The epidemic size in a population with one group (dotted curve, same as in Fig. 2) or two groups (plain, thick curve) with and p = 0.02 as a function of the basic reproduction ratio (obtained by varying q in the first case and q₂ in the second case). The other lines correspond to simulations of r(t) for t = 30, 60, 90 in the model with two groups. With two groups, we recover a phase transition as in Figure 2, corresponding to q₂ ≈ 0.96.