Theorem 3.2. Let one consider the unvaccinated version of model (2.1) and (2.2), that is, with and , and let us further assume that .
If , then the disease-free equilibrium with and exists and is globally stable.
Proof. In view of Remark 3.1, this theorem is a direct consequence of [4, Theorem 1].
Turning to the more general model (2.1) and (2.2), with vaccination and without the assumption , we can identify three possible equilibrium points.
Proposition 3.3. () If , then the disease-free equilibrium is locally asymptotically stable and its coordinates ar