Before revision:  Although often used to compare outbreaks and forecast pandemic risk, this single number belies the complexity that two different pathogens can exhibit, even when they have the same R_0. Here, we show how predicting outbreak size requires both an estimate of R_0 and an estimate of the heterogeneity in the number of secondary infections.
After revision:  Although often used to compare outbreaks and forecast pandemic risk, this single number belies the complexity that two different pathogens can exhibit, even when they have the same R_0. Here, we show how to predict outbreak size using estimates of the distribution of secondary infections, leveraging both its average R_0 and an estimate of the heterogeneity in the number of secondary infections.