Results from a large study of HIV transmission risk among homosexual male couples with differing HIV status have been published in The Lancet HIV.

The study, led by the Kirby Institute at UNSW Sydney, is called Opposites Attract, and contributes to global evidence demonstrating that when the HIV-positive partner is on daily antiretroviral therapy (ART) and has an undetectable viral load, the risk of sexual transmission to the HIV-negative partner is effectively zero.

“Opposites Attract shows that HIV treatment as prevention works,” said Dr Benjamin Bavinton from the Kirby Institute, who is the study’s project leader. “Not only is this information vital to inform HIV prevention in gay, bisexual, and other men who have sex with men, it provides strong evidence to help dismantle some of the stigma still associated with HIV”.

Opposites Attract

Researchers tracked the sexual behaviour of 343 couples in Australia, Brazil and Thailand over a four-year period, specifically acts of condomless anal intercourse, along with testing the HIV-negative partner for HIV, and the HIV-positive partner’s viral load. 

Despite over 12 000 acts of condomless anal intercourse over the course of the study where the HIV-negative partner was not taking PrEP and the HIV-positive partner was virally suppressed, there were no new HIV infections due to sex between Opposites Attract study partners. 

Since it was first presented at last year’s IAS Conference on HIV Science in Paris, the Opposites Attract data has added to a previously limited body of evidence in this area. 

“These results form a significant part of the evidence base for the international community-led Undetectable=Untransmissible, or U=U campaign, which highlights the fact that people living with HIV can now live long and healthy lives, with effectively zero chance of sexually transmitting the virus to others, provided their viral load is undetectable due to effective ART,” said Professor Andrew Grulich, head of the Kirby Institute’s HIV Epidemiology and Prevention Program and chief investigator on the study. 

The study has been cited in the Consensus Statement endorsed by HIV global leaders and organisations on risk of sexual transmission of HIV from a person living with HIV who has an undetectable viral load.