Global behavior of a multi-group SIS epidemic model with age structure

We study global dynamics of a system of partial differential equations. The system is motivated by modelling the transmission dynamics of infectious diseases in a population with multiple groups and age-dependent transition rates. Existence and uniqueness of a positive (endemic) equilibrium are established under the quasi-irreducibility assumption, which is weaker than irreducibility, on the function representing the force of infection. We give a classification of initial values from which corresponding solutions converge to either the disease-free or the endemic equilibrium. The stability of each equilibrium is linked to the dominant eigenvalue s(A), where A is the infinitesimal generator of a "quasi-irreducible" semigroup generated by the model equations. In particular, we show that if s(A) < 0 then the disease-free equilibrium is globally stable; if s(A) > 0 then the unique endemic equilibrium is globally stable.