Inclusion Criteria - for VESIP:

• Admission to PICU or SDU at any point during illness due to severity (not due to bed availability).
• 8 months to 17 years old, inclusive.
• Severe Acute Respiratory Infection:
  • > 1 sign of acute infection: fever or hypothermia (≥38°C or ≤5°C), WBC elevated or decreased for age, elevated CRP or procalcitonin, altered mental status); and
  • > 1 sign of acute respiratory illness: cough, shortness of breath, tachypnea or retractions, invasive or non-invasive mechanical ventilation, need for FiO2 to maintain SpO2 >92%, pulmonary infiltrate or hyperinflation on chest imaging.
• Onset of symptoms < 10 days prior to enrollment.
• Respiratory specimen for testing of influenza collected or performed within 72 hours of hospital admission.
• At least 1 parent or legal guardian able and willing to provide permission.

Exclusion Criteria - for VESIP:

• Previous enrollment in this study during the current season.
• Chronic mechanical ventilator support through a tracheostomy for > 12 hours per day.
• End stage lung disease awaiting transplant.
• Evidence of current pregnancy from testing or other documentation.
• > 72 hours since hospital presentation before enrollment, unless an upper respiratory sample that was collected ≤ 72 hours since hospital presentation (for clinical purposes) is available for testing for influenza status.