[A follow up study of total IgM, IgG, nucleoprotein and spike protein antibodies against severe acute respiratory syndrome (SARS) coronavirus in patients with SARS]

Abstract

Objective: To survey the dynamic changing and persistence of the special antibodies, including total IgM, IgG, nucleocapsid protein and spike protein antibodies, against severe acute respiratory syndrome coronavirus (SARS-CoV) in patients with SARS.

Methods: 146 cases, all clinically diagnosed as SARS with positive SARS-CoV IgG, were followed up. 362 serum samples were collected from the onset of the disease to 660 days afterward. Total IgM and IgG against SARS-CoV were tested with commercial ELISA kits. For recombinant nucleoprotein and spike protein, we developed an ELISA to test these two antibodies.

Results: Within 20 days of the onset, the positive rate of anti-SARS-CoV IgM was 46.5% (20/43); it reached a peak after 21 - 40 days (80.6%, 25/31). Then, the positive rate of IgM went down gradually to 8.2% (6/73) until 550 days after the onset. The patient's IgG positive rate was lower (34.9%, 15/43) than that of IgM within 20 days of the onset. Then it went up rapidly to 100%. It remained positive (98.6%, 70/71) until 600 - 660 days after the onset. When N-IgG and S-IgG were tested 40 days after the onset of the disease at three different times, the positive rate of N-IgG (92.5%, 37/40) was higher than that of S-IgG (67.5%, 27/40), but the two structure protein antibodies were always lower than the total IgG.

Conclusions: In SARS patients with definite clinical and etiological diagnosis, the highest positive rate of the antibodies against SARS-CoV was found at 21 - 40 days after the onset. IgM disappeared almost 500 days (91.8%) after the onset. Total IgG positive rate could reach 100% and 98.6% and the positivity might persist nearly two years. So it is speculated that the total IgG antibody may be positive 3 to 5 years after infection, but it seems that N-IgG and S-IgG keep positive shorter in time than total IgG antibody.