Abstract: Objective To investigate and analyze the infection in family cluster outbreaks of COVID-19, so as to provide scientific basis for preventing and controlling the spread of the epidemic. Methods The nucleic acid of SARS-CoV-2 was detected by RT-PCR in respiratory tract samples. Serum samples were tested for antibody of SARS-CoV-2 by the colloidal gold method. The field epidemiological method was used to investigate the exposure condition of cases and family close contacts in 3 family cluster outbreaks in County Y, exploring the source of infection in each family and analyzing transmission chain. Results Seven confirmed cases, 1 case of asymptomatic infection, and 2 cases with positive SARS-CoV-2 antibody were reported from 3 families. The time intervals between the date of infection and the date of onset of the confirmed cases were within the incubation period of COVID-19. There was no epidemiological history of COVID-19 affected area in each family′s index case. The case in Family 1 had the earliest onset. The source of infection in Family 2 had a history of dining together with the source of infection in Family 1. The source of infection in Family 3 had close contact with the second generation case in Family 1. Conclusion The infectious source （first generation case） of the 3 family cluster COVID-19 outbreaks was case C in Family 1. At the early stage of prevention and control of the epidemic, isolation of the source of infection, less aggregation, and strengthening personal protection can effectively contain the spread of the epidemic.

Key words: COVID-19, Familial clustering, Infection source, Transmission chain, Epidemiology