LIU Zi-yan, HU Shi-xiong, LUO Kai-wei, DAI Zhi-hui, ZHAO Shan-lu, ZENG Ge, YANG Hao, SUN Qian-lai, ZHANG Heng-jiao, DENG Zhi-hong, ZHANG Si-yu, HUANG Yi-wei, XIAO Jie-hua, GAO Li-dong
Hunan Provincial Center for Disease Control and Prevention/Hunan Workstation for Emerging Infectious Disease Control and Prevention, Chinese Academy of Medical Sciences, Changsha, Hunan 410008, China
Abstract：Objective To analyze the serial interval of coronavirus disease 2019 (COVID-19) in 87 cluster epidemics caused by close contact so as to provide a basis for formulating prevention and control strategies and measures. Methods Data regarding COVID-19 cluster cases reported in Hunan Province from January to February 2020 were collected. The serial interval of COVID-19 cases was estimated by median according to the illness onset of primary cases and secondary cases. The differences in serial intervals between different generations and in primary cases with different onset-quarantine intervals were compared by rank sum test, and the information about 2 cluster epidemics with negative serial interval was described. Results The median serial interval of COVID-19 was 6 days, and the quartile interval was 7 days. There was no statistically significant difference in serial interval between different generations. The serial intervals of primary cases with the onset-quarantine interval being 11 days and above and 6-10 days were 10 and 8 days, which were both longer than 4 days in primary cases with the onset-quarantine interval being 0-5 days (P<0.05). Transmission might occur as early as 7 days before the onset of illness. Conclusion The median serial interval of COVID-19 is shorter than those of severe acute respiratory syndrome and Middle East respiratory syndrome, and the effective transmission is faster. Early quarantine and treatment of primary cases will shorten the serial interval. Studies in the early stages of epidemic reflect the actual ability of transmission before intervention. COVID-19 cases are highly infectious at the time of illness onset. A proportion of secondary transmission occurs prior to the illness onset of primary cases, and time range for investigation and determination of close contacts can be expanded in the areas where conditions permit.
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