Although the widespread use of antiretroviral therapy could be expected to make HIV-positive gay men less likely to pass on HIV during unprotected sex than in the early 1990s, the risk of transmission per-sexual-act is actually quite similar, Australian researchers report in AIDS.
The authors put forward a number of suggestions to explain this surprising finding - that transmission during primary infection is a more significant factor than before 1996, that sexually transmitted infections are now more common, or that viral load is a less important factor for anal transmission than for vaginal transmission.
Their study also suggests that circumcised men who have unprotected insertive sex are less likely to acquire HIV than men who are not circumcised, and that ejaculation inside the rectum increases the risk of transmission during unprotected receptive anal intercourse.
How the estimates are produced
There are considerable methodological challenges in producing reliable estimates for the risk of HIV transmission during a single sexual contact. Cohort studies which follow large numbers of individuals at risk of HIV infection for several years are required. These individuals must regularly provide information about their sexual behaviour (which may not always be completely accurate) and regularly take HIV tests.
Researchers attempt to produce an estimate of the risk of infection during sex with an HIV-positive person, but most people’s sexual partners will be reported to be HIV negative or of unknown HIV status. The researchers must therefore use an estimate of what proportion of these sexual partners are likely to have in fact have had HIV (although it may have been undiagnosed or undisclosed).
Moreover, the estimate that the researchers produce can only be an average. A wide range of individual factors make infection more or less likely, including stage of infection, antiretroviral medication, sexually transmitted infections in either partner, and how much body fluid is transferred.
For transmission between gay men, the last estimates were published by Vittinghoff and colleagues in 1999, and were based on data collected in the United States between 1992 and 1994.
Since then, the use of highly active antiretroviral therapy (HAART) has become widespread in rich countries, with the result that more people have an undetectable viral load. As this could have a significant impact on the risk of transmission, Fengyi Jin and colleagues analysed data from the Australian Health in Men (HiM) cohort in order to produce more up-to-date estimates. This cohort of gay and bisexual men was followed between 2001 and 2007.
Aidsmap.com has previously reported findings from the cohort on HIV transmission in relation to risk reduction practices, circumcision and sexually transmitted infections, especially gonorrhoea and anal warts.
A total of 1,136 participants reported unprotected anal intercourse at least once, and so were included in the analysis. Forty six of these men acquired HIV.
Over a quarter of a million acts of unprotected anal intercourse were reported, and these were broken down by partner’s HIV status, sexual position and (for receptive intercourse only) whether ejaculation occurred inside the rectum. The respondent’s circumcision status was also recorded.
For the receptive partner, the transmission risk per-contact was found to be higher if ejaculation occurred inside the rectum:
- Receptive, with ejaculation - 1.43% (95% confidence interval 0.48 - 2.85)
- Receptive, without ejaculation - 0.65% (95% confidence interval 0.15 - 1.53)
For the insertive partner, the risk was lower if he was circumcised:
- Insertive, not circumcised - 0.62% (95% confidence interval 0.07 - 1.68)
- Insertive, circumcised - 0.11% (95% confidence interval 0.02 - 0.24)
It’s also notable that for a man who is not circumcised, it appears that the risks of insertive intercourse are broadly similar to the risks of receptive intercourse without ejaculation.
However in almost all of these estimates, the confidence intervals are wide. This was also the case for the estimates produced in the 1990s, and is because the figures are based on a relatively small number of participants acquiring HIV. This highlights the uncertainty that remains over the figures produced, with the confidence intervals showing the range of possible true results.
The results produced were based on an assumption of HIV prevalence being 0.5% in sexual partners thought to be HIV negative and 10% in sexual partners of unknown status. The researchers re-ran the analysis with a range of estimates (0.5-2%, and 5-15%) and found that similar figures were produced.
Comparison with the pre-HAART era
The researchers warn that their study is not directly comparable to the one conducted in the United States in the 1990s. Nonetheless they say that it is surprising that the results are broadly similar.
For example, whereas the new study has found a transmission risk during receptive sex of 0.65% (without ejaculation) or 1.43% (with ejaculation), the 1990s estimate was 0.82% (with or without ejaculation).
The authors suggest some possible explanations, which would all need to be explored in future research. Sexually transmitted infections (which increase the risk of HIV transmission) were more prevalent in Sydney in the last decade than in the US in the early 1990s. Moreover, primary infection (when people are exceptionally infectious) may play a larger part in the dynamics of HIV transmission than expected.
Also: “It may be possible that HIV transmission by anal intercourse is not as closely related to viral load as it is in vaginal transmission. There is a paucity of data on HIV transmission at low viral loads, and there are almost no data on transmission and viral load in homosexual men.”
The authors believe that their estimates “are useful for understanding the average magnitude of transmission risk” in the era HAART. However they recommend caution in using them to consider an individual’s risk of transmission or infection.
They found a great deal of individual variation in their cohort. Twelve individuals who had had unprotected sex ten times or less acquired HIV. On the other hand, six men who reported a total of 502 episodes of unprotected receptive intercourse with ejaculation remained HIV negative.
Jin F et al. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS, published online ahead of print, 2010.