The values of antibodies and complements in blood serum in Qin's [13] study showed that immunoglobulins (IgA, IgG and IgM) and complement proteins (C3 and C4) of COVID-19 patients are within the normal range. Compared with the non-severe group, the IgM of the severe group was only slightly decreased, and there was no significant difference in other immunoglobulins and complement, which was consistent with the results of Feng's study [14].