The differential infectivity and staged progression models for the transmission of HIV

Recent studies of HIV RNA in infected individuals show that viral levels va ry widely between individuals and within the same individual over time. Ind ividuals with higher viral loads during the chronic phase tend to develop A IDS more rapidly. If RNA levels are correlated with infectiousness, these v ariations explain puzzling results from HIV transmission studies and sugges t that a small subset of infected people may be responsible for a dispropor tionate number of infections. We use two simple models to study the impact of variations in infectiousness. In the first model, we account for differe nt levels of virus between individuals during the chronic phase of infectio n, and the increase in the average time from infection to AIDS that goes al ong with a decreased viral load, The second model follows the more standard hypothesis that infected individuals progress through a series of infectio n stages, with the infectiousness of a person depending upon his current di sease stage. We derive and compare threshold conditions for the two models and find explicit formulas of their endemic equilibria. We show that formul as for both models can be put into a standard form, which allows for a clea r interpretation. We define the relative impact of each group as the fracti on of infections being caused by that group.We use these formulas and numer ical simulations to examine the relative importance of different stages of infection and different chronic levels of virus to the spreading of the dis ease. The acute stage and the most infectious group both appear to have a d isproportionate effect, especially on the early epidemic. Contact tracing t o identify super-spreaders and alertness to the symptoms of acute HIV infec tion may both be needed to contain this epidemic. (C) 1999 Published by Els evier Science Inc. All rights reserved.