A total of 103 patients were enrolled in this randomized clinical trial. They were assigned to either the convalescent plasma group or the control group in a 1:1 ratio and were categorized as follows: 23 patients in the convalescent plasma group and 22 patients in the control group had severe COVID-19, and 29 patients in the convalescent plasma group and 29 patients in the control group had life-threatening COVID-19. The median age was 70 years (IQR, 62-78 years) among all patients, 71 years (IQR, 66-82 years) for patients with severe COVID-19, and 69 years (IQR, 61-76 years) for patients with life-threatening COVID-19. Of the patients included in the study, 60 (58.3%) were men, and the percentages of men with severe and life-threatening COVID-19 were 53.3% and 62.1%, respectively. A total of 89.2% of the patients had a normal body temperature at the time of participation, and the median body temperature was 36.5 °C (IQR, 36.2-36.7 °C). Primary Clinical Outcome: For all patients combined, there was no significant difference in the primary outcome of time to clinical improvement within 28 days: 51.9% (27/52) in the convalescent plasma group vs 43.1% (22/51) in the control group. Secondary Clinical Outcomes: There was no significant difference in the secondary outcome of 28-day mortality (15.7% in the convalescent plasma group vs 24.0% in the control group).