The authors were interested in estimating the size of the 2019-novel coronavirus epidemic in Wuhan based on the number of cases exported from Wuhan to cities outside mainland China in order to forecast the extent of its domestic and global public health risks. They used data from January 2020 on the number of cases exported from Wuhan internationally to infer the number of infections in Wuhan from the start of December 2019. Cases exported domestically were then estimated. They then forecasted the national and global spread of 2019-nCoV while accounting for the metropolitan-wide quarantine of Wuhan and its surrounding cities starting on 23rd January. In their baseline scenario, the authors estimated that the basic reproductive number for 2019-nCoV was 2·68 and that 75,815 patients will have been infected in Wuhan by 25th January. The epidemic doubling time was 6·4 days. They estimated that in the baseline scenario, Chongqing, Beijing, Shanghai, Guangzhou and Shenzhen will have imported 461, 113, 98, 111 and 80 infections from Wuhan, respectively. If the transmissibility of 2019-nCoV remains the same across the country then they inferred that the epidemic will be growing exponentially in multiple major cities, with a lag time behind the Wuhan outbreak of about 1–2 weeks. Therefore self-sustaining outbreaks are likely to occur in major cities in China.
Read more here