Long-term behavior of stochastic SIQRS epidemic models.

In this paper we analyze and classify the dynamics of SIQRS epidemiological models with susceptible, infected, quarantined, and recovered classes, where the recovered individuals can become reinfected. We are able to treat general incidence functional responses. Our models are more realistic than what has been studied in the literature since they include two important types of random fluctuations. The first type is due to small fluctuations of the various model parameters and leads to white noise terms. The second type of noise is due to significant environment regime shifts in that can happen at random. The environment switches randomly between a finite number of environmental states, each with a possibly different disease dynamic. We prove that the long-term fate of the disease is fully determined by a real-valued threshold $\lambda$ . When $\lambda <0$ the disease goes extinct asymptotically at an exponential rate. On the other hand, if $\lambda >0$ the disease will persist indefinitely. We end our analysis by looking at some important examples where $\lambda$ can be computed explicitly, and by showcasing some simulation results that shed light on real-world situations.