Stability, Bifurcation, and a Pair of Conserved Quantities in a Simple Epidemic System with Reinfection for the Spread of Diseases Caused by Coronaviruses

Figure 10

Phase plane of system (9) for , with ; the other rates are , , and . The vector field and some trajectories are indicated by lines and arrows in gray. Two nullclines are shown: one horizontal (blue line) and other vertical (red line); the horizontal and vertical remaining pair coincides with the -axis. The disease-free equilibrium point is stable, and the endemic is an unstable node of two “straight-line” solutions; both points are indicated by a black circle.