Lead
Respiratory syncytial virus (RSV: a common respiratory virus that causes mild cold-like illness in older children but can cause severe bronchiolitis or pneumonia in infants) is so common that almost every child has been infected at least once by age 2 [1]. In older children and adults, it usually produces a bad cold and not much more. In infants under six months, the same virus can cause bronchiolitis: inflammation and obstruction of the small airways in the lungs, causing wheezing and breathing difficulty, mainly in infants or pneumonia requiring hospitalization [2]. What drives the difference in severity, and how has the landscape of prevention changed? This article covers both.
Epidemiology and Risk Factors
RSV circulates mainly in autumn and winter; in the Northern Hemisphere, the peak typically falls between October and March [1]. Transmission is by droplet and contact, and infants are frequently infected by caregivers or older siblings carrying the virus.
Groups consistently reported to be at higher risk for severe disease include: preterm infants (born before 37 weeks), children with congenital heart disease, children with chronic lung disease (including bronchopulmonary dysplasia), immunocompromised children, and otherwise healthy infants under three months of age [2,3]. Across all infants under six months, approximately 2–3% of infections are estimated to follow a severe course requiring hospitalization [2].
The anatomical explanation for infant severity is straightforward. The bronchioles in a young infant are extremely narrow; inflammation, mucosal edema, and secretions can occlude them readily. An immature immune response that takes longer to clear the virus compounds the problem.
Symptoms — From Runny Nose to Respiratory Distress
The typical course has two phases. For the first two to four days, symptoms are upper respiratory: nasal congestion, mild fever, and cough. In a subset of infants, the inflammation then extends to the lower airways, producing wheezing, tachypnea: abnormally rapid breathing rate; in infants, defined as more than 60 breaths per minute at rest (respiratory rate above 60 per minute), chest retractions, and difficulty feeding [2].
Signs widely cited as indicators for hospitalization include: persistent oxygen saturation below 95%, feeding intake below 50% of baseline, and significant tachypnea with retractions [2]. Cyanosis — bluish discoloration of the lips — represents greater urgency. Any of these signs warrants prompt medical evaluation.
Prevention — The Arrival of Nirsevimab
For years, palivizumab (brand name Synagis) was the standard preventive option for high-risk infants. Given as monthly intramuscular injections throughout RSV season, it reduces RSV-related hospitalizations in high-risk infants by approximately 55% [3]. The indication was limited, and monthly clinic visits were a significant burden.
A newer monoclonal antibody: a laboratory-produced antibody that targets a specific protein, used to prevent or treat diseases, nirsevimab, attracted attention from large clinical trials in 2022–2023. A single pre-season injection provides passive immunity lasting one full RSV season. In the HARMONIE trial, conducted in healthy term and late-preterm infants, nirsevimab reduced RSV-related hospitalizations by 83% [4]. As its indicated population has expanded, it is now being considered beyond the traditional high-risk categories.
There is also a maternal vaccination approach. A bivalent prefusion F protein vaccine (Abrysvo) administered during pregnancy transfers antibody across the placenta, protecting the infant during the most vulnerable window of the first three to six months of life. The MATISSE trial (2023) reported a 69% reduction in severe RSV lower respiratory tract disease in infants under 90 days of age [5]. Approval status varies by country; confirm the current options with your obstetrician or pediatrician.
Treatment and Home Care
No specific antiviral drug is currently in general use for RSV. Management is supportive [2].
At home, regular nasal suctioning helps reduce nasal obstruction, and keeping up with fluid intake is the primary goal. A visible drop in feeding is a meaningful warning sign. When hospitalization is indicated, oxygen therapy is the mainstay; in cases of severe respiratory distress, high-flow nasal cannula oxygen (high-flow therapy) may be used.
Putting It Into Practice
- Before the first autumn and winter after birth, ask your pediatrician about nirsevimab and the maternal vaccine option
- If your child is in a high-risk category, learn the three key signs to watch for: wheeze, fast breathing, and difficulty feeding
- The autumn when a child starts daycare often overlaps with RSV season, and older siblings are a common source of transmission
- Logging the date symptoms start and any progression — with specific attention to feeding amounts and breathing — makes it easier to convey the timeline accurately when you contact the doctor
Summary
RSV infects nearly every child by age 2, but the severity profile in infants is categorically different from what adults experience. Nirsevimab and maternal RSV vaccination represent a meaningful expansion of prevention options. Knowing the signs of deterioration gives parents a clearer threshold for when to seek care.
References
- Glezen WP, Taber LH, Frank AL, Kasel JA. Risk of primary infection and reinfection with respiratory syncytial virus. Am J Dis Child. 1986;140(6):543–546. doi:10.1001/archpedi.1986.02140200053026. PMID: 3706232.
- Ralston SL, Lieberthal AS, Meissner HC, et al.; American Academy of Pediatrics. Clinical practice guideline: the diagnosis, management, and prevention of bronchiolitis. Pediatrics. 2014;134(5):e1474–e1502. doi:10.1542/peds.2014-2742. PMID: 25349312.
- Simões EAF, Carbonell-Estrany X, Fullarton JR, et al. A predictive model for respiratory syncytial virus severity in children. Pediatr Infect Dis J. 2008;27(9):791–797. doi:10.1097/INF.0b013e318172fa80. PMID: 18679152.
- Drysdale SB, Cathie K, Flamein F, et al.; HARMONIE Trial Group. Nirsevimab for prevention of hospitalizations due to RSV in infants. N Engl J Med. 2023;389(26):2425–2435. doi:10.1056/NEJMoa2309189. PMID: 38048191.
- Kampmann B, Madhi SA, Munjal I, et al.; MATISSE Study Group. Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants. N Engl J Med. 2023;388(16):1451–1464. doi:10.1056/NEJMoa2216480. PMID: 37018468.