Temporal dynamics in viral shedding and transmissibility of COVID-19

Xi He; Eric HY Lau; Peng Wu; Xilong Deng; Jian Wang; Xinxin Hao; Yiu Chung Lau; Jessica Y Wong; Yujuan Guan; Xinghua Tan; Xiaoneng Mo; Yanqing Chen; Baolin Liao; Weilie Chen; Fengyu Hu; Qing Zhang; Mingqiu Zhong; Yanrong Wu; Lingzhai Zhao; Fuchun Zhang; Benjamin J Cowling; Fang Li; Gabriel M Leung

We used a published estimate of the incubation period distribution to infer infectiousness with respect to symptom onset, from the first 425 COVID-19 patients in Wuhan with detailed exposure history. 2 We considered that infected cases would become infectious at a certain time point before or after illness onset (tS1). Infectiousness, i.e. transmission probability to a secondary case, would then increase until reaching its peak (Figure 1 ). The transmission event would occur at time tI with a probability described by the infectiousness profile β(tI -tS1) relative to the illness onset date, assuming a gamma distribution. The secondary case would then show symptoms at time tS2, after the incubation period that is assumed to follow a lognormal distribution g(tS2 -tI). Hence the observed serial intervals distribution f(tS2 -tS1) would be the convolution between the infectiousness profile and incubation period distribution. We constructed a likelihood function based on the convolution which was fitted to the observed serial intervals, allowing for the start of infectiousness around symptom onset. Parameters were estimated using maximum likelihood. We also performed sensitivity analyses by fixing the start of infectiousness at day 4 and 7 respectively before symptom onset and inferred the infectiousness profile.