It is important to exactly identify the difference in the changes in a receptor C5aR1-levels at onset and Immunoglobulin G (IgG)-levels after recovery between severe (acute respiratory distress syndrome) and nonsevere (pneumonia) coronavirus disease 2019 patients to reduce the severity of the disease and prevent reinfection with the causative virus. 2012 literature studies were selected from the PubMed Central® databases with keywords of (((covid-19) AND individual) NOT review) AND C5a / IgG. Three studies showing individual expression levels of C5a/C5aR1 / antibody in the patients before and after the passage of time without significance were selected by assessing the literatures. We extracted dynamics where the slope values of the regression line between the initial (X) and changed level (C) with passage of time were not zero significantly (p < 0.05) using SAS-JMP-10. We examined the significance of C in the patients with Ishida’s t-test1 and that of the difference in C between the severe and nonsevere patients with Ishida’s t-test2, which used the values on the above regression line as the expected values of C. These tests suggested a greater increase in C5aR1-levels at onset and a smaller decrease in IgG-levels after recovery in severe patients than in nonsevere patients.