The effective reproduction number, R, calculated by our method changes over time, because of decreases in the fraction of susceptibles, S(t)/N(t), and the availability of more information, as more cases are observed. Thus, we use the values obtained for R at early times, when S(t)/N(t) approximates its initial value, to estimate R 0 of the simulation by assuming that max(R) = R 0 . As shown in Figure 3 , in all circumstances, the method gives an excellent estimation of R 0 as outbreaks unfold, usually making accurate predictions when supplied with a mere two or three observation points. Uncertainty, measured by the width of the 95% credible interval, is reduced by larger case numbers, so that it typically remains higher the smaller the R 0 . In all instances uncertainty is reduced as more cases are reported over time.