Biological correlates of sexual transmission of HIV: practical consequences and potential targets for public health

The probability of sexual transmission of HIV depends on the infectiousness of the index case and the susceptibility of the sexual contact. The risk o f HIV transmission is heterogeneous and may be greatest during the initial sexual contacts in a steady partnership. Several factors, including systemi c and mucosal acquired protective immune response, might be responsible for the apparent decrease of per-sex-act risk of transmission in a given partn ership over time. Biological studies can be used to understand better the c omplex information obtained from epidemiological surveys. The infectiousnes s of HIV depends both on the inoculum and on virologic factors. The genital tract viral load of the index case is probably the most important determin ant of transmission. At the population level, interventions that reduce the genital shedding of HIV by reducing systemic blood viral load and/or local inflammatory processes are likely to have a beneficial impact on HIV incid ence. Antiretroviral drugs are likely to reduce sexual transmission of HIV; however, these drugs may not do so equally. Compartmentalised HIV replicat ion in the male and female genital tract has been observed. Treatment with antiretroviral drugs that penetrate the genital tract poorly pose the risk of local production and spread of resistant viruses. In addition, increased risk-taking behaviour could offset the benefits of reduced probability of transmission at the population level. Biological data about HIV transmissio n must be used to inform public health policies and optimise HIV prevention strategies. (C) 2001 Lippincott Williams & Wilkins.