Figure 4.

(a) Prevalence or fraction of infected individuals, $I_i=I_i^{\mathit{\text{NP}}}+I_i^P$ and (b) fraction of protected individuals $P_i=S_i^P+I_i^P$ in the old (red) and young (blue) groups, evolving in time, with initial conditions fixed as in figure 3 for both groups. The adoption rate is fixed at $\alpha =50$ and the infection costs are $T_O=2000$ and $T_Y=500,$ respectively. (c) Fraction of protected individuals and (d) fraction of infected individuals in both groups displayed in a shorter time window. (e) The fraction of cases belonging to the old group in time. All results are obtained by numerically integrating equations (2.5)–(2.9) using a standard ODE solver, with fixed $\mathrm{\Delta }t=0.005$, and in both groups, $\beta =0.6$, $\mu =0.2$, $\gamma =0.4$ and $c=1$. (Online version in colour.)