Definition of moderate and severe COVID-19 infection
All patients included in the present study had: i) A positive nasopharyngeal swab for COVID-19, ii) thoracic CT findings compatible with COVID-19 pneumonia and iii) hospitalized due to COVID-19 infection. Patients fulfilling one or more of the following criteria were accepted as severe COVID-19 infection: i) A respiratory rate >30 breaths/minute signifying respiratory distress, ii) a resting oxygen saturation 93% or less, iii) ratio of partial arterial oxygen saturation to the fraction of inspired oxygen <300 mmHg and iv) respiratory failure or a critical life-threatening complication of COVID-19 necessitating admission to intensive care unit. Patients who did not fulfill these criteria were accepted as having a moderate COVID-19 infection. Subjects within the control group had a negative nasopharyngeal swab for COVID-19, with or without a negative CT scan for COVID-19 pneumonia.