Every parent with a child who has had bronchiolitis remembers the rapid breathing, the fast rise and fall of the chest, the crackling sound with each breath, and the fear that came with watching a small baby work hard to breathe. RSV – respiratory syncytial virus – causes the majority of bronchiolitis cases and sends more infants to hospital than any other single infectious cause in the UK each winter.
For most children, RSV is a cold. For a proportion of infants – particularly those under 3 months, those born preterm, and those with congenital heart disease or chronic lung disease – it causes something considerably more serious. Understanding who is at highest risk, what the early warning signs of respiratory distress are, and what the new preventive options are allows parents to respond quickly and appropriately.
Healthbooq (healthbooq.com) covers respiratory illness and infection in infants.
What RSV Is
RSV is a single-stranded RNA virus in the Pneumoviridae family. It is highly contagious and spreads via respiratory droplets and contact with contaminated surfaces, where it can survive for several hours. RSV has a marked winter seasonality in the UK, with a peak in November to February.
By age 2, virtually all children have been infected with RSV at least once. Reinfection occurs throughout life (RSV does not produce lasting immunity), though subsequent infections in healthy older children and adults are typically mild colds.
In infants, the virus infects the lower respiratory tract – the bronchioles and alveoli. The infected epithelium becomes inflamed and swollen, producing excess mucus. Small airways that are already narrow become narrower still, creating the characteristic wheeze and crackle of bronchiolitis.
Who Is at Highest Risk
The highest-risk groups for severe RSV disease include:
Infants under 3 months: the younger the infant, the more severe the illness tends to be and the more likely to require hospital admission. Infants under 6 weeks can deteriorate rapidly and may develop apnoeic episodes (brief pauses in breathing) with RSV infection.
Preterm infants: born before 32 weeks, or between 32-36 weeks with additional risk factors. Immature lungs and reduced passive immunity from maternal antibodies at earlier gestational ages increase susceptibility.
Infants with congenital heart disease: particularly those with haemodynamically significant lesions who cannot compensate for the increased work of breathing.
Infants with chronic lung disease (bronchopulmonary dysplasia from preterm birth).
Immunocompromised infants.
Recognising Respiratory Distress
Most RSV infections in infants present initially like a cold: runny nose, cough, possibly a low-grade fever. In bronchiolitis, the illness progresses over 2-5 days to lower airway involvement.
Signs that require same-day medical assessment: respiratory rate persistently above 60 breaths per minute (normal infant: 30-40); visible recession – the chest wall pulling in between or below the ribs with each breath; nasal flaring; head bobbing; grunting sounds with breathing.
Signs requiring immediate emergency care (call 999): very fast breathing (over 70 breaths per minute), severe recession, persistent low oxygen (blue or pale lips and tongue), apnoea (stopping breathing), a baby who is very difficult to rouse or who is limp.
Feeding difficulties are an early warning sign: an infant who is struggling to breathe cannot coordinate breathing and feeding effectively and may take significantly less milk than usual. An infant taking less than half their usual feeds is a pragmatic threshold for same-day assessment.
Treatment of Bronchiolitis
There is no specific antiviral treatment for RSV. Treatment of bronchiolitis in hospital is supportive: supplemental oxygen to maintain oxygen saturation above 92-94%, and nasogastric or intravenous fluids if the infant cannot take sufficient feeds. Bronchodilators (salbutamol) are not effective in bronchiolitis (the obstruction is inflammatory and mucus-related, not bronchospasm) and are not recommended by NICE (NG9). Antibiotics are not indicated unless there is confirmed secondary bacterial infection.
Most infants with bronchiolitis improve within 7-10 days, though cough can persist for 3-4 weeks.
Nirsevimab (Beyfortus): New Preventive Option from 2023
Nirsevimab is a long-acting monoclonal antibody that provides passive immunity against RSV by directly supplying RSV-F protein neutralising antibodies, rather than stimulating the immune system (it is not a vaccine). A single dose lasts approximately 5 months, covering an RSV season.
The MELODY trial (Griffin et al., NEJM 2020) and Harmonie trial (Drysdale et al., NEJM 2023) demonstrated that nirsevimab reduced RSV-associated lower respiratory tract infection and hospitalisation by approximately 80% in healthy term infants. Based on this evidence, NHS England introduced nirsevimab into the infant immunisation programme in October 2023, offering a single dose to all babies born during or entering their first RSV season.
Previously, palivizumab (monthly IM injections) was offered only to high-risk infants (preterm, congenital heart disease, chronic lung disease); nirsevimab is now offered more widely and replaces palivizumab in most clinical contexts.
Key Takeaways
Respiratory syncytial virus (RSV) is the most common cause of serious lower respiratory tract infection in infants and young children, and the leading cause of hospital admission in infants under 1 year in the UK. Almost all children are infected with RSV by age 2, but most have only a mild upper respiratory illness. In infants under 6 months, RSV can cause bronchiolitis – inflammation of the small airways – which in some cases causes significant respiratory distress requiring hospital admission. A new monoclonal antibody (nirsevimab, brand name Beyfortus) was introduced in England in October 2023 for infants entering their first RSV season, providing passive immunity without vaccination.
