Backward bifurcation and saddle-node bifurcation in virus-immune dynamics

Recently, Wang and Xu [ Appl. Math. Lett. 78 (2018) 105-111] studied thresholds and bi-stability in virus-immune dynamics. In this paper, we show there also exist backward bifurcation and saddle node bifurcation in this model. Our investigation demonstrates the existence of post-bifurcation phenomenon in the system when the immune strength was selected as the bifurcation parameter. The bifurcation of saddle knot is then analyzed. The values of backward bifurcation point and saddle node bifurcation point are the elite control threshold and post-treatment control threshold respectively. The time interval between the two thresholds is the bi-stable interval.