Schemata of model components.

<p>(A) Schema of London-student model. <i>S</i>: Susceptible state; <i>I</i>: Infected state; <i>λ</i>_<i>low</i>: incidence rate of low-risk group; <i>λ</i>_<i>high</i>: incidence rate of high-risk group; <i>γ</i>: recovery rate. (B) Schema of the succession of Infected and Susceptible states. <i>S</i>: Susceptible state; <i>I</i>: Infected state; <i>t</i>₁: time at baseline; <i>t</i>₂: time at follow-up; <i>T</i>_<i>F</i>: time to follow-up; in this example, the individual was <i>Infected</i> at baseline and, again, at follow-up.