Dynamic changes of IgM and IgG antibodies in asymptomatic patients as an effective way to detect SARS-CoV-2 infection.

Background: COVID-19 has become a global pandemic, and close contacts and asymptomatic patients are worthy of attention.

Methods: A total of 1844 people in close contacts with 76 COVID-19 patients were investigated, and nasopharyngeal swabs and venous blood were collected for centralized medical quarantine observation. Real-time fluorescence was used to detect SARS-CoV-2 nucleic acid in nasopharyngeal swabs of all close contacts, and the colloidal gold method was used to detect serum-specific antibodies. Levels of IgM- and IgG-specific antibodies were detected quantitatively through chemiluminescence from the first nucleic acid turned negative date (0 week) and on weekly intervals of ≤1 week, 1-2 weeks, 2-3 weeks, 3-4 weeks, 4-5 weeks, 5-6 weeks, and 6-7 weeks.

Results: The total positive rate of the colloidal gold method (88.5%, 23/26) was significantly higher (χ  = 59.182, p < 0.001) than that of the healthy control group (2.0%, 1/50). There was significant difference in IgG concentration at different time points (0-7 weeks) after negative nucleic acid conversion (χ  = 14.034, p = 0.029). Serum IgG levels were significantly higher at weekly time points of 4-5 weeks (Z = -2.399, p = 0.016), 5-6 weeks (Z = -2.049, p = 0.040), and 6-7 weeks (Z = -2.197, p = 0.028) compared with 1-2 weeks after negative nucleic acid conversion. However, there was no significant difference (χ  = 4.936, p = 0.552) in IgM concentration between time points tested (0-7 weeks) after negative nucleic acid conversion. The positive rates of IgM and IgG in asymptomatic patients (χ  = 84.660, p < 0.001) were significantly higher than those in the healthy control group (χ  = 9.201, p = 0.002) within 7 weeks of negative nucleic acid conversion.

Conclusions: The IgG concentration in asymptomatic cases remained at a high level after nucleic acid turned negative. Nucleic acid detection combined with IgM and IgG antibody detection is an effective way to screen asymptomatic infections.