Antibodies and antigen kinetics in patients with COVID-19

The antigen and antibody kinetics of SARS-CoV-2 infected patients remains unclear, and the clinical values of the serological test have not been completely elucidated. A total of 154 serum samples from 13 patients with COVID-19 infection were collected at about three-day intervals during hospitalization. Samples were screened for SARS-CoV-2-specific total antibodies (TAb), IgA, IgM, IgG, and antigen (Ag) using chemiluminescent microparticle immunoassays (CMIA). The overall seroconversion and/or four-fold increase rates of TAb, IgA, IgM, and IgG during hospitalization were 92.31%, 92.31%, 84.62%, and 92.31%, respectively. However, within a week of onset, antibodies were present in <50% of the patients. The combination of “Ag and/or TAb” maintained the positive rate at 81.82% during the first three days after symptom onset and quickly enhanced to 92.31% during 4–6 days after the symptom onset. The seropositive median day of Ag was two days after symptom onset. Among patients who underwent IgM and IgG seroconversion, the seroconversion median days of IgA, TAb, IgM, and IgG were 9.5 days, 10 days, 11 days, and 11.5 days after the symptom onset, respectively. Serological testing, especially virus-specific antigen testing, may be helpful for early identification of suspected patients and asymptomatic infections.