4.3 Bronchiolitis, RSV Prevention (Nirsevimab, Clesrovimab, Palivizumab) & Viral Lower Respiratory Infections
Key Takeaways
- AAP bronchiolitis guidelines recommend strictly supportive care (oxygen only if persistent SpO2 <90%, gentle suctioning, hydration); routine use of bronchodilators, systemic corticosteroids, hypertonic saline in the ED, and antibiotics is explicitly not recommended.
- Nirsevimab is a long-acting monoclonal antibody targeting the prefusion F protein with an engineered YTE Fc region (half-life 69-100 days), dosed as a single IM injection (50 mg for <5 kg; 100 mg for >=5 kg) providing at least 5 months of universal protection for infants <8 months.
- Clesrovimab (Enflonsia), FDA-approved June 9, 2025 and ACIP-recommended June 25, 2025, is an alternative to nirsevimab for infants under 8 months entering their first RSV season, given as a single 105 mg IM dose that is identical regardless of body weight; it is not recommended at or after 8 months, so second-season high-risk children must receive nirsevimab.
- Maternal RSV vaccination with bivalent RSVpreF (Abrysvo) at 32-36 weeks gestation provides passive transplacental infant protection through 6 months; infant nirsevimab is not needed unless birth occurs <14 days post-vaccination or maternal response is impaired.
- Pediatric influenza pharmacotherapy with oseltamivir requires tiered dosing (3 mg/kg BID for infants 0-8 months; 3.5 mg/kg BID for 9-11 months; weight-tiered 30 to 75 mg BID for >=1 year) for 5 days, initiated as early as possible within 48 hours.
4.3 Bronchiolitis, RSV Prevention & Viral Respiratory Infections
Quick Answer: Pediatric bronchiolitis is managed with supportive care alone: oxygen if $\text{SpO}_2 < 90%$, gentle suctioning, and hydration. The AAP guideline explicitly advises against routine bronchodilators, systemic corticosteroids, and hypertonic saline in the emergency department. For RSV immunoprophylaxis, nirsevimab (Beyfortus) has superseded palivizumab as the standard of care for all infants <8 months entering their first RSV season (dosed 50 mg IM for <5 kg, 100 mg IM for ≥5 kg), providing ≥5 months of protection via an engineered YTE Fc domain. If the mother received Abrysvo (RSVpreF) at 32–36 weeks gestation ≥14 days prior to delivery, infant nirsevimab is not needed. Pediatric influenza is treated with weight-tiered oseltamivir for 5 days.
1. Acute Viral Bronchiolitis: Pathophysiology & Clinical Course
Bronchiolitis is the leading cause of hospitalization in infants younger than 12 months in the United States, peaking between December and March.
Etiology & Pathogenesis
- Respiratory Syncytial Virus (RSV) accounts for 50% to 80% of cases.
- Other viral agents: Human rhinovirus, human metapneumovirus (hMPV), parainfluenza virus types 1–3, influenza, adenovirus, and coronavirus.
- Pathophysiology: Direct viral infection and destruction of ciliated respiratory epithelial cells in the bronchioles triggers lymphocytic infiltration, submucosal edema, epithelial necrosis, and excessive mucus production. The accumulation of cellular debris and viscous mucus causes ball-valve luminal obstruction, leading to hyperinflation, patchy atelectasis, dynamic airway narrowing, and severe ventilation-perfusion ($V/Q$) mismatch.
Clinical Presentation
Infants typically experience a 2- to 3-day viral upper respiratory prodrome (rhinorrhea, low-grade fever, sneezing), progressing on days 3 to 5 to lower respiratory tract involvement: tachypnea (respiratory rate >50–70 breaths/min), persistent wheezing, diffuse fine rales/crackles, intercostal and subcostal retractions, nasal flaring, grunting, and feeding intolerance.
2. American Academy of Pediatrics (AAP) Guidelines: The "Do Not Do" Core
The 2014 AAP clinical practice guideline (reaffirmed 2019/2023) transformed bronchiolitis management by explicitly designating most historical pharmacotherapies as non-beneficial or harmful.
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| AAP Bronchiolitis Guideline Recommendations |
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| Intervention | AAP Recommendation | Clinical Rationale |
|----------------------------|---------------------------------|--------------------------|
| Diagnostic Viral / X-ray | DO NOT routinely order viral | Clinical diagnosis. Does |
| Testing | panels or chest radiographs | not alter management. |
| Inhaled Bronchodilators | DO NOT routinely administer | No reduction in admission|
| (Albuterol, Epinephrine) | (albuterol or epinephrine) | or length of stay (LOS). |
| Systemic Corticosteroids | DO NOT administer corticosteroids| Ineffective; does not |
| (Dexamethasone, Prednisone)| in any formulation | alter airway pathology. |
| Nebulized Hypertonic Saline| DO NOT administer in emergency | Does not reduce admission|
| (3% NaCl) | department setting | rates from ED. |
| Antibacterials | DO NOT administer unless proven | Secondary bacterial rate |
| | concurrent bacterial infection | is extremely low (<1-2%).|
| Supplemental Oxygen | Administer ONLY if SpO2 < 90% | Avoid over-monitoring and|
| | continuously | delayed discharge. |
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Why Bronchodilators and Corticosteroids Fail in Bronchiolitis
- Inhaled Bronchodilators: Unlike asthma, in which airway obstruction is caused by smooth muscle bronchospasm, bronchiolitis obstruction is mechanical—driven by sloughed necrotic epithelium, mucosal edema, and mucus plugging. Infant bronchiolar smooth muscle is poorly developed and lacks adequate beta-2 receptor coupling. Meta-analyses demonstrate albuterol does not reduce hospitalization rates or hospital length of stay, while inducing tachycardia, tremor, and paradoxical hypoxemia via worsening $V/Q$ mismatch.
- Systemic Corticosteroids: The inflammatory cascade in viral bronchiolitis is predominantly neutrophilic and monocytic epithelial destruction rather than eosinophilic allergic inflammation. Multiple large multicenter trials (e.g., Corneli et al.) demonstrated that oral dexamethasone or prednisolone produces no clinical improvement, does not reduce admissions, and may prolong viral shedding.
Supportive Care Fundamentals
- Oxygen Therapy: Indicated only if $\text{SpO}_2$ persistently falls below 90% during quiet, awake states. Continuous pulse oximetry should be avoided in clinically stable infants weaning oxygen, as it artificially prolongs hospitalization.
- Hydration & Nutrition: Assess hydration status. If respiratory rate exceeds 60 to 70 breaths/min, oral feeding carries an aspiration risk. Provide isotonic IV fluids (e.g., $D_5$ 0.9% NaCl) or nasogastric enteral feeds.
- Airway Clearance: Gentle superficial nasal suctioning with saline drops before feeding and sleep. Deep nasopharyngeal suctioning causes traumatic mucosal edema and should be avoided.
3. RSV Immunoprophylaxis: Nirsevimab, Clesrovimab & Palivizumab
The introduction of nirsevimab-alip (Beyfortus) marked a paradigm shift from selective high-risk prophylaxis to universal infant passive immunization, and a second long-acting monoclonal antibody, clesrovimab-cfor (Enflonsia), joined it in 2025.
[!IMPORTANT] ACIP recommends that every infant be protected by one of three routes: (1) maternal RSV vaccination during pregnancy, (2) infant nirsevimab, or (3) infant clesrovimab. Nirsevimab and clesrovimab are alternatives to each other — never give both.
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| Nirsevimab vs Palivizumab Pharmacologic Comparison |
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| Characteristic | Nirsevimab (Beyfortus) | Palivizumab (Synagis) |
|----------------------------|---------------------------------|--------------------------|
| Molecular Structure | Recombinant human IgG1kappa | Humanized mouse-human |
| Target Antigen | Prefusion F protein (Site 0) | F protein (Sites A/II) |
| Fc Modification | YTE mutation (M252Y/S254T/T256E)| Wild-type Fc domain |
| Elimination Half-Life (t1/2)| 69 to 100 days | 18 to 20 days |
| Duration of Protection | >= 5 months (entire RSV season) | ~30 days per injection |
| Dosing Regimen | Single IM injection once | Monthly IM x up to 5 doses
| Target Population | ALL infants <8 months entering | Restricted high-risk only|
| | Season 1; select high-risk S2 | (prematurity, CLD, CHD) |
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Clesrovimab (Enflonsia): The Weight-Independent Alternative
The FDA approved clesrovimab on June 9, 2025, and ACIP recommended it on June 25, 2025 as an alternative to nirsevimab for infants aged <8 months born during or entering their first RSV season who are not protected by maternal vaccination (MMWR 2025;74:No. 32).
- Dose: A single 105 mg IM injection (0.7 mL prefilled syringe) — the same dose for every infant regardless of body weight, which removes the <5 kg / ≥5 kg weight-tier calculation step that drives nirsevimab dosing errors.
- Target: Binds a distinct site on the RSV fusion (F) protein; unlike nirsevimab it does not use an Fc half-life-extension mutation, relying instead on high potency and slow clearance.
- Trials: Phase 2b/3 CLEVER and Phase 3 SMART demonstrated reductions in RSV-associated medically attended lower respiratory infection and in RSV hospitalization.
- Hard Age Limit: Clesrovimab is not recommended at or after 8 months of age. Children aged 8 through 19 months who remain at increased risk and are entering their second RSV season must receive nirsevimab, not clesrovimab — a frequent distractor.
The YTE Technology Explained
Nirsevimab incorporates a triple amino-acid substitution (M252Y / S254T / T256E, designated YTE) into the constant ($C_H2$) region of its Fc domain. This mutation dramatically increases the antibody's binding affinity to the neonatal Fc receptor (FcRn) at an acidic pH (6.0) inside endosomes, while preserving dissociation at neutral physiological pH (7.4) in extracellular fluid. As a result, endocytosed nirsevimab is efficiently diverted away from lysosomal degradation and recycled back into the systemic circulation, prolonging its terminal elimination half-life to 69–100 days.
Nirsevimab Dosing & ACIP Indications
- Season 1 (Universal): All infants <8 months of age born during or entering their first RSV season whose mothers did not receive the maternal RSV vaccine:
- Body weight < 5 kg: 50 mg IM as a single dose.
- Body weight ≥ 5 kg: 100 mg IM as a single dose.
- Season 2 (High-Risk Only): Children aged 8 through 19 months entering their second RSV season who remain at increased risk of severe RSV disease (chronic lung disease of prematurity requiring medical support, severe immunocompromise, cystic fibrosis with severe pulmonary involvement or weight-for-length <10th percentile, American Indian and Alaska Native children):
- Dose: 200 mg IM administered as two 100 mg injections at separate anatomical sites.
Palivizumab Restricted Modern Indications
Palivizumab (15 mg/kg IM monthly for up to 5 doses) is now reserved exclusively for clinical situations where nirsevimab is unavailable or contraindicated. Under traditional AAP guidance, qualifying criteria included:
- Infants born at <29 weeks, 0 days gestation who are <12 months old at season start.
- Infants with chronic lung disease of prematurity (CLD) (<32 weeks GA requiring >21% oxygen for ≥28 days after birth) who require medical support within 6 months.
- Infants with hemodynamically significant congenital heart disease (CHD) (cyanotic lesions, congestive heart failure requiring inotropes/diuretics, moderate-to-severe pulmonary hypertension).
4. Maternal RSV Immunization: Abrysvo (RSVpreF)
In 2023, the FDA approved and CDC/ACIP recommended the unadjuvanted bivalent RSV prefusion F protein vaccine (Abrysvo) for pregnant individuals.
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| Maternal Vaccine vs Infant Monoclonal Algorithm |
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| Clinical Scenario | Infant Recommendation |
|-------------------------------------------------------|---------------------------------|
| Mother received Abrysvo at 32-36 weeks GA | NO nirsevimab needed. |
| >=14 days prior to delivery | Infant protected by maternal IgG|
| Mother received Abrysvo <14 days prior to delivery | Give nirsevimab OR clesrovimab |
| | (Inadequate transplacental IgG) |
| Mother immunocompromised (e.g., solid organ transplant)| Administer nirsevimab to infant |
| Mother did NOT receive Abrysvo during pregnancy | Give nirsevimab OR clesrovimab |
| Infant undergoes cardiopulmonary bypass or ECMO | Re-dose nirsevimab (loss in CPB)|
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Maternal Timing Rationale
Abrysvo is administered strictly between 32 weeks, 0 days and 36 weeks, 6 days of gestation using seasonal administration (September through January in most of the continental US). This window maximizes transplacental IgG transport via FcRn receptors into the fetal circulation while eliminating the theoretical risk of preterm birth observed when given earlier in gestation.
5. Pediatric Influenza Pharmacotherapy
Antiviral treatment is recommended as early as possible for any child hospitalized with suspected or confirmed influenza, children with severe, complicated, or progressive illness, or children at high risk of influenza complications (e.g., age <2 years, chronic pulmonary disorders including asthma, congenital heart disease, neurodevelopmental conditions, immunosuppression, sickle cell disease, chronic kidney disease).
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| Pediatric Influenza Antiviral Pharmacotherapy |
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| Drug | Mechanism & Spectrum | Approved Age & Dosing |
|----------------------------|---------------------------------|--------------------------|
| Oseltamivir (Tamiflu) | Neuraminidase inhibitor | Treatment: Any age |
| (Oral suspension/capsule) | Influenza A and B | (including preterm/term) |
| | | Dosing tiered by weight/age
| Baloxavir marboxil | Cap-dependent endonuclease | Treatment & Prophylaxis: |
| (Xofluza) (Oral tablet) | inhibitor (Influenza A & B) | Age >= 5 years |
| | | Single oral dose |
| Zanamivir (Relenza) | Neuraminidase inhibitor | Treatment: >= 7 years |
| (Oral dry powder inhaler) | Influenza A and B | Prophylaxis: >= 5 years |
| | | CONTRAINDICATED in asthma|
| Peramivir (Rapivab) | Neuraminidase inhibitor | Treatment: >= 6 months |
| (Intravenous infusion) | Influenza A and B | Single IV dose: 12 mg/kg |
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Oseltamivir Pediatric Dosing Protocol (Treatment Duration: 5 Days)
- Preterm Infants (Postmenstrual age <38 weeks): 1 mg/kg/dose orally BID.
- Preterm Infants (Postmenstrual age 38 to 40 weeks): 1.5 mg/kg/dose orally BID.
- Term Infants 0 to 8 Months: 3 mg/kg/dose orally BID.
- Term Infants 9 to 11 Months: 3.5 mg/kg/dose orally BID.
- Children ≥1 Year (Weight-Tiered Dosing):
- ≤15 kg: 30 mg orally BID.
- >15 to 23 kg: 45 mg orally BID.
- >23 to 40 kg: 60 mg orally BID.
- >40 kg: 75 mg orally BID (standard adult dose).
Prophylaxis: Administer the same weight-tiered doses once daily for 7 to 10 days.
Baloxavir Marboxil (Xofluza) Clinical Nuances
- Approved for treatment and post-exposure prophylaxis in children ≥5 years of age.
- Weight-based single dose: 2 mg/kg for children <20 kg; 40 mg for 20 to <80 kg; 80 mg for ≥80 kg.
- Drug Interaction Warning: Co-administration with polyvalent metal cations (calcium, iron, magnesium, aluminum, zinc, dairy products, antacids, sucralfate) causes severe chelation, drastically reducing baloxavir bioavailability and leading to treatment failure.
Zanamivir Inhalation Warning
Zanamivir is delivered via an oral dry-powder diskhaler. It is strictly contraindicated in children with asthma, reactive airway disease, or chronic obstructive lung conditions due to documented cases of severe, fatal bronchospasm.
An infant born at 39 weeks gestation on November 10th is scheduled for a 2-week well-child check. The infant's mother received the bivalent unadjuvanted RSVpreF vaccine (Abrysvo) at 34 weeks gestation, exactly 28 days prior to delivery. The infant weighs 3.8 kg, is exclusively breastfed, and is clinically well. According to ACIP guidelines, what is the appropriate RSV immunoprophylaxis recommendation for this infant?
A 4-month-old infant weighing 6 kg is brought to the emergency department with a 3-day history of clear rhinorrhea and low-grade fever, progressing to tachypnea (respiratory rate 62 breaths/min), diffuse wheezing, intercostal retractions, and a room-air pulse oximetry of 93%. The infant is diagnosed with acute viral bronchiolitis. In accordance with the American Academy of Pediatrics (AAP) bronchiolitis clinical practice guidelines, which of the following represents the most appropriate management plan?
A 10-month-old infant weighing 8 kg presents to the clinic with a 24-hour history of sudden fever (39.5°C), cough, lethargy, and decreased oral intake during peak influenza season. Rapid molecular testing confirms Influenza A infection. The infant has a history of mild asthma. What is the most appropriate antiviral pharmacotherapy regimen for this infant?