ABSTRACT
Objective To explore the transmission chain of COVID-19 by serum antibody detection, and to provide scientific evidence for the prevention and control of the epidemic. Methods Field epidemiological investigation was used to determine the COVID-19 cases and their close contacts. The 2019-nCoV nucleic acid in throat swabs and anal swabs were examined by RT-PCR. Serum specimens were collected for anti-2019-nCoV IgM antibody detection and combined IgM/IgG detection. Results Case A had no confirmed exposure to COVID-19. However, case C and D had dinner and lived together with case A; they reported contact history and dinner history with other confirmed COVID-19 cases(H, L). Case A tested positive for 2019-nCoV nucleic acid, whereas case C and D were negative. Moreover, case A and C were IgM antibody positive, while case D was negative. Case A, C and D were all positive for combined IgM/IgG. In addition, case D had clinical symptom, while case C did not. Conclusion Serum antibody detection can be used as an effective supplement to the inference of transmission chain of COVID-19, which may facilitate determining the source of infection and improving the evidence.