Department of Mathematics
Some pathogens change their antigenic properties at an intermediate rate in the sense that antigenes remain constant during a single infectious episode while antigenic change is fast enough that a host during its lifespan may experience several reinfections with antigenically cross-reacting strains. Thus the pathogens utilize a distinct life history strategy where evolutionary changes in the pathogen allow them to recolonize hosts in several infection episodes. The most well documented example of intermediate antigenic change is Influenza A during a period of antigenic drift . Extensions of the SIR-model can describe the epidemiology of such diseases as well as the natural selection on the pathogen strains [2-4]. In general models of the natural selection are rather complicated because it is necessary to keep track of all possible host immune histories. However if cross-reaction acts by reducing the infectivity of a second infection, the model may be simplified considerably by subdividing for each strain k, the uninfected hosts according to their degree of cross-immunity to k .