in patients with more severe respiratory symptoms, or when the diagnosis remains uncertain.
• Pulse Oximetry: A simple, non-invasive measurement of blood oxygen levels, important in assessing severity and the need for hospitalisation, particularly in infants and older adults.

Preventing RSV

Until recently, RSV prevention was limited largely to good hygiene practices. That has changed substantially: over the past two to three years, several new maternal vaccines, long-acting infant antibody injections, and vaccines for older adults have become available, together representing one of the most significant advances in respiratory infection prevention in a generation.

Steps you can take to prevent RSV include:

• Frequent handwashing with soap and water, especially before touching the face, feeding an infant, or after being in public spaces
• Avoiding close contact with people who have cold-like symptoms, particularly around infants and older or immunocompromised individuals
• Covering coughs and sneezes, and disposing of tissues promptly
• Regularly cleaning frequently touched surfaces such as toys, doorknobs and countertops
• Avoiding smoking around infants and children, as second-hand smoke exposure increases the risk and severity of RSV infection
• Where possible, limiting infants' exposure to crowded settings during periods of high respiratory virus circulation

Two complementary approaches are now available to protect infants during their most vulnerable early months, working by providing ready-made antibodies rather than relying on the infant's own immune system to respond.

• Maternal RSV vaccination: Given to pregnant women, typically between 32 and 36 weeks of pregnancy, this vaccine prompts the mother to produce RSV antibodies that cross the placenta and protect the baby from birth through approximately the first six months of life — covering exactly the period of highest risk.
• Infant monoclonal antibody injection: A long-acting monoclonal antibody given as a single injection directly to the infant, rather than a vaccine that stimulates the infant's own immune response. It provides immediate, ready-made protection against severe RSV disease for at least five months, and is recommended for infants entering their first RSV season whose mothers did not receive the maternal vaccine, as well as for certain older infants and young children with risk factors for severe disease entering their second RSV season.

These two approaches are complementary rather than duplicative — most infants need only one, chosen based on the timing of the mother's pregnancy and vaccination status — and both have shown strong real-world effectiveness in preventing RSV-related hospitalisation in infants since their introduction.

Several RSV vaccines are now available for older adults, each using a different technology to prompt the body to produce protective antibodies before natural infection occurs. A single dose has been shown to provide strong protection against severe RSV disease for at least two RSV seasons, and vaccination is generally recommended for:

• All adults aged 60 and above (some guidelines now emphasise those 75 and above as a particular priority, given their substantially higher risk)
• Adults aged 50 to 59 who have chronic heart, lung, kidney or metabolic disease, or are otherwise at increased risk of severe RSV
• Adults who are immunocompromised, for whom specific vaccine options may be recommended

As with any vaccine, mild side effects such as injection site soreness, fatigue and headache are common and generally resolve within a day or two. Rare but serious side effects have been identified with ongoing safety monitoring, and are best discussed individually with a doctor, who can weigh personal risk factors against the substantial protection these vaccines offer against severe RSV illness and hospitalisation.