Groundbreaking research shows 95 percent reduction in HIV transmission between sex partners when antiretroviral therapy is started early

The world of HIV prevention changed last week when it was announced that a recent National Institutes of Health (NIH)-funded study was stopped early when it was determined that the risk of HIV transmission is 95 percent less if the person with HIV is on treatment and has his/her viral load (the amount of virus present in blood) completely suppressed than if they are not on antiretroviral therapy.

The study, carried out by the HIV Prevention Trials Network and designated HPTN 052, started in 2005 and involved 1,763 couples, 95 percent of whom were heterosexual, in which one partner had HIV and the other did not. Subjects were enrolled at 13 sites across the Americas, Asia and Africa. All HIV-infected participants had CD4 counts between 350 and 550/mm3 at enrollment and had not yet taken anti-HIV therapy. (CD4 count is the number of CD4, or T-helper lymphocytes, from a blood sample.) One group of study participants received anti-HIV drugs at the time of enrollment; the other, comparison group did not. Over the course of the study 28 partners of study participants acquired HIV — only one partner in the group receiving treatment and 27 in the group who were not on therapy.

While the HPTN 052 study did not include large numbers of persons with same sex partners, it is an important demonstration of principle, and it seems likely that substantial protection against transmission would occur in same-sex couples. The study results support the decision to start anti-HIV therapy in persons with CD4 counts higher than currently recommended, and serve to re-enforce the message stated again and again on the MedHelp forums and communities that it is important to ask partners if they have HIV and, if they have HIV, whether they are on effective therapy, before initiating sexual activity. While a 95 percent reduction is not the same as the virtually complete reduction of risk when condoms are used consistently and correctly and remain intact throughout the sex act, it is an important, newly recognized element of HIV prevention that has been proved now through rigorous scientific study.

Previous research had suggested that HIV infected persons on treatment were less likely to infect their sex partners. However, HPTN 052 studied much larger numbers of patients and was a randomized controlled trial, the most powerful kind of research design. In addition, the finding of 95 percent protection was a surprise to the researchers. In designing the study, they counted on a substantially lower level of protection. This is why they anticipated continuing to follow the research subjects and their partners through 2015; it was thought it might take that long to accurately measure a difference in transmission risk between those taking anti-HIV treatment and the untreated control couples. That the protection was so good that the trial could be stopped 4 years ahead of schedule was a very pleasant surprise to the investigators, as well as to other HIV prevention experts.

What This Means for You

For HIV infected persons, the results mean that antiviral treatment not only improves health and survival but goes a long way in preventing sexual transmission of the virus to uninfected partners. But there also is an important "take-home" message for sexually active persons without HIV: if a person's sex partner has HIV and is taking anti-HIV therapy, the chance of sexual transmission is much lower than it otherwise would be. This is especially so if the infected person has a low viral load, meaning they have small amounts of HIV in their blood, or none at all. Of course it remains wise to discuss mutual HIV status prior to sex with a new partner and avoid vaginal or anal sex with partners known to have HIV, and to use condoms routinely for vaginal or anal sex outside mutually monogamous relationships. But these very important research results should help reassure persons who, after an exposure, learn their partners have HIV but are on treatment for it.

In the future, we can anticipate additional helpful research outcomes from HPTN 052. Since the inception of MedHelp's HIV Prevention forum and Sexually Transmitted Disease forum, and more recently the HIV - International forum, one of the most common questions we field regards the probability of catching HIV after a sexual exposure. The available data are imprecise. We often quote figures like 1 chance in 1,000 to 2,000 for transmission by a single, unprotected vaginal sex exposure, if one partner is infected. Such data, which have been published by the U.S. Centers for Disease Control and Prevention (CDC), are rough estimates at best, because they are based primarily on people's recollections of sexual exposures in the months and years before they tested positive. In HPTN 052, the research subjects were asked to keep careful records of their sexual exposures. Analysis of these data should give more precise estimates of transmission risk for vaginal intercourse. Whether there was sufficient frequency of oral or anal sex for accurate calculation remains to be seen — but perhaps we will have improved information for those practices as well.