Criteria

World Health Organization: COVID-19 disease severity[85]

Mild illness

  • Symptomatic patients meeting the case definition for COVID-19 without evidence of hypoxia or pneumonia.

  • Common symptoms include fever, cough, fatigue, anorexia, dyspnoea, and myalgia. Other non-specific symptoms include sore throat, nasal congestion, headache, diarrhoea, nausea/vomiting, and loss of smell/taste. Additional neurological manifestations reported include dizziness, agitation, weakness, seizures, or findings suggestive of stroke. Children may not report fever or cough as frequently as adults.

  • Older people and immunosuppressed people may present with atypical symptoms (e.g., fatigue, reduced alertness, reduced mobility, diarrhoea, loss of appetite, delirium, absence of fever).

  • Symptoms due to physiological adaptations of pregnancy or adverse pregnancy events (e.g., dyspnoea, fever, gastrointestinal symptoms, fatigue) or other diseases (e.g., malaria) may overlap with COVID-19 symptoms.

Moderate disease

  • Adolescent or adult: clinical signs of pneumonia (i.e., fever, cough, dyspnoea, fast breathing) but no signs of severe pneumonia, including blood oxygen saturation levels (SpO₂) ≥90% on room air.

  • Children: clinical signs of non-severe pneumonia (i.e., cough or difficulty breathing plus fast breathing and/or chest indrawing) and no signs of severe pneumonia. Fast breathing is defined as:

    • <2 months of age: ≥60 breaths/minute

    • 2-11 months of age: ≥50 breaths/minute

    • 1-5 years years of age: ≥40 breaths/minute.

  • While the diagnosis can be made on clinical grounds, chest imaging may assist in diagnosis and identify or exclude pulmonary complications.

Severe disease

  • Adolescent or adult: clinical signs of pneumonia (i.e., fever, cough, dyspnoea, fast breathing) plus one of the following:

    • Respiratory rate >30 breaths/minute

    • Severe respiratory distress

    • SpO₂ <90% on room air.

  • Children: clinical signs of pneumonia (i.e., cough or difficulty in breathing) plus at least one of the following:

    • Central cyanosis or SpO₂ <90%

    • Severe respiratory distress (e.g., fast breathing, grunting, very severe chest indrawing)

    • General danger signs: inability to breastfeed or drink, lethargy or unconsciousness, or convulsions

    • Fast breathing (<2 months: ≥60 breaths per minute; 2-11 months: ≥50 breaths per minute; 1-5 years: ≥40 breaths per minute).

  • While the diagnosis can be made on clinical grounds, chest imaging may assist in diagnosis and identify or exclude pulmonary complications.

Critical disease

  • Presence of acute respiratory distress syndrome (ARDS), sepsis, septic shock, acute thrombosis, or multisystem inflammatory syndrome in children.

World Health Organization: hospitalisation risk for patients with non-severe disease[398]

Several recommendations for people with non-severe disease, specifically for the use of antiviral drugs (i.e., nirmatrelvir/ritonavir, molnupiravir, remdesivir), are stratified by the likelihood of hospital admission. The criteria for deciding whether a patient is at high, moderate, or low risk of hospitalisation are detailed below.

  • High risk (6%) of hospitalisation

    • Diagnosed immunodeficiency syndromes.

    • History of solid organ transplant and receiving immunosuppressants.

    • Autoimmune illness and receiving immunosuppressants.

  • Moderate risk (3%) of hospitalisation

    • Age ≥65 years.

    • Obesity.

    • Diabetes and/or chronic cardiopulmonary disease.

    • Chronic kidney or liver disease.

    • Active cancer.

    • Disabilities.

    • Comorbidities of chronic disease.

  • Low risk (0.5%) of hospitalisation

    • Not at moderate or high risk of hospitalisation (most patients are low risk).

National Institutes of Health: clinical classification of COVID-19[644]​

Asymptomatic or presymptomatic infection

  • People who test positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) using a virological test but have no symptoms consistent with COVID-19.

Mild illness

  • People who have any of various signs and symptoms (e.g., fever, cough, sore throat, malaise, headache, muscle pain, nausea, vomiting, diarrhoea, loss of taste and smell) without shortness of breath, dyspnoea, or abnormal chest imaging.

Moderate illness

  • People who have evidence of lower respiratory disease by clinical assessment or imaging and an oxygen saturation (SpO₂) ≥94% on room air at sea level.

Severe illness

  • People who have respiratory rate >30 breaths per minute, SpO₂ <94% on room air at sea level, ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂) <300 mmHg, or lung infiltrates >50%.

Critical illness

  • People who have respiratory failure, septic shock, and/or multiple organ dysfunction.

Infectious Diseases Society of America: disease severity definitions[465]

Mild-to-moderate illness

  • Patients with a SpO2 >94% not requiring supplemental oxygen.

Severe illness

  • Patients with SpO2 ≤94% on room air, including patients on supplemental oxygen.

Critical illness

  • Patients on mechanical ventilation and extracorporeal mechanical oxygenation (ECMO).

  • Includes end-organ dysfunction as is seen in sepsis/septic shock. The most commonly reported form of end-organ dysfunction is acute respiratory distress syndrome (ARDS).

Case definitions

Various case definitions are available:

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