Designing HIV vaccination policies: Subtypes and cross-immunity

We developed and used mathematical models to assess vaccine programs for controlling two subtypes of HIV, both for developing countries where more than one subtype is present and for countries where only one subtype is present but other subtypes may invade. We began by formulating a model of the intrinsic transmission dynamics of the two HIV subtypes and then extended this model to include the effects of a prophylactic vaccine that provides a degree of protection against infection by one subtype and vaccine-induced cross-immunity against infection by the second subtype. Using these models, we assessed the potential impact of using a prophylatic vaccine when one subtype of HIV is endemic and a second subtype is introduced into the community. In each case, mass vaccination could result in one of four possible outcomes: (1) both subtypes are eradicated, (2) the endemic subtype persists and the invading subtype is eradicated, (3) the endemic subtype is eradicated and the invading subtype persists, or (4) both subtypes coexist.