To explore the hypotheses that HIV-1 infected patients receiving suppressive antiretroviral therapy believe that they cannot transmit their virus, that this translates to increasing rates of unprotected anal intercourse and new infections, and that sexual practice may be influenced by the HIV-1 serostatus of ones sexual partner. We developed a Bernoulli model for HIV-1 transmission amongst a cross-sectional cohort of 119 HIV-1 positive treated and untreated men having sex with men attending a metropolitan HIV-1 clinic. The model included transmission probabilities for different types of anal intercourse, the local HIV-1 seroprevalence rate, and behavioural data from a validated sexual practice questionnaire and scales of treatment optimism-scepticism and sexual beliefs pertaining to the previous 3 months. Subjects reported partner serostatus as HIV-1 negative or unknown. There were no differences in optimism-scepticism score (p = 0.295) and total sexual belief scores (p = 0.211) according to antiretroviral therapy status. Sex with unknown serostatus partners was higher risk for transmission than sex with HIV-1 negative partners. Most unprotected anal intercourse was practiced by a minority of men, independent of antiretroviral treatment status. Compared to subjects with HIV-1 negative partners, subjects with unknown serostatus partners had more UAI, a 2-fold higher probability of transmitting HIV-1 and were responsible for 2.6 as many new infections. Overall, there were 4 times as many infections predicted to occur amongst unknown serostatus men cf. HIV-1 negative men. A sub-group reported frequent unprotected anal intercourse with their partners. Our model may be used to quantify risk behavior during individual counselling or to develop targeted prevention programmes at the public health level.
Keywords: Anal intercourse, HIV-1 infection, infectivity, model, probability, transmission