The largest study of HIV transmission to date finds that HIV-positive men whose virus levels are undetectable as a result of treatment do not transmit the virus to their sexual partners.

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A new, large-scale study reports zero cases of HIV transmission among male homosexual couples with differing HIV statuses.

Almost 37 million people worldwide currently have HIV, and 1.2 million adults in the United States are living with the virus. Although the death rate from HIV has dropped significantly over the past few decades due to public awareness campaigns and the success of antiretroviral therapy (ART), the transmission rates for the virus remain high.

According to the National Institutes of Health (NIH), the year 2015 recorded 2.1 million new infections all over the world, a level that the NIH deem “unacceptable.”

However, recent results from a landmark study offer hope for a drastic reduction in HIV transmission. In fact, according to the new research, HIV-positive homosexual men who have an undetectable viral load – as a result of diligently taking their medication – do not transmit the virus to their partners after having unprotected sex.

The study, called “Opposites Attract,” was carried out by the Kirby Institute in Sydney, Australia, and the results were presented at the ninth International AIDS Society conference, held in Paris, France.

This is the largest study to have ever investigated HIV transmission risk among male homosexual couples with different HIV statuses.

HIV treatment known as ART reduces the virus levels in the body, stopping the virus from damaging the patient’s immune system, and in some cases, even reversing this damage.

Additionally, ART treatment can prevent the transmission of the virus if the medication is taken daily and as prescribed. If medication is taken consistently, virus levels are reduced to the point that they become undetectable.

Two previous studies have investigated HIV transmission rates among couples with differing HIV statuses.

In 2011, one study showed that ART treatment reduced the chances of HIV transmission from an HIV-positive person to their HIV-negative sexual partner by 96 percent. However, these results overwhelmingly regarded heterosexual couples.

Therefore, the so-called PARTNER study aimed to investigate whether the same applied to male homosexual couples. This study examined heterosexual and gay couples alike, and found zero HIV transmissions from partners with an undetectable viral load.

Now, the Opposites Attract study has looked at homosexual couples exclusively.

Opposites Attract is a large cohort study that consisted of 358 gay male couples, all of which comprised one partner who was HIV-positive, and one who was HIV-negative.

The participants came from various social backgrounds and from three different countries, which were Thailand, Brazil, and Australia.

The study was carried out between 2012 and 2016. Throughout this period, the couples reported a total of nearly 17,000 acts of unprotected anal sex.

During the 4 years, zero cases of HIV transmission were reported. This shows that when HIV-positive gay men manage to maintain undetectable levels of the virus, the risk of passing it on to an HIV-negative partner is negligible.

This is especially encouraging given that, compared with vaginal intercourse, the risk of HIV transmission during anal sex has been estimated to be 18 times higher.

Lead researcher Prof. Andrew Grulich, from the Kirby Institute, summarizes the findings, saying, “Undetectable virus level effectively prevents HIV transmission among gay couples.”

“Opposites Attract is the first study to show that these results apply in both high- and middle-income countries,” he adds. “Our research adds to the evidence from a small number of other international studies of heterosexual and homosexual couples.”

[W]e can say, with confidence, that effectively treated HIV blocks transmission in couples of differing HIV status […] This is life-changing news for couples of differing HIV status.”

Prof. Andrew Grulich

However, he cautions, “It is important that the HIV-positive partner is under regular medical care and does not miss any of their antiretroviral medication in order to ensure they maintain an undetectable viral load.”