The authors describe the data on the first 425 confirmed cases of 2019-novel coronavirus (2019-nCoV) in Wuhan to determine its epidemiology. The median age of the affected patients was 59 years and 56% were male. 55% of cases with onset before the start of 2020 were linked to the Huanan Seafood Wholesale Market, compared with 8.6% of subsequent cases. The mean incubation period was 5.2 days, with the 95th percentile of the distribution at 12.5 days. In the early stages, the epidemic doubled in size every 7.4 days. With a mean serial interval of 7.5 days, the basic reproductive number is estimated to be 2.2. Therefore there is evidence that human-to-human transmission has been occurring among close contacts since the middle of December 2019.
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