Summary: An SIS epidemic model with vaccination, temporary immunity, and varying total population size is studied. Three threshold parameters , , and are identified. The disease-free equilibrium is globally stable if and unstable if , the endemic equilibrium is globally stable if . The disease cannot break out if , the disease may break out when the fractions of the susceptible and the infectious satisfy some condition if and . The population becomes extinct ultimately and the disease always exists in the population if and . There is a really endemic disease if and .
The global stability of the disease-free equilibrium and the existence and global stability of the endemic equilibrium are proved by means of LaSalle’s invariance principle, the method of estimating values and Stokes’ theorem, respectively. The results with vaccination and without vaccination are compared, and the measures and effects of vaccination are discussed.