Fig. 2.

Schematics of trade-offs. The first stage of an infection may present as less symptomatic, leading to a longer first stage but at the cost of reduced transmission. (A) Interpretations of latency. Since our model formulation considers bilinear mass action incidence, latency can be viewed on individual-status or population-level bases. Each circle denotes a host in the first stage $I_1$ of an infection, and the shading corresponds to symptoms. The former implies mild symptoms for each host, whereas the latter implies a fraction of hosts in $I_1$ fully asymptomatic and the others fully symptomatic. As a function of latency $\lambda$, we model (B) the initial stage transmission rate ${\alpha }_1$ and (C) the rate of progression from the first infectious stage to the second infectious stage in general terms as ${\alpha }_1\left[\lambda \right]=b_1{\left(\lambda +1\right)}^{-b_2}+{\alpha }_{1,\infty }$ and ${\nu }_1\left[\lambda \right]=c_1{\left(\lambda +1\right)}^{-c_2}+{\nu }_{1,\infty }$, respectively, with $b_1,c_1,b_2,c_2>0$. (D) Illustration of the possible shapes of the scaled initial transmission rate as a function of the scaled initial-stage progression rate, depending on values of $b_2$ and $c_2$. Here, the scaled rates are simply $\frac{{\alpha }_1-{\alpha }_{1,\infty }}{b_1}$ and $\frac{{\nu }_1-{\nu }_{1,\infty }}{c_1}$, respectively.