“…The amount of literature studying the properties of solutions of such models is huge, see e.g. Brauer and Castillo-Chávez, 2012 , Ji and Jiang, 2017 , Jiao et al, 2020 , Kuniya and Nakata, 2012 , Li and Muldowney, 1995 , Zhang and Teng, 2007 , Zhao et al, 2017 and the references therein among many others. In the present, much effort is taken in order to estimate the evolution of the COVID-19 pandemic, see e.g.…”