Determinants of sexual transmission of HV: implications for control

The extent to which ART (anti-retroviral therapy) reduces HIV transmission has received attention in recent years. Using data on the relationship between transmission and viral load we show that transmission saturates at high viral loads. We fit a power-law model and an exponential converging to an asymptote. Using data on the viral load in HIV-positive people we show that ART is likely to reduce transmission by 91.6% (81.7%−96.2%) under the first and 99.5% (98.5%−99.8%) under the second model. The role of the acute phase in HIV transmission is still debated. High levels of transmission during the acute phase have been used to argue that failure to identify people in the acute phase of HIV may compromise the impact of treatment on preventing new infections and that having concurrent sexual partners during the acute phase is an important driver of the epidemic. We show that the acute phase probably accounts for less than 1% of overall transmission. We also show that even if a significant proportion of infections are transmitted during the acute phase, this will not compromise the impact of treatment on population levels of transmission given the constraint implied by the doubling time of the epidemic. This analysis leads to other relevant conclusions. First, it is likely that discordant-couple studies significantly underestimate the risk of infection. Second, attention should be paid to the variability in set point viral load which determines both the infectiousness of HIV-positive people and the variability in the susceptibility of HIV-negative people. Third, if ART drugs are in short supply those with the highest viral load should be given priority, others things including age, gender and opportunistic infections being equal, but to reduce transmission ART should be offered to all those with a viral load above about 10k/mm.3