2.4. Data synthesis

Baseline characteristics were compared with primary outcomes describing the efficacy and safety of CPT in patients. For pooled analysis, the parameters considered as primary outcomes included status at 7 and 30 days after intervention and serious transfusion‐related adverse effects. Status after intervention is classified into four groups: discharged, hospitalized, deceased and alive. Outcomes are defined as additional data used to assess patients' improvement after intervention, including laboratory findings, time to negative viral titre and oxygenation.

In meta‐analysis, primary outcomes included mortality and discharge rates, while secondary outcomes included clinical improvement and viral nucleic acid negative rates in the treated groups (CPT‐receiving patients) versus control groups (standard care alone). Mortality is defined as a cumulative number of deaths after 30‐day intervention. Discharge rate is defined as the number of patients discharged from the hospital after 7 or 28 days after intervention. Clinical improvement is defined as an increase by 6 or 8 points in WHO disease severity scale, 26 and improvement of oxygenation at 14 days after intervention. Viral nucleic acid negative rate is defined as the number of patients with undetectable viral load via polymerase chain reaction (PCR) assay during 1, 2, 3 or 7 days after intervention. The above‐mentioned analysis was performed for all diseases severity, time‐to‐transfusion, and antibody titre; furthermore, we also did meta‐analysis of possible confounding factors which might affect CPT outcomes, including diseases severity and convalescent plasma antibody titre. Disease severity was classified into mild, moderate, severe and critical based on its clinical manifestations. Mild symptoms in COVID‐19 patients are characterized by fever <38°C, with or without cough, no dyspnoea, no gasping, no chronic disease and no imaging findings of pneumonia. Moderate symptoms of COVID‐19 are when patients developed fever, respiratory symptoms, with imaging findings of pneumonia. Severe signs of COVID‐19 in patients are characterized by respiratory distress, suggested by tachypnoea of ≥30 breaths per minute in resting state, oxygen saturation of 93% or less in room air, or arterial partial pressure of oxygen (PaO₂)/fraction of inspired oxygen (FiO₂) of 300 or less. Critical symptoms of COVID‐19 are characterized by respiratory failure requiring mechanical ventilation, shock or other organ failure (apart from lung), leading to the necessity of intensive care unit (ICU) monitoring. 27 , 28 Subgroup analysis was also performed according to the types of diseases (COVID‐19, influenza, SARS and MERS).