BACKWARD BIFURCATION IN A DISCRETE SIS MODEL WITH VACCINATION
Abstract
A simple discrete SIS model with vaccination is proposed. Its dynamics depend on a lumped parameter Rvac. The model exhibits the classical threshold behavior when vaccination is totally ineffective. When vaccination is partially effective, a backward transcritical bifurcation may occur at Rvac = 1. In this case, the model also undergoes a saddle–node bifurcation at certain parameter values when Rvac < 1. The disease can persist for Rvac > 1 and can be eradicated for Rvac < 1 if a forward transcritical bifurcation occurs at Rvac = 1. However, the disease may persist even when Rvac < 1 if a backward bifurcation occurs at Rvac = 1.