In contrast to HIV transmission risk in heterosexuals (2
), data on HIV transmission in homosexual men are limited (5
). There have been no publications estimating per-contact probability of HIV transmission between homosexual men in the era of HAART. The participants recruited in the current study came from a setting with high coverage of HAART. Despite this, our estimates of HIV transmission probabilities were found to be similar to those reported from developed settings prior to HAART. For receptive UAI, we estimated the per-contact risk to be 1.43% if ejaculation occurred and 0.65% if withdrawal occurred without ejaculation. We estimated the per-contact risk for insertive UAI to be 0.11% in men who were circumcised and 0.62% in uncircumcised men. Due to differences in sampling and mathematical methods in different studies, it is difficult to directly compare results between studies. Nevertheless, our estimate of the per-contact risk of receptive UAI is very similar to that from a cohort of homosexual men recruited in the US in the early 1990s (of 0.82%, which did not differentiate whether or not withdrawal was involved) (6
Our estimate of the per-contact risk for insertive UAI in uncircumcised men was similar to that for receptive UAI with withdrawal, but was 80% lower in those who were circumcised. In comparison, among heterosexual men per-contact transmission risk was reduced by 50-60% in three randomized controlled trials of circumcision in African settings (13
). Our estimate of transmission risk for insertive UAI is approximately twice previous estimates (6
Our finding that the per-contact probability of HIV transmission is similar to that in the pre-HAART era was unexpected given the close correlation between HIV viral load and its infectiousness in heterosexual and vertical transmission (27
). In Australia, homosexual men have very high rates of recent HIV testing (28
), about 70% of HIV-positive men are receiving HAART, and 75% of those on treatment have undetectable viral load (16
). Thus it is surprising that our estimates of HIV transmission risk were similar to those in an era when few HIV-positive men would have had undetectable viral load.
There are some potential explanations for this unexpected finding. First, primary HIV infection, which is associated with higher viral load and thus higher infectiousness (29
), may have a larger role in the dynamics of HIV transmission than expected. In addition, individuals with primary HIV infection are usually unaware of their HIV status. It is likely that some of the partners not identified as HIV-positive could have had primary HIV infection. Second, the proportion of undiagnosed HIV infections or prevalence in the population could be higher than we expected (31
). We assumed that the prevalence of HIV among sexual partners thought to be HIV-negative and among those with unknown HIV status were 0.5% and 10%, respectively. However, we conducted a sensitivity analysis and found our estimates to be consistent across broad assumptions. Third, it may be possible that HIV transmission by anal intercourse is not as closely related to viral load as it is in vaginal transmission (27
). There is a paucity of data on HIV transmission risk at low viral loads, and there are almost no data on transmission and viral load in homosexual men (18
). Fourth, the prevalence of other sexually transmissible infections (STIs) in Sydney, as in many parts of the developed world, was higher during the timeframe of this study than the levels during the pre-HAART era. The presence of other STIs may increase the risk of HIV transmission (32
Our samples were recruited from a large variety of community-based sources, and the only behavioral criterion was that participants needed to report having sex with another man in the last five years. Compared with the previous mentioned US study (6
), which required participants to report risky behavior, our estimate could be more representative of gay community-attached men in general. Being one of the largest cohort studies examining incident HIV infection in homosexual men, only 46 HIV seroconverters who reported at least one episode of UAI were included in the analyses. Almost a quarter of a million episodes of UAI were reported by study participants, although only around 10,000 of these were with partners who were reported to be HIV-positive. Due to limited power, co-variates such as STIs and recreational drug use could not be included in the current estimations.
As with other observational studies relying on participants’ self-report, recall bias could influence the accuracy of the results. The study implemented six-monthly telephone interviews between annual face-to-face visits to minimize the possible inaccuracy of self-reported sexual behavior due to the long interview interval. The use of face-to-face interviews might have also reduced social desirability bias arising from the studies that collect sensible sexual behaviors.
Despite a more than ten-year gap from the last estimation of HIV transmission risk in homosexual men and the substantially improved treatment availability, the per-contact risk of HIV transmission with an HIV-positive partner does not seem to have reduced. While these updated estimates are valuable in determining the risk of HIV transmission, caution should be exercised before interpreting the results at the level of individual men. There is considerable heterogeneity between individuals including various biological and genetic factors associated with HIV infectiousness and susceptibility. This is emphasized by the occurrence of twelve seroconversion cases in the cohort of this study as a result of fewer than ten episodes of UAI per person and six cases that did not seroconvert despite extremely large numbers of receptive UAI episodes with HIV-positive partners. However, our estimates are useful for understanding the average magnitude of transmission risk due to different types of sexual exposures among homosexual men in the era of HAART.