It is now 6 weeks since Chinese health authorities announced the discovery of a novel coronavirus (2019-nCoV) [1] causing a cluster of pneumonia cases in Wuhan, the major transport hub of central China. The earliest human infections had occurred by early December 2019, and a large wet market in central Wuhan was linked to most, but not all, of the initial cases [2]. While evidence from the initial outbreak investigations seemed to suggest that 2019-nCoV could not easily spread between humans [3], it is now very clear that infections have been spreading from person to person [2]. We recently estimated that more than 75,000 infections may have occurred in Wuhan as at 25 January 2020 [4], and increasing numbers of infections continue to be detected in other cities in mainland China and around the world. A number of important characteristics of 2019-nCoV infection have already been identified, but in order to calibrate public health responses we need improved information on transmission dynamics, severity of the disease, immunity, and the impact of control and mitigation measures that have been applied to date.