HIV epidemics driven by late disease stage transmission.

How infectious a person is when infected with HIV depends on what stage of the disease the person is in. We use 3 stages, which we call primary, asymptomatic, and symptomatic. It is important to have a systematic method for computing all 3 infectivities so that the measurements are comparable. Using robust modeling, we provide high-resolution estimates of semen infectivity by HIV disease stage. We find that the infectivity of the symptomatic stage is far higher, hence more potent, than the values that prior studies have used when modeling HIV transmission dynamics. The stage infectivity rates for semen are 0.024, 0.002, and 0.299 for the primary, asymptomatic, and symptomatic stages, respectively. Implications of our infectivity estimates and modeling for understanding heterosexual epidemics such as that in sub-Saharan African are explored.