Figure 7.

The threshold (β₀) and lower bound (${\beta }_c^q$, for q = 0 and 1) transmission coefficients as a function of the incubation rate γ, using p = 0.01; all other values are those given in Table 1. β₀, ${\beta }_c^0$ and ${\beta }_c^1$ are decreasing functions, with ${\beta }_0>{\beta }_c^0>{\beta }_c^1$. When q = 1, they assume the same value (${\beta }_0={\beta }_c^1$ = 205 years^-1) at γ = 0.0001595 years^-1, and for q = 0, they assume the same value (${\beta }_0={\beta }_c^0$ = 207 years^-1) at γ = 0.000158 years^-1 (a). At a given γ, the difference between β₀ and ${\beta }_c^1$ (or ${\beta }_c^0$, which is practically the same) corresponds to the range of β in which two positive solutions are found (b).