The estimated reproduction number of human-to-human transmission for the novel influenza A(H7N9). Each line shows the estimated reproduction number of human-to-human transmission for novel influenza A(H7N9). If the reproduction number exceeds 1, it indicates that a pandemic in human population can be caused with a certain probability. Our estimates depend on the proportion of cases caused by bird-to-human transmissions among a total of confirmed cases, and thus, the actual number of confirmed cases with bird contact. As of 1 May 2013, seventeen cases have been reported to have had contact with bird. Panels A and B show the results from the first scenario in which we analysed only the cases with a known history of exposure (i.e. 17 contacts with bird and 3 cases without bird contact). Panels C and D show the results from the second scenario, which adhered to the latest contact tracing information , considering that 99 out of 129 confirmed cases had a history of exposure to bird. Panels A and C additionally show the 95%confidence intervals in dashed lines, while panels B and D vary the mean generation time from 2 to 4 days (baseline: 3 days). In all panels, solid line represents the maximum likelihood estimates of the reproduction number for human-to-human transmission for the baseline scenario. The horizontal grey line corresponds to the reproduction number at unity above which a pandemic could occur. An arrow in each panel on the horizontal axis points the baseline assumption of the number of contacts with bird.