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ART reduces HIV transmission risk in anal sex to 4%

Wednesday, 5 March, 2014

Initial results from the PARTNER study, released at the Conference on Retroviruses and Opportunistic Infections (CROI) this week, reported no sexual transmission of HIV in couples where one partner was HIV-negative, and the other HIV-positive, on treatment, and with a viral load of less than 200 ml/copy. The researchers also calculated that the greatest likelihood of HIV transmission during anal sex was only four percent per year, as long as the HIV-negative partner was on treatment and had a suppressed viral load.

There have been several trials that have looked at the efficacy of HIV treatment in reducing the risk of transmission. The PARTNER study is the second largest study conducted that looks at HIV transmission risk between serodiscordant couples (when one partner is HIV-negative and the other HIV-positive) and the infectiousness of the HIV-positive partner whilst on treatment. In 2008, a group of Swiss scientists asserted that an HIV-positive person who is taking effective antiretroviral therapy, has an undetectable viral load and is free from STIs, has a negligible risk of infecting others with the virus. In 2011, another major trial was conducted – the HPTN 052 study involved serodiscordant heterosexual couples, and found a 96 percent reduction in risk of HIV transmission after commencing treatment.

However, until now there has been a notable gap in the research around the infectiousness of the HIV-positive partner and the likelihood of HIV transmission during anal sex, which the PARTNER study fills. In this study, 40 percent of the couples were gay, recording around 16,400 occasions of gay sex and 14,000 occasions of heterosexual sex. In order to be eligible for the study, the couples had to have unprotected sex at least some of the time, and the HIV-positive partner had to have an undetectable viral load.

The researchers found that there was no transmission of HIV during either vaginal or anal sex in the first two years of the study. They estimate that over a ten-year period, there is a maximum five percent chance that one in ten of the HIV-negative partners would contract HIV through unprotected anal sex with their partner. However, the authors of the study state that it was more likely that this percentage was actually lower. At the conference, presenter Alison Rodger was asked what the likelihood was of someone who had an undetectable viral load transmitting the virus, to which she replied: “Our best estimate is zero.”

Interestingly, the study also shows that despite a high level of STIs, particularly among gay couples, this did not impact upon the likelihood of HIV transmission – as stated in the 2008 Swiss Statement.

The PARTNER study is still ongoing, and continues to recruit volunteers. The authors note that these are only preliminary results, and that the full results from the study should be expected in 2017.