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Definition of disease severity of COVID19

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Authoring team

Definition of Disease Severity of COVID19 is detailed below:

COVID-19 disease severity definitions according to the World Health Organization (WHO COVID-19 clinical management: living guidance).

Mild disease


Patients with symptoms meeting the case definition for COVID-19 without evidence of viral pneumonia or hypoxia.
See the WHO website for the most up-to-date case definitions.
Presenting signs and symptoms of COVID-19 vary:

  • Most people experience fever (8% to 99%), cough (59% to 82%), fatigue (44% to 70%), anorexia (40% to 84%), shortness of breath (31% to 40%), myalgias (11% to 35%). Other non-specific symptoms, such as sore throat, nasal congestion, headache, diarrhoea, nausea and vomiting, have also been reported.
  • Loss of smell (anosmia) or loss of taste (ageusia) preceding the onset of respiratory symptoms have also been reported.
  • Additional neurological manifestations reported include dizziness, agitation, weakness, seizures or findings suggestive of stroke including trouble with speech or vision, sensory loss, or problems with balance when standing or walking.
  • Older people and people who are immunosuppressed in particular may present with atypical symptoms such as reduced alertness, reduced mobility, diarrhoea, loss of appetite, confusion and absence of fever.
  • Symptoms such as dyspnoea, fever, gastrointestinal symptoms or fatigue because of physiological adaptations in women who are pregnant, adverse pregnancy events or other diseases such as malaria, may overlap with symptoms of COVID-19.
  • Children may report fever or cough less frequently than adults.

Moderate disease


Adolescents or adults with clinical signs of pneumonia (fever, cough, dyspnoea, fast breathing) but no signs of severe pneumonia, including SpO2 90% or more on room air.

Children with clinical signs of non-severe pneumonia (cough or difficulty breathing plus fast breathing or chest indrawing) and no signs of severe pneumonia.

Fast breathing (in breaths per minute): under 2 months: 60 or more; 2 months to 11 months: 50 or more; 1 year to 5 years: 40 or more.

While the diagnosis can be made on clinical grounds, chest imaging (radiograph, CT scan or ultrasound) may assist in diagnosis, and may identify or exclude pulmonary complications.

Severe disease

Severe pneumonia

Adolescents or adults with clinical signs of pneumonia (fever, cough, dyspnoea, fast breathing) plus 1 of the following: respiratory rate more than 30 breaths per minute; severe respiratory distress; or SpO2 less than 90% on room air.

Children with clinical signs of pneumonia (cough or difficulty in breathing) plus at least 1 of the following:

  • Central cyanosis or SpO2 less than 90%; severe respiratory distress (for example, fast breathing, grunting, very severe chest indrawing); general danger sign: inability to breastfeed or drink, lethargy or unconsciousness, or convulsions.
  • Fast breathing (in breaths per minute): less than 2 months: 60 or more; 2 months to 11 months: 50 or more; 1 year to 5 years: 40 or more.

While the diagnosis can be made on clinical grounds, chest imaging (radiograph, CT scan or ultrasound) may assist in diagnosis and identify or exclude pulmonary complications.

Critical disease

Acute respiratory distress syndrome (ARDS)

Onset: within 1 week of a known clinical insult (that is, pneumonia) or new or worsening respiratory symptoms.

Chest imaging (radiograph, CT scan or ultrasound): bilateral opacities, not fully explained by volume overload, lobar or lung collapse, or nodules.

Origin of pulmonary infiltrates: respiratory failure not fully explained by cardiac failure or fluid overload. Need objective assessment (for example, echocardiography) to exclude hydrostatic cause of infiltrate or oedema if no risk factor present.

Oxygenation impairment in adults:

  • Mild ARDS: 200 mmHg less than PaO2/FiO2 300 mmHg or less (with PEEP or CPAP 5 cmH2O or more).
  • Moderate ARDS: 100 mmHg less than PaO2/FiO2 200 mmHg or less (with PEEP 5 cmH2O or more).
  • Severe ARDS: PaO2/FiO2 100 mmHg or less (with PEEP 5 cmH2O or more).

Oxygen impairment in children: note OI and OSI. Use OI when available. If PaO2 is not available, wean FiO2 to maintain SpO2 97% or less to calculate OSI or SpO2/FiO2 ratio:

  • Bi-level (NIV or CPAP) more than or equal to 5 cmH2O via full face mask: PaO2/FiO2 300 mmHg or less or SpO2/FiO2 264 or less.
  • Mild ARDS (invasively ventilated): 4 <=OI < 8 or 5 <=OSI < 7.5. (OI greater than or equal to 4 and less than 8, or OSI greater than or equal to 5 and less than 7.5).
  • Moderate ARDS (invasively ventilated): 8 <=OI < 16 or 7.5 <=OSI < 12.3. (OI greater than or equal to 8 and less than 16, or OSI greater than or equal to 7.5 and less than 12.3).
  • Severe ARDS (invasively ventilated): OI >= 16 or OSI >= 12.3. (OI greater than or equal to 16, or OSI greater than or equal to 12.3).

Critical disease


Adults with acute life-threatening organ dysfunction caused by a dysregulated host response to suspected or proven infection. Signs of organ dysfunction include: altered mental status, difficult or fast breathing, low oxygen saturation, reduced urine output, fast heart rate, weak pulse, cold extremities or low blood pressure, skin mottling, laboratory evidence of coagulopathy, thrombocytopaenia, acidosis, high lactate and hyperbilirubinaemia.

Children with suspected or proven infection and 2 or more age-based systemic inflammatory response syndrome (SIRS) criteria, of which 1 must be abnormal temperature or white blood cell count.

Critical disease

Septic shock

Adults with persistent hypotension despite volume resuscitation, requiring vasopressors to maintain MAP 65 mmHg or more and serum lactate level of more than 2 mmol/litre.

Children with any hypotension (SBP below fifth centile or more than 22 SD below normal for age) or 2 or 3 of the following: altered mental status; bradycardia or tachycardia (HR less than 90 bpm or more than 160 bpm in babies and heart rate less than 70 bpm or more than 150 bpm in children); prolonged capillary refill (more than 2 seconds) or weak pulse; fast breathing; mottled or cool skin or petechial or purpuric rash; high lactate; reduced urine output; hyperthermia or hypothermia.


[1] If altitude is higher than 1000 m, then the correction factor should be calculated as follows: PaO2/FiO2 x barometric pressure/760. [2] When PaO2 is not available, SpO2/FiO2 315 or less suggests ARDS (including in non-ventilated patients). [3]Oxygenation index (OI) is an invasive measurement of the severity of hypoxaemic respiratory failure and may be used to predict outcomes in children. It is calculated as follows: percentage of fraction of inhaled oxygen multiplied by the mean airway pressure (in mmHg), divided by the partial pressure of arterial oxygen (in mmHG). Oxygen saturation index (OSI) is a non-invasive measurement and has been shown to be a reliable surrogate marker of OI in children and adults with respiratory failure. OSI replaces PaO2 with oxygen saturation as measured by pulse oximetry (SpO2) in the OI equation. [4] SIRS criteria: abnormal temperature (more than 38.5°C or less than 36°C); tachycardia for age or bradycardia for age if less than 1 year; tachypnoea for age or need for mechanical ventilation; abnormal white blood cell count for age or more than 10% bands.

For full details then see:

NICE guideline [NG191].COVID-19 rapid guideline: managing COVID-19

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