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

Figure 11

Phase plane of system (9), for the case where ; 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 and the endemic coincide at the point , the latter being a doubly degenerate equilibrium; such points are indicated by a black circle.