On January 23, 2020, China enacted a travel quarantine of Wuhan and nearby cities to curb the geographic expansion of a novel coronavirus (2019-nCoV). Here, we use ground, rail and air travel data to estimate the probability that 2019-nCoV cases were imported from Wuhan to 370 cities throughout China in the three weeks prior to the quarantine. Based on an estimated R0 of 2.56 [95% CI: 2.09, 2.78], we find that the probability of a pre-quarantine introduction exceeds 50% in 131 [95% CI 72, 232] cities, including six cities with populations over two million people with no reported cases as of January 27th.
