This randomized, double-blind, placebo-controlled phase 2 trial will be conducted at a single tertiary pediatric intensive care unit (PICU). The study will include children with RSV infection who were admitted to the pediatric intensive care unit and require respiratory support via positive pressure ventilation (invasive and noninvasive).

Eligible participants include all children admitted to the PICU at Children's of Alabama with a diagnosis of RSV infection and requiring positive pressure ventilation, invasive or noninvasive, including bilevel positive airway pressure (BiPAP) or high flow nasal cannula (HFNC) oxygen (ie, >1 L/kg/min of flow, with 5 L/min flow for children weighing <5 kg). During hospitalization, all patients will be treated according to the American Academy of Pediatrics guidelines for the management of bronchiolitis, primarily supportive care. Participants will then be randomized according to a permuted-block design to receive either placebo (saline) or AZM (Fresenius Kabi) at 10mg/kg/d (ie, standard dose) or 20mg/kg/d (ie, high dose) intravenously every 24 hours for 3 days. All biologic samples collected will be analyzed in the PI's lab at the University of Alabama at Birmingham. Drug pharmacokinetics will be performed at the Pharmaceutical Sciences Research Institute of Samford University, Birmingham, AL.

Inclusion Criteria:

• Admission to the PICU with RSV infection
• Need for positive pressure ventilation (invasive and non-invasive)
• Randomization and drug/placebo initiation within 48 hours of admission to Pediatric Intensive Care Unit

Exclusion Criteria:

• Azithromycin use within 7 days of PICU admission

• Contraindication to azithromycin use including:

  • Patients with electrocardiogram QT interval corrected for heart rate (Qtc) ≥ 450 ms
  • Patients with significant hepatic impairment (direct bilirubin >1.5 mg/dL)
  • Known hypersensitivity to azithromycin, erythromycin, any macrolide, or ketolide drug
  • Cardiac arrhythmia

• History of pyloric stenosis

• Immunocompromised children (any cause)

• Current use of any medication known to cause QT prolongation

Participant, clinical providers and study staff are masked to the intervention. The random assignment of treatment was determined by random permutation. A statistician generated the randomization schedule and provided the randomization schedule (with randomization IDs) in an excel file to the study pharmacist in charge of dispensing the appropriate treatment.