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(2) = 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(2) = 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(2) = 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(2) = 84.660, p < 0.001) were significantly higher than those in the healthy control group (χ(2) = 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.