[Investigation of transmission chain of a cluster COVID-19 cases]

Objective: To analyze of the transmission characteristics of a cluster of COVID-19 cases in Chongqing and evaluate the infectivity of COVID-19 in the incubation period. Methods: A retrospective survey was conducted by using unified questionnaire through field and telephone interviews among 129 close contacts of COVID-19 cases. The relationship of transmission was indicated by transmission chain, and the infectivity was analyzed by the contact history. Results: This cluster of COVID-19 cases occurred after a classmate party involving members in three families and work fellows in a factory (R(0)=3.8). The infection rate during the incubation period was 17.57%. On average, it was infectious three days before onset. There was significant difference in infection rate among different contact modes (χ(2)=15.10, P<0.01),There was significant difference in infection rate among single exposureswith different time length ( χ(2)=25.08, P<0.01). Conclusions: COVID-19 is highly infectious in the incubation period. The more confined the space is, the higher the risk is,and the longer the single exposure is, the higher the risk of transmission is. Indirect contact transmission still exists.