Estimation of incubation period distribution of COVID-19 using disease onset forward time: a novel cross-sectional and forward follow-up study

Qin Jing; Chong You; Qiushi Lin; Taojun Hu; Shicheng Yu; Xiao-Hua Zhou

To the best of our knowledge, there is only a handful of studies estimating the incubation period of COVID-19. Among them are Li et al (2020) , Guan et al (2020) , Backer et al (2020) , and Linton et al (2020) . [3] [4] [5] [6] In Li et al, the first 425 lab-confirmed cases, reported as of January 22, 2020, were included in the study, while only ten cases could be identified with the exact dates of exposure. 3 The distribution of the incubation period was subsequently approximated by fitting a lognormal distribution to these ten data points, resulting in a mean incubation period of 5·2 days (95% CI: 4·1-7·0), and the 95th percentile is 12·5 days. However, given the limited sample size, it is challenging to make a solid inference on the distribution of the incubation period. A different result was reported by Guan et al, based on 291 patients who had clear information regarding the specific date of exposure as of January 29, 2020, stating that the median incubation period was 4·0 days (interquartile range, 2 to 7). 4 However, such study of the incubation period can be highly influenced by the individuals' recall bias or interviewers' judgement on the possible dates of exposure rather than the actual dates of exposure that, in turn, might not be accurately monitored and determined, thus leading to a high percentage of error. In Backer et al, 88