Mathematical model simulations have demonstrated the potential for reducing the incidence of new HIV infections through biomedical and behavioural interventions. The benefits of early interventions and of targeting interventions at core groups have been emphasized [13,14]. Targeted interventions are liable to be less effective if introduced after high levels of HIV prevalence have been reached. Their success will also depend on the level of heterogeneity in sexual activity which exists within a population and the predominant pattern of mixing between different risk groups [15]. Combinations of intervention approaches can act additively and even synergistically [16].