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Howdy, PARTNER: Gay Men and HIV Transmission Editor Bob Leahy in Conversation With CATIE's James Wilton on What the PARTNER Study Tells Us About the Risk of HIV Transmission in Gay Men Where One Partner Is HIV-Positive but Who's Viral Load Is Undetectable

June 3, 2014

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Howdy, PARTNER: Gay Men and HIV Transmission

Hi James. Thanks for agreeing to talk to again, this time on the PARTNER study. Now, I read your analysis of the interim results -- Positive published it here -- which took me back to the day the news first broke from CROI that we finally had data about the risk of HIV transmission involving gay men with an undetectable viral load. I was really excited that day. Would you agree that this was ground-breaking stuff?

Yes, I think the preliminary results from this study are very important. Ever since the HPTN052 study released its results in 2011 -- showing that antiretroviral treatment can significantly reduce the risk of HIV transmission through vaginal sex -- there have been unanswered questions with regards to its implications for populations which mostly have anal sex, such as some gay men and men who have sex with men. The preliminary results from the PARTNER study help answer some of these questions.

But we did have some pretty good ideas about the very slim likelihood of transmission occurring involving undetectable viral loads prior to PARTNER, didn't we. The fact that so few known transmissions were occurring in the studies of heterosexuals that Mona Loutfy looked at was well established, wasn't it? And many people were talking about it.

Well, we knew from HPTN 052 that the risk of HIV transmission through vaginal sex could be drastically reduced by starting treatment, but it was unclear exactly what this risk was reduced to. Because of this, Mona Loutfy -- a physician and researcher from Woman's College Hospital in Toronto -- conducted a review of the research on serodiscordant couples to determine how many HIV transmissions had occurred when the viral load was undetectable.


Her research team identified six studies of heterosexual serodiscordant couples where the HIV-positive partner was on ART. In these studies, no HIV transmissions occurred when the viral load was undetectable. This was a significant finding because the couples in these studies were followed for a long period of time, a total of almost 3,000 couple years of follow up (equivalent to following 3,000 couples for a year with no HIV transmissions).

The findings from this review got a lot of attention and it was definitely good news. However, the review also had its limitations. The main one being that the couples in these studies reported using condoms the majority of the time. This made it difficult to know the extent to which having an undetectable viral load, and not condoms, was responsible for the absence of HIV transmissions. In other words, it didn't answer the question of what the risk of HIV transmission is when the viral load is undetectable and no condom is used. This, as you know, is a question that many people want the answer to.

Also, Mona's review didn't provide any information on what the risk is reduced to for anal sex, as the studies her team identified only enrolled heterosexual couples. There is the concern that -- even if the reduction in risk is the same for anal sex as it is for vaginal sex -- that the risk when undetectable may still be higher for receptive anal sex (where the HIV-negative partner takes the receptive position during anal sex, also known as bottoming) compared to other types of sex. This is because the risk of HIV transmission through receptive anal sex is much higher than other types of sex when the viral load is detectable. Therefore, there is the possibility that the risk could also be much higher when the viral load is undetectable.

In the face of these research gaps, uncertainties, and unanswered questions, we have seen expert consensus statements playing an important guiding role in translating the research we have available into useful messaging. You may remember that, shortly after the HPTN 052 results were released in 2011, the World Health Organization released an expert consensus concluding that treatment also reduces the risk of HIV transmission through anal sex, but the risk-reduction may or may not be the same as for vaginal sex.

More recently, but prior to the results of the PARTNER results, the British HIV Association released an expert consensus saying that the risk of HIV transmission through both anal and vaginal sex is "extremely low" when the viral load is undetectable and other conditions are met. These conditions included the absence of STIs, having had an undetectable viral load for at least 6 months, regular viral load testing and complete disclosure within the couple about sexual relationships outside the partnership.

So on to PARTNER. Do you want to summarize briefly what PARTNER told us, James?

Sure. The purpose of the PARTNER study was to answer the question: "what is the risk of HIV transmission through anal and vaginal sex when the viral load is undetectable and no condom is used?" This was a question that had not previously been answered.

To do this, the study enrolled heterosexual and gay male serodiscordant couples that had previously made the decision -- for whatever reason -- to stop using condoms consistently. In fact, the couples who ended up enrolling had already been having condomless sex for an average of two years prior to entering the study. It is also important to note that the study only enrolled couples in which the HIV-positive partner was on treatment. The study did not enroll a "control" group of couples who were not on treatment.

The preliminary analysis of the PARTNER study was presented in March 2014 at a conference in the United States and included information on 767 couples followed between September 2010 and November 2013. During this time, couples engaged in approximately 44,000 condomless sex acts when the viral load was undetectable. This included 13,728 receptive vaginal sex acts, 14,295 insertive vaginal sex acts, 7,738 receptive anal sex acts, and 11,749 insertive anal sex acts. No HIV transmissions occurred.

Interpreting these results is challenging. Since there was no control group, it is difficult to know how many HIV transmissions would have been expected if the HIV-positive partner had not been on treatment and had a detectable viral load. However, based on results from previous studies, I would have expected about 10 infections each through receptive vaginal sex, insertive vaginal sex and insertive anal sex. So, for these sex acts, the preliminary results weren't super compelling because not many infections would have been expected anyways. Also, for vaginal sex, these results are less compelling than the results for vaginal sex from the HPTN 052 study.

The most compelling result from the preliminary analysis is that no infections occurred despite participants engaging 7,738 receptive anal sex acts. As I mentioned already, when the viral load is detectable, receptive anal sex is much higher risk than other types of sex. In the PARTNER study, I would have expected about 80 HIV infections through receptive anal sex if the viral load of the HIV-positive partner had been detectable. This is a significant number of HIV infections and therefore the preliminary analysis provided the first direct evidence that treatment can dramatically reduce the risk of HIV transmission through receptive anal sex.

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See Also
Fact Sheet: HIV/AIDS and Young Men Who Have Sex With Men
More on HIV Prevention Research in Gay Men

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