HIV treatment prevented transmission of the virus in gay couples when only one partner had the virus, a new study shows.

The research included 358 such gay couples in Australia, Brazil and Thailand. The HIV-positive partners received treatment that reduced the amount of virus in the blood to undetectable levels.

The participants were followed from 2012 to 2016. During that time, the couples reported nearly 17,000 instances of anal intercourse without a condom. None resulted in HIV transmission, the study authors said.

“Undetectable virus level effectively prevents HIV transmission among gay couples,” said chief study investigator Andrew Grulich, a professor at the Kirby Institute at the University of New South Wales in Australia.

“This is life-changing news for couples of differing HIV status. But it is important that the HIV positive partner is under regular medical care and does not miss any of their anti-retroviral medication in order to ensure they maintain an undetectable viral load,” Grulich said in a news release from amFAR, the Foundation for AIDS Research.

The study was to be presented Tuesday at the IAS Conference on HIV Science, in Paris. Findings presented at meetings are typically viewed as preliminary until they’ve been published in a peer-reviewed journal.

This “is the first study to show that these results apply in both high- and middle-income countries,” Grulich said. “Our research adds to the evidence from a small number of other international studies of heterosexual and homosexual couples and means that we can say, with confidence, that effectively treated HIV blocks transmission in couples of differing HIV status.

“Our data add to previous studies which show that there has never been a recorded case of HIV transmission from an HIV-positive person to their HIV-negative sexual partner when the HIV-positive partner had undetectable viral load,” he said.

More information

The U.S. Centers for Disease Control and Prevention has more on HIV/AIDS.

Source: HealthDay