The global stability of an SIRS model with infection age

Infection age is an important factor affecting the transmission of infectious diseases. In this paper, we consider an SIRS model with infection age, which is described by a mixed system of ordinary differential equations and partial differential equations. The expression of the basic reproduction number R0 is obtained. If R0≤1 then the model only has the disease-free equilibrium, while if R0>1 then besides the disease-free equilibrium the model also has an endemic equilibrium. Moreover, if R0<1 then the disease-free equilibrium is globally asymptotically stable otherwise it is unstable; if R0>1 then the endemic equilibrium is globally asymptotically stable under additional conditions. The local stability is established through linearization. The global stability of the disease-free equilibrium is shown by applying the fluctuation lemma and that of the endemic equilibrium is proved by employing Lyapunov functionals. The theoretical results are illustrated with numerical simulations.