Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in Wuhan, China in the end of 2019, and soon spread oversea A comprehensive and timely review summarized the scientific research in estimating the basic reproduction number (R0) released from 1 January to 7 February 2020 [1] During the early outbreak, when the key epidemiological features of COVID-19 were uncovered, the R0 estimation largely relied on the growth rate of the epidemic curve and the estimation of the serial interval (SI) Here, we demonstrated that an overlarge SI would lead to overestimation of R0 We adopted the growing process proposed in [2] deterministically with a population of 11 million in Wuhan, 1 case initially onset on 5 December 2019 and a fixed step at 1 day We consider two values of the mean serial interval (SI) that are  SI at 4 6 days estimated based on 28 records of transmission chains [3], which was largely consistent with the SI estimate at 4 4 days based on 71 records [4];and  SI at 8 days, which was closer to the SI of the severe acute respiratory syndrome (SARS, 8 4 days), SI of the Middle East respiratory syndrome (MERS, 7 6 days) As for demonstration that a larger SI could lead to overestimation in R0, we conducted the simulation with two schemes that are  Scheme (I): R0 = 2, and SI = 4 6 days;and  Scheme (II): R0 = 2, 3, 4 and 3 3 as summarized in [1], and SI = 8 days We also compared the simulation results with the previous estimates of the cumulative number of COVID-19 infection in Wuhan In Fig 1, the simulation results of the scheme (I) had almost the same growing trends as those of scheme (II) with R0 = 3 3 Although a higher R0 could force the epidemic curve increasing rapidly, a shorter SI could increase iteration of transmission generation, i e , transmission may occur shortly post infection According to the simple approximated formula that R0 = exp(γ∙SI), where γ was the exponential growth rate calculated from the incidence data directly, a longer SI would lead to a higher R0 estimate theoretically With a shorter SI at 4 6 days, which was supported by richer datasets in [3, 4], the R0 of COVID-19 could be lower than previous estimates based on longer SI By using the growth rate (γ) at 0 15 per day, the R0 was found at 2 0 with SI at 4 6 days, whereas 3 3 with SI at 8 days Although the effects of public health control were ignored in this analysis, our model could be Downloaded from https://academic oup com/jtm/advance-article-abstract/doi/10 1093/jtm/taaa033/5803291 by guest on 13 March 2020 4 extended by introducing an effective reproduction number accounting for the effectiveness of the control measures, and we remarked this modification would not affect the main conclusion Furthermore, as pointed out in [3], provided that the SI of COVID-19 might be shorter than its incubation period, pre-symptomatic transmission may occur shortly after being infected [5] This implies that a fraction of transmissions cannot be prevented solely through isolating the symptomatic cases, since the time when contact tracing is conducted, they may have already been infectious and generated secondary cases Therefore, the effectively quarantine of suspected (and probable) cases, as well as close contacts, and timely contact tracing were crucial in successful outbreak mitigation
Serial interval in determining the estimation of reproduction number of the novel coronavirus disease (COVID-19) during the early outbreak ;Journal of Travel Medicine ;Oxford Academic