HIV transmission risk is zero with suppressive treatment

07 May 2019

PARTNER2 trial results confirm that gay men can also benefit from U=U (undetectable=untransmittable), as Lancet publishes final study report.

Gay men chatting

Zero new HIV infections occurred in more than 76,000 condomless sex acts among mixed-status gay couples, where one partner is HIV-negative and the other is living with HIV and adhering to their antiretroviral treatment (ART).

The findings were published by the Lancet last week, after being previously reported during a late-breaker session at the International AIDS Conference in Amsterdam (AIDS 2018) last July. The news has since made global headlines.

The study in question, PARTNER2, was the follow-up study to the landmark PARTNER1 study, the results of which were released back in 2016. PARTNER1 reported data from 75 clinical sites in 14 European countries and revealed zero linked HIV transmissions among 888 mixed-status couples who reported condomless penetrative sex during 1,238 couple-years (the total number of years of follow up across all couples included in the study).

The findings of PARTNER 1 were revolutionary, providing real-world evidence of the preventative benefit of effective ART on top of the benefit for keeping the person living with HIV healthy. When the level of virus is reduced to such low levels that they are ‘undetectable’, there is zero chance that HIV can be transmitted.

But what this meant for gay and other men who have sex with men was, at the time, less clear. Penetrative anal sex is associated with a higher risk of HIV, but too few gay couples were included in the original PARTNER study to provide a similar level of evidence as for heterosexual couples. Of the gay couples who did participate in PARTNER1, however, the rate of within-couple transmission was zero.

From June 1, 2014, to July 31, 2017, the second phase of PARTNER2 was launched to fill the research gap, recruiting gay male mixed-status couples only. Couples were allowed to take part if they were over 18, had condomless anal sex in the last month, if the HIV-positive partner intended to stay on ART, and the couple expected to have sex with each other again in the coming months. Follow-up was stopped if the partnership ended, the couple moved away, or if either partner withdrew consent.

Data from time periods where the partner was on pre-exposure prophylaxis (PrEP), which is a treatment taken by an HIV-negative partner to further eliminate the chance of getting HIV, was not included in the analysis. 

In the end, 782 couples were followed for almost 1,600 eligible couple-years of follow-up which included more than 76,000 reports of condomless sex. Without ART, researchers predicted that 472 HIV transmissions would have taken place through receptive condomless anal sex acts alone, on the basis of frequency and type of sex. However, zero took place.

In their conclusion, the authors noted the importance of reducing the number of people who don’t know their status to fully realise the benefits of undetectable=untransmittable (U=U), “sustained effort is required to increase rates of testing and HIV diagnosis with early initiation of ART and full support to maintain high levels of adherence.”

“The results from the PARTNER studies support wider dissemination of the message of the U=U campaign that risk of transmission of HIV in the context of virally suppressive ART is zero. This dissemination is necessary to promote the benefits of early testing and treatment and to tackle stigma, discrimination, and criminalisation laws that continue to affect HIV-positive people.”

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Written by Caitlin Mahon

Content Specialist - HIV & Sexual Health