The HIV transmission gradient: relationship patterns of protection.

We describe a gradient of potential HIV transmission from HIV-infected persons to their partners and thence to uninfected populations. The effect of this newly discovered transmission gradient is to limit the spread of HIV. We roughly estimate a 2% long-term transmission probability for sex and 14% for drug injection for two-step transmission. Then we test theories to account for this pattern on a network sample of 267 inner city drug users and nonusers. Although HIV positive persons engaged in a high level of risk with one another, they engaged in less risk with HIV negative partners, and these partners engaged in even lower levels of risk with other HIV negative persons. Analyses suggest that the primary motivation for sexual risk reduction is partner protection, while emotional closeness is the major barrier. Hypotheses accounting for risk in terms of self protection, social norms, gender power, and drug use were weakly supported or unsupported.