A singularly perturbed model emerges when we combine a SEIRS epidemiological model whose time scale is the day, with a juvenile/adult demographic model whose time scale is the year. In this article, the combined model is used to describe the dynamics of the corona virus COVID-19, taking into account the rhythm of the start of this epidemic, which seemed to affect only adults. The 5 dimension model is reduced, by applying Tikhonov’s Theorem for slow-fast systems, to a linear or affine planar model, depending on the initial conditions, whose solutions provide an approximation of the initial solutions in their slow phases, for arbitrarily large times.