Severity classification | Description |
Tier 1: Mild illness | No new or increased* supplemental oxygen requirements |
Tier 2: Moderate illness | New or increased* requirement for supplemental oxygen via nasal cannula |
Tier 3: Severe illness | New or increased* requirement for supplemental oxygen via high-flow nasal cannula or noninvasive ventilation¶ |
Tier 4: Critical illness | Supplemental oxygen via mechanical ventilation or ECMO |
ECMO: extracorporeal membrane oxygenation; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2.
* Increased from baseline requirement.
¶ Continuous or bilevel positive airway pressure.