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Table 2 COVID-19 severity classifications across different guidelines harmonized according to WHO’s classification of severity, guided by WHO case definition

From: Role of adjunctive treatment strategies in COVID-19 and a review of international and national clinical guidelines

WHO classification and case definitionNHC (China)SIMIT Lombardy section (Italy)SWAB (Netherlands)Sciensano (Belgium)
Mild“Mild”“Mild respiratory symptoms”“Mild”“Mild to moderate”
- Uncomplicated upper respiratory tract viral infection- Mild clinical symptoms
- No radiologic signs of pneumonia
- Fever (> 37.5 °C), cough, no dyspnoea- No supplemental oxygen required- no oxygen requirement or no evidence of pneumonia
Pneumonia“Common” or “Regular”“Moderate respiratory symptoms”Nil 
- Pneumonia but no signs of severe pneumonia
- No need for supplemental oxygen
- Fever, symptoms of respiratory tract infection
- Signs of pneumonia on imaging
- Fever (> 37.5 °C), cough, mild to moderate dyspnoea, and/or
- Pneumonia on chest x-ray
- Mild respiratory symptoms in age >70 years and/or co-morbidities with increased mortality risk
Severe“Severe”Nil“Moderately severe”“Severe”
- Fever or suspected respiratory infection, plus 1 of the following:
• RR > 30/min;
• Severe respiratory distress
• SpO2 ≤ 93% on room air
- Dyspnoea, respiratory rate > 30/min
- SpO2 < 93% at rest, or
- PaO2/FiO2 ratio < 300 mmHg
 - Requires monitoring in ward
- Supplemental oxygen required
≥1 of the following:
- RR ≥30/min
- SpO2 ≤ 93%
- PaO2/FiO2 ratio < 300 mmHg
- Lung infiltrates > 50% of the lung field within 24–48 h
Critical“Critical”“Critically ill”“Very severe”“Critical”
- ARDS, or
- Sepsis with acute organ dysfunction
- Respiratory failure requiring mechanical ventilation, or
- Presence of shock, or
- Multi-organ failure requiring monitoring in ICU
- Respiratory failure
- Haemodynamic failure
(re-classified under WHO’s “critical” category)
- Monitoring in ICU required, or
- ECMO required, or
- Clinical deterioration from moderate severity with initial anti-viral therapy
≥1 of the following:
- Sepsis
- Altered consciousness
- Multi-organ failure
  1. WHO World Health Organization, NHC National Health Commission, SIMIT Società Italiana di Malattie Infettive e Tropicali (Italian Society of Infectious and Tropical Diseases), SWAB Stichting Werkgroep Antibiotica Beleid (Working Party on Antibiotic Policy), RR Respiratory rate, SpO2 Peripheral oxygen saturation, PaO2 Partial pressure of arterial oxygen, FiO2 Fraction of inspired oxygen, ARDS Acute respiratory distress syndrome, ICU Intensive care unit, ECMO Extra-corporeal membrane oxygenation