In this paper, SEIS epidemic models with varying population size are considered. Firstly, we consider the case when births of population are throughout the year. A threshold σ is identified, which determines the outcome of disease, that is, when σ < 1, the disease dies out; whereas when σ > 1, the disease persists and the unique endemic equilibrium is globally asymptotically stable; when σ = 1, bifurcation occurs and leads to "the change of stability". Two other thresholds σ′ and [Formula: see text] are also identified, which determine the dynamics of epidemic model with varying population size, when the disease dies out or it is endemic. Secondly, we consider the other case, birth pulse. The population density is increased by an amount B(N)N at the discrete time nτ, where n is any non-negative integer and τ is a positive constant, B(N) is density-dependent birth rate. By applying the corresponding stroboscopic map, we obtain the existence of infection-free periodic solution with period τ. Lastly, through numerical simulations, we show the dynamic complexities of SEIS epidemic models with varying population size, there is a sequence of bifurcations, leading to chaotic strange attractors. Non-unique attractors also appear, which implies that the dynamics of SEIS epidemic models with varying population size can be very complex.
Periodicity and stationary distribution of two novel stochastic epidemic models with infectivity in the latent period and household quarantine
