Serial interval and generation interval for respectively the imported and local infectors estimated using reported contact-tracing data of COVID-19 in China

Li, Menghui; Liu, Kai; Song, Yukun; Wang, Ming; Wu, Jinshan

Backgrounds: The emerging virus, COVID-19, has caused a massive outbreak worldwide. Based on the publicly available contact-tracing data, we identified 337 transmission chains from 10 provinces in China and estimated the serial interval (SI) and generation interval (GI) of COVID-19 in China. Methods: Inspired by possibly different values of the time-varying reproduction number for the imported cases and the local cases in China, we divided all transmission events into three subsets: imported (the zeroth generation) infecting 1st-generation locals, 1st-generation locals infecting 2nd-generation locals, and others transmissions among 2+ generations. The corresponding SI (GI) is respectively denoted as $\mbox{SI}^{0}_{1}$ ($\mbox{GI}^{0}_{1}$), $\mbox{SI}^{1}_{2}$ ($\mbox{GI}^{1}_{2}$), and $\mbox{SI}^{2+}_{3+}$ ($\mbox{GI}^{2+}_{3+}$). A Bayesian approach with doubly interval-censored likelihood is employed to fit the lognormal, gamma, and Weibull distribution function of the SI and GI using the identified 337 transmission chains. Findings: It is found that the estimated $\mbox{SI}^{0}_{1}$ = 6.27 (95%CI : 5.62 - 6.98), $\mbox{SI}^{1}_{2}$ = 5.92 (95%CI : 5.27 - 6.66), $\mbox{SI}^{2+}_{3+}$ = 4.36 (95%CI : 3.65 - 5.20), and $\mbox{GI}^{0}_{1}$ = 8.15 (95%CI : 6.73 - 9.91), $\mbox{GI}^{1}_{2}$ = 5.26 (95%CI : 4.25 - 6.69), $\mbox{GI}^{2+}_{3+}$ = 3.85 (95%CI : 2.18 - 6.72), thus overall both SI and GI decrease when generation increases.