Epidemiology and Transmission of COVID-19 in Shenzhen China: Analysis of 391 cases and 1,286 of their close contacts

Qifang Bi; Yongsheng Wu; Shujiang Mei; Chenfei Ye; Xuan Zou; Zhen Zhang; Xiaojian Liu; Lan Wei; Shaun A Truelove; Tong Zhang; Wei Gao; Cong Cheng; Xiujuan Tang; Xiaoliang Wu; Yu Wu; Binbin Sun; Suli Huang; Yu Sun; Juncen Zhang; Ting Ma; Justin Lessler; Teijian Feng

In Shenzhen, SARS-CoV-2 transmission is most likely between very close contacts, such as individuals sharing a household. However, even in this group less than 1 in 6 contacts were infected; and, overall, we observed far less than one (0.4) onward transmission per primary case. As noted above, low transmission levels may in part be due to the impact of isolation and surveillance; but it is equally likely unobserved transmission is playing some rule. We also estimate reasonably high rates of overdispersion in the number of cases each individual causes, leaving open the possibility that large COVID-19 clusters occur even if surveillance and isolation are forcing R below one; events that could potentially overwhelm the surveillance system. This work has numerous limitations. As in any active outbreak response, the data were collected by multiple teams under protocols that, by necessity, changed as the situation developed. Hence, there may be noise and inconsistency in definitions. Of note, the definition of a confirmed case changed to require symptoms near the end of our analysis period (Feb. 7); but sensitivity analyses show that truncating the data at this point does not qualitatively impact results. It is, likewise, impossible to identify every potential contact an individual has, so contact tracing focuses on those close contacts most likely to be infected; hence our observed R is assuredly less than the true R in the population. Asymptomatic travellers will be missed by symptom-based surveillance; and, even if tested, some asymptomatic contacts may be missed due to the imperfect sensitivity of the PCR test 20 .