Results and discussion
The exponential growth fitting results are shown in Figure 1(b), (d), (f), (h), (j) and (l). The coefficient of determination, R-squared, ranges from 0.91 to 0.92 for all reporting rate changing scenarios, which implies that the early outbreak data were largely following the exponential growth. In Table 1, we estimated that the R 0 ranges from 2.24 (95%CI: 1.96-2.55) to 5.71 (95%CI: 4.24-7.54) associated with an 8-fold to 0-fold increase in the reporting rate. All R 0 estimates are significantly larger than 1, which indicates the potential of 2019-nCoV to cause outbreaks. Since the official diagnostic protocol was released by WHO on January 17 (World Health Organization, 2020b), an increase in the diagnosis and reporting of 2019-nCoV infections probably occurred. Thereafter, the daily number of newly reported cases started increasing around January 17, see Figure 1, which implies that more infections were likely being diagnosed and recorded. We suggested that changes in reporting might exist, and thus it should be considered in the estimation, i.e., 8-, 4- and 2-fold changes are more likely than no change in the reporting efforts. Although six scenarios about the reporting rate were explored in this study, the real situation is difficult to determine given limited data and (almost) equivalent model fitting performance in terms of R-squared. However, with increasing reporting rate, we found the mean R 0 is likely to be between 2 and 3.
Our analysis and estimation of R 0 rely on the accuracy of the SI of 2019-nCoV, which remains unknown as of January 25. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV. The determination of SI requires knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up (Cowling et al., 2009), and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an insight to the transmission potential of 2019-nCoV at the early stage of the outbreak. We reported that the mean R 0 of 2019-nCoV is likely to be from 2.24 (8-fold) to 3.58 (2-fold), and it is largely in the range of those of SARS, i.e., 2-5 (Bauch et al., 2005, Lipsitch et al., 2003, Wallinga and Teunis, 2004), and MERS, i.e., 2.7-3.9 (Lin et al., 2018).
We note that WHO reported the basic reproduction number for the human-to-human (direct) transmission ranged from 1.4 to 2.5 (World Health Organization, 2020b), which is marginally lower than ours. However, many of the existing online preprints estimate the mean R 0 ranging from 2 to 5 (Imai et al., 2020, Riou and Althaus, 2020, Read et al., 2020, Shen et al., 2020), which is largely consistent with our results.