RSV Bronchiolitis

  1. Overview and Epidemiology
  2. Pathophysiology
  3. Clinical Presentation
  4. Diagnosis
  5. Treatment — Supportive Care
  6. Indications for Hospitalization
  7. Prevention — Nirsevimab and Maternal RSV Vaccine
  8. Post-RSV Wheezing and Long-Term Outcomes
  9. Key Research Papers
  10. Connections
  11. Featured Videos

Overview and Epidemiology

Respiratory syncytial virus (RSV) bronchiolitis is the leading cause of lower respiratory tract infection in infants and young children worldwide. RSV is a single-stranded negative-sense RNA virus belonging to the family Pneumoviridae (formerly classified within Paramyxoviridae); it is the most common cause of bronchiolitis and the number-one cause of infant hospitalization in the United States, accounting for an estimated 57,000 to 200,000 hospitalizations per year in children under 5 years of age.

Virtually all children are infected with RSV by age 2, and reinfections occur throughout life because the immune response after primary infection is incomplete and short-lived. Two major antigenic subtypes — RSV-A and RSV-B — co-circulate each season, with one or the other typically dominant. In the northern hemisphere, RSV season runs from October through March, peaking in December and January. RSV is transmitted via large respiratory droplets and direct contact with contaminated secretions; it survives for several hours on hard surfaces, making hand hygiene the most important infection-control measure.

Approximately 1–3% of all infants require hospitalization during their first RSV season. High-risk groups with substantially greater hospitalization and mortality risk include:

Despite high prevalence, RSV bronchiolitis is fatal in fewer than 1% of otherwise healthy term infants hospitalized in high-resource settings. The global burden is far greater in low- and middle-income countries, where RSV is estimated to cause approximately 120,000 child deaths per year, predominantly in sub-Saharan Africa and South Asia.

Back to Table of Contents

Pathophysiology

Understanding why bronchiolitis produces its characteristic clinical picture requires knowing both the direct viral cytopathic effects and the host inflammatory response that amplifies them.

Viral entry and fusion. RSV encodes two major surface glycoproteins: the attachment protein G, which binds nucleolin and other receptors on airway epithelial cells, and the fusion protein F, which drives viral-host membrane fusion. The prefusion conformation of F protein is the target of the most effective neutralizing antibodies — a discovery that underpins nirsevimab's design. Upon fusion, RSV injects its RNA genome and replication machinery into the host cell cytoplasm.

Syncytia formation — the hallmark. As the name "respiratory syncytial" indicates, RSV F protein causes adjacent infected epithelial cells to fuse into multinucleated giant cells (syncytia). This cytopathic effect destroys the ciliated pseudostratified epithelium lining the bronchioles, abolishing mucociliary clearance and triggering an intense peribronchiolar lymphocytic infiltrate.

Small airway obstruction. The combination of epithelial necrosis, mucosal edema, and profuse mucus hypersecretion fills the bronchioles (1–2 mm diameter) with cellular debris and secretions. This creates two patterns of obstruction:

The resulting ventilation-perfusion (V/Q) mismatch drives hypoxemia, the most important physiological consequence of bronchiolitis. Work of breathing increases substantially as the infant fights against hyperinflated, non-compliant lungs, eventually risking respiratory muscle fatigue.

Why bronchodilators don't reliably help. Unlike asthma, the primary obstruction in RSV bronchiolitis is mechanical (mucus plugging and edema) rather than bronchospasm. The airways of infants have relatively less smooth muscle than older children, and beta-2 adrenergic receptors are sparser in the peripheral airways. This anatomical reality explains why multiple high-quality randomized trials have shown that albuterol (salbutamol) and epinephrine provide no consistent clinical benefit in the average bronchiolitis patient.

Immune evasion. RSV non-structural proteins NS1 and NS2 actively suppress the innate interferon response, allowing prolonged viral replication and contributing to the clinical duration of 7–14 days. The resulting inflammatory state — mediated by IL-4, IL-13, and leukotriene C4 — may prime the developing airway for subsequent hyperreactivity.

Back to Table of Contents

Clinical Presentation

RSV bronchiolitis follows a characteristic biphasic progression. Recognizing the normal time course helps parents and clinicians anticipate the peak of illness and identify infants who are deteriorating beyond expectation.

Days 1–2: Upper respiratory prodrome. The illness begins indistinguishably from a common cold — clear rhinorrhea, mild nasal congestion, low-grade fever (typically 38–38.5°C / 100.4–101.3°F), and a dry cough. Infants may feed slightly less well due to nasal obstruction. This phase is easily dismissed as a routine viral URI.

Days 3–5: Lower tract extension and peak severity. In vulnerable infants — particularly those under 3 months, born prematurely, or with comorbidities — the virus spreads to the lower airways. Parents notice rapid worsening:

Apnea as a presenting sign. In infants under 6 weeks of age (especially former premature infants), apnea — cessation of breathing for >20 seconds or shorter episodes with bradycardia or color change — can be the presenting manifestation of RSV bronchiolitis, even before wheeze or significant respiratory distress develops. This "apnea of RSV" occurs because the virus directly suppresses central respiratory drive and because upper airway secretions trigger reflex laryngospasm. Any infant under 3 months with unexplained apnea should be tested for RSV.

Days 5–14: Gradual recovery. Symptoms peak around day 3–5 and then improve steadily over 7–14 days. Cough is typically the last symptom to resolve and may persist for 3–4 weeks as the airway epithelium regenerates. The "happy wheezer" — a wheezing infant who is comfortable, feeding adequately, maintaining SaO2 >95%, and whose parents can reliably monitor for deterioration — does not require hospitalization.

Back to Table of Contents

Diagnosis

RSV bronchiolitis is a clinical diagnosis. The American Academy of Pediatrics (AAP) 2014 clinical practice guideline explicitly states that routine laboratory and radiologic testing is not recommended in the typical case because results do not change management. This principle reflects both cost-effectiveness and the reality that supportive care is the treatment regardless of which virus is causative.

When viral testing is indicated:

Test options:

Chest X-ray. CXR is NOT routinely indicated in bronchiolitis. When obtained, typical findings include bilateral hyperinflation (flattened diaphragms, increased anterior-posterior diameter), peribronchial thickening ("dirty hila"), and patchy atelectasis — most often in the right upper lobe or right middle lobe. Importantly, atelectasis is frequently mistaken for pneumonia; this misreading drives inappropriate antibiotic prescribing. CXR should be obtained when the diagnosis is uncertain, when severe or atypical disease is present, or when bacterial pneumonia is in the differential (e.g., persistent high fever, toxic appearance, unilateral lobar changes).

Blood tests. CBC, blood cultures, and CRP are not routinely needed. The rate of serious bacterial infection (SBI) in febrile infants with bronchiolitis is low — RSV appears to partially protect against concurrent bacteremia through its own fever mechanism. Exceptions include infants under 30 days of age, toxic-appearing infants, or those with persistent high fever in whom a full sepsis evaluation is warranted.

Back to Table of Contents

Treatment — Supportive Care

There is no specific antiviral therapy for RSV bronchiolitis in immunocompetent infants. The AAP 2014 clinical practice guideline — the most comprehensive evidence synthesis on this topic — establishes that supportive care is the standard of treatment. This means clinicians must resist the pressure to prescribe medications that do not improve outcomes and focus on the interventions that do matter.

What works:

What does NOT work (and should NOT be routinely used):

Back to Table of Contents

Indications for Hospitalization

Most infants with RSV bronchiolitis can be safely managed at home with careful parental education about warning signs. The decision to hospitalize is individualized and guided by clinical severity, physiological parameters, age, comorbidities, and social factors.

Clinical criteria for hospitalization:

Patient-specific risk factors driving a lower threshold for admission:

Social and logistic factors:

PICU admission criteria: FiO2 >0.5 to maintain target saturation; failure of HFNC (persistent tachypnea >70–80/min, SaO2 <90%, increasing distress); mechanical ventilation requirement for refractory apnea or respiratory failure. Ventilated infants with RSV bronchiolitis typically require lung-protective strategies accounting for the severe air trapping (short inspiratory time, long expiratory time, low PEEP to avoid worsening hyperinflation).

Discharge criteria: SaO2 ≥90–92% in room air while awake and asleep; able to maintain adequate oral feeds (>50% of usual intake); respiratory rate <60 breaths/min; caregiver education and comfort with outpatient monitoring; close follow-up arranged within 24–48 hours for infants at continuing risk.

Back to Table of Contents

Prevention — Nirsevimab and Maternal RSV Vaccine

The prevention landscape for infant RSV disease was transformed between 2023 and 2024 by two new products: a long-acting monoclonal antibody for direct infant immunization and a maternal vaccine that confers passive protection to the newborn. These represent the most significant advance in RSV prevention since the introduction of palivizumab (Synagis) in 1998.

Nirsevimab (Beyfortus, Sanofi/AstraZeneca). FDA-approved August 2023. Nirsevimab is a recombinant human IgG1 monoclonal antibody engineered to bind the prefusion conformation of RSV F protein with extremely high affinity — the same epitope targeted by the most potent naturally occurring neutralizing antibodies. Three modifications distinguish it from older antibodies: a triple amino acid substitution (YTE modification) extends the half-life to approximately 70 days (compared to 20 days for palivizumab), enabling a single injection to provide approximately 150 days of protection through an RSV season.

The ACIP recommends nirsevimab for all infants under 8 months entering their first RSV season (October–March in the northern hemisphere) — a universal recommendation, not limited to high-risk infants. For older infants with CHD, BPD, or immunodeficiency entering their second RSV season, a second-season dose (200 mg) is recommended through 24 months of age.

Phase 3 efficacy data from the MELODY trial (Hammitt LL et al., NEJM 2022): nirsevimab reduced medically attended RSV-associated lower respiratory tract infections by 74.5% compared to placebo in healthy late-preterm and term infants. Hospitalization for RSV-associated lower respiratory tract disease was reduced by 77.3%. The safety profile was comparable to placebo.

Nirsevimab supersedes palivizumab for the general infant population. Palivizumab remains appropriate for infants with CHD with hemodynamic significance, BPD on medical treatment, or gestational age ≤28 weeks during their second RSV season when nirsevimab is not available or accessible.

Maternal RSV vaccine: Abrysvo (Pfizer). FDA-approved May 2023 for administration to pregnant persons at 32–36 weeks of gestation. Abrysvo contains bivalent prefusion-stabilized RSV F proteins (RSV-A and RSV-B). A single intramuscular dose primes the maternal immune system; the resulting IgG antibodies cross the placenta during the third trimester, conferring passive immunity to the newborn for the first 6 months of life — the window of maximum vulnerability.

The phase 3 HARMONY trial (Kampmann B et al., NEJM 2023): maternal Abrysvo vaccination reduced severe RSV-associated lower respiratory infections in infants during the first 90 days of life by 81.8%, and during the first 180 days of life by 69.4%. All-cause hospitalization in the first 90 days was reduced by 56%.

Choosing between maternal vaccine and nirsevimab: Both options are endorsed by ACIP; the choice is context-dependent. The maternal vaccine is administered prenatally and requires no postnatal intervention; nirsevimab is a postnatal injection but can be timed precisely to the RSV season and covers infants born to mothers who were not vaccinated during pregnancy. ACIP currently recommends against using both in the same infant unless born <32 weeks gestation (in which case both are recommended regardless). When maternal vaccination status is unknown at delivery, nirsevimab should be offered.

Back to Table of Contents

Post-RSV Wheezing and Long-Term Outcomes

For most families, the most enduring question after an RSV hospitalization is: "Will my child develop asthma?" The epidemiological association between early severe RSV bronchiolitis and subsequent recurrent wheeze and asthma is robust, but the causal interpretation remains actively debated.

The epidemiological signal. Approximately 30–40% of infants hospitalized with RSV bronchiolitis develop recurrent wheezing episodes during early childhood. Compared to infants without RSV hospitalizations, those with severe early RSV bronchiolitis have an approximately 2–3-fold higher odds of an asthma diagnosis by school age in multiple cohort studies. This association is independent of atopic family history, though atopy amplifies it.

Two competing hypotheses:

Rhinovirus vs. RSV. Data from the COAST (Childhood Origins of ASThma) birth cohort and other studies demonstrate that rhinovirus-associated wheezing illness in the first 3 years of life — particularly in atopic children — is an even stronger predictor of school-age asthma than RSV bronchiolitis. This suggests the causal picture is complex: it is likely that severe early viral lower respiratory illness of any type, in a genetically susceptible host, interacts with aeroallergen sensitization to shape the asthma trajectory.

Long-term outcomes by severity. Premature infants hospitalized with RSV bronchiolitis have the highest rates of subsequent obstructive airway disease; their baseline airway hyperreactivity and reduced lung reserve from prematurity are compounded by RSV-driven inflammation. School-age children with a history of severe RSV bronchiolitis in infancy show measurable differences in spirometry (lower FEV1/FVC ratios) even in the absence of a clinical asthma diagnosis.

Does any intervention during RSV illness modify long-term wheeze risk? No. Despite the plausible biological rationale, no RCT of any treatment during the acute RSV episode — including azithromycin, corticosteroids, or montelukast — has demonstrated a reduction in subsequent wheezing or asthma diagnoses. The post-RSV wheezing trajectory appears to be determined by factors that precede or are independent of the acute illness.

Counseling families. Most children with post-RSV wheezing substantially improve by school age; the majority do not have persistent asthma. Parents should watch for recurrent wheezing with subsequent respiratory illnesses and seek timely evaluation. The presence of post-RSV wheeze does not mean the child has asthma — but it does warrant close monitoring and, if episodes are frequent or severe, formal evaluation by a pediatric pulmonologist or allergist.

Back to Table of Contents

Key Research Papers

  1. Hammitt LL et al., 2022 — Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants (MELODY trial) — New England Journal of Medicine — PMID: 36541062 — Phase 3 randomized controlled trial demonstrating 74.5% efficacy of a single dose of nirsevimab against medically attended RSV-associated lower respiratory tract infection in healthy late-preterm and term infants, leading to FDA approval in 2023.
  2. Kampmann B et al., 2023 — Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants (HARMONY trial) — New England Journal of Medicine — PMID: 37214276 — Phase 3 trial of maternal RSV vaccination with Abrysvo (Pfizer) showing 81.8% efficacy against severe RSV-associated lower respiratory infections in the first 90 days of infant life, establishing the maternal immunization strategy.
  3. Ralston SL et al. (American Academy of Pediatrics), 2014 — Clinical Practice Guideline: The Diagnosis, Management, and Prevention of Bronchiolitis — Pediatrics — PMID: 25070057 — The foundational AAP clinical practice guideline establishing evidence-based recommendations for the diagnosis and management of bronchiolitis; the primary source for the "supportive care only" standard in RSV bronchiolitis management.
  4. Shay DK et al., 1999 — Bronchiolitis-Associated Hospitalizations among US Children, 1980–1996 — JAMA — PMID: 9399950 — Landmark epidemiological study quantifying the national burden of bronchiolitis hospitalizations in the United States; established RSV bronchiolitis as the leading cause of infant hospitalization and documented rising hospitalization trends.
  5. Griffin MP et al., 2020 — Single-Dose Nirsevimab for Prevention of RSV in Preterm Infants — New England Journal of Medicine — PMID: 34570984 — Phase 2–3 trial of nirsevimab in preterm infants (29–35 weeks gestational age); demonstrated 70.1% efficacy against medically attended lower respiratory tract infections, establishing the dose and safety profile in the highest-risk preterm population.
  6. Zorc JJ, Hall CB, 2010 — Bronchiolitis: Recent Evidence on Diagnosis and Management — Pediatrics — PMID: 20980254 — Comprehensive clinical review synthesizing evidence on RSV bronchiolitis diagnosis, management, and prevention; widely used as a reference for the evidence base underlying supportive-care-only guidelines.
  7. Fernandes RM et al., 2013 — Glucocorticoids for Acute Viral Bronchiolitis in Infants and Young Children (Cochrane Review) — Cochrane Database of Systematic Reviews — PMID: 22778122 — Systematic review and meta-analysis of 17 RCTs examining corticosteroid use in bronchiolitis; demonstrated no significant benefit on hospitalization rates, length of stay, or clinical severity scores, supporting guideline recommendations against routine corticosteroid use.
  8. Hartling L et al., 2011 — Epinephrine for Bronchiolitis (Cochrane Review) — Cochrane Database of Systematic Reviews — PMID: 21310841 — Comprehensive meta-analysis comparing epinephrine and albuterol to placebo in bronchiolitis; found short-term clinical score improvements with outpatient epinephrine but no durable benefit on hospitalization rates or length of stay, supporting current recommendations against routine epinephrine use.
  9. Stein RT et al., 1999 — Respiratory Syncytial Virus in Early Life and Risk of Wheeze and Allergy by Age 13 Years — Lancet — PMID: 10382693 — Tucson Children's Respiratory Study long-term follow-up demonstrating that RSV lower respiratory illness in the first 3 years of life significantly increased wheeze risk at age 6, with the excess risk declining by age 13; seminal paper in the post-RSV wheeze literature.
  10. Lemanske RF Jr. et al., 2005 — Rhinovirus Illnesses during Infancy Predict Subsequent Childhood Wheezing (COAST study) — Journal of Allergy and Clinical Immunology — PMID: 16210074 — Childhood Origins of ASThma (COAST) birth cohort demonstrating that rhinovirus-associated wheezing in the first year of life, particularly in sensitized infants, was the strongest predictor of asthma at age 6, providing important context to the RSV-asthma association.
  11. PubMed: PARIS trial high-flow nasal cannula RSV bronchiolitis — Franklin D et al. 2018 NEJM PARIS trial; RCT of high-flow nasal cannula oxygen vs. standard care in bronchiolitis (743 infants); no significant reduction in treatment failure (the primary outcome) with HFNC vs. standard flow, though secondary analyses suggested benefit in a subgroup with higher severity scores.
  12. PubMed: hypertonic saline nebulization bronchiolitis meta-analysis — Zhang L et al. Cochrane systematic review of nebulized hypertonic saline for acute bronchiolitis; early meta-analyses suggested modest reductions in hospitalization length; larger subsequent trials showed no consistent benefit, informing the AAP recommendation against routine use.

Back to Table of Contents

Connections

Back to Table of Contents