The Novel Coronavirus, 2019-nCoV, is Highly Contagious and More Infectious Than Initially Estimated

Steven Sanche; Yen Ting Lin; Chonggang Xu; Ethan Romero-Severson; Nick Hengartner; Ruian Ke

We integrated the travel data into our inferential models using two approaches. The rationale of the first model, the 'first-arrival' approach, is that an increasing fraction of people infected in Wuhan increases the likelihood that one such case is exported to the other provinces. Hence, how soon new cases are observed in other provinces can inform disease progression in Wuhan (Fig. 2C ). This has similarities with earlier analyses to estimate the size of the 2019-nCoV outbreak in Wuhan based on international travel data (5, 7, 8) , though inference based infected cases outside of China may suffer large uncertainty due to the low volume of international travel. In our model, we assumed exponential growth for the infected population I* in Wuhan, * ( ) = ( − 0 ) , where is the exponential growth rate and 0 is the time of the exponential growth initiation, i.e. * ( 0 ) = 1. Note that 0 is likely to be later than the date of the first infection event, because multiple infections may be needed before the onset of exponential growth (9) . We used travel data to each of the provinces (Table S3 ) and the earliest times that an infected individual arrived at a province across a total of 26 provinces (Fig. 2D ) to infer and 0 (see Supplementary Materials for details). Model predictions of arrival times in the 26 provinces fitted the actual data well (Fig. S2 ). We estimated that the date of the beginning of an exponential growth is December 20, 2019 (CI: December 11 to 26). This suggests that human infections in early December may be due to spillovers from the animal reservoir or limited chains of transmission (10, 11) . The growth rate of the outbreak is estimated to be 0.29 per day (CI: 0.21 to 0.37 per day), a much higher rate than two recent estimates (1, 5). This growth rate corresponds to a doubling time of 2.4 days. We further estimated that the total infected population size in Wuhan was approximately 4,100 (CI: 2,423 to 6,178) on January 18, which is remarkably consistent with a recently posted estimate (7) . The estimated number of infected individuals is 18,700 (CI: 7,147, 38,663) on January 23, i.e. the date when Wuhan started lock down. We projected that without any control measure, the infected population would be approximately 233,400 (CI: 38,757 to 778,278) by the end of January (Fig. S3 ).