European study of nearly 1,000 gay male couples who had sex without using condoms – where one partner was HIV positive and on suppressive antiretroviral therapy (ART) and the other HIV negative – reports no cases of within-couple HIV transmission over 8 years.

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A European study of 972 gay male serodifferent couples, in which one partner was on suppressive antiretroviral therapy (ART), has found no cases of HIV transmission within couples over 8 years of follow up.

The observational study, which is the largest of its kind and published in The Lancet, provides conclusive evidence that having an undetectable viral load (less than 200 copies/mL) on ART means that the virus is untransmittible.


Over 8 years of follow up in the study, 15 HIV-negative men became infected with HIV, but none of the viruses screened in the newly infected partner were genetically linked to the HIV virus that had infected their main partners, ruling out any within-couple HIV transmissions.

Importantly, the researchers estimate that effective ART prevented around 472 HIV transmissions during the 8 years of the study.

Regular testing is still important

The authors emphasise the importance of regular monitoring and supporting people with long-term adherence to therapy and caution that the effectiveness of ART in preventing HIV transmission is dependent on ensuring that the virus remains undetectable in the blood.


The results support the international U=U (undetectable equals untransmittable) campaign, that has been endorsed by more than 780 HIV organisations in 96 countries, to raise awareness of how scientific evidence shows that effective ART means people living with HIV can have sex without fear of transmitting the HIV virus to others.

“Our findings provide conclusive evidence for gay men that the risk of HIV transmission with suppressive ART is zero,” says Professor Alison Rodger from UCL, UK, who co-led the research. “Our findings support the message of the international U=U campaign, that an undetectable viral load makes HIV untransmittable. This powerful message can help end the HIV pandemic by preventing HIV transmission, and tackling the stigma and discrimination that many people with HIV face. Increased efforts must now focus on wider dissemination of this powerful message and ensuring that all HIV-positive people have access to testing, effective treatment, adherence support and linkage to care to help maintain an undetectable viral load.” [1]

The new study, PARTNER2, assessed the risk of HIV transmission between serodifferent (one partner is HIV-positive, one is HIV-negative) gay male couples not using condoms. The results add to an earlier phase of the PARTNER study, which found that HIV transmission risk for serodifferent heterosexual couples was zero.


The first phase of the PARTNER study, which took place between September 2010 and May 2014 and was published in 2016, recruited and followed up 1,116 serodifferent couples who were not using condoms. Of these, 888 couples, both heterosexual (548 couples) and gay men (340 couples), provided 1,238 eligible years of follow up from 14 European countries. The second phase of the study (PARTNER2) included gay male couples only.

Between September, 2010 and July, 2017, 972 gay couples were recruited, of which 782 provided 1,593 eligible years of follow up—the main reasons couples weren’t included in the analysis were because no condomless sex was reported, they had used pre-exposure or post-exposure prophylaxis (PrEP or PEP), or HIV viral load or HIV test results were not available.

Couples had frequent follow-up (every 6-12 months) to complete confidential questionnaires on their sexual behaviour, to test HIV-negative partners for HIV, and to measure plasma HIV-1 viral load in the HIV-positive partner. If HIV-negative partners became infected with HIV, genetic testing was done to see whether the virus was genetically similar to their study partner’s virus, or was acquired from another sexual partner.


At the start of the study, the couples had already been having sex without condoms for a median of 1 year, and the HIV-positive partners had been on ART for a median of 4 years, with high adherence (98% participants reported 90% adherence).

During an average of 2 years follow-up (per couple), couples had anal sex without condoms a total of 76,088 times (equivalent to about once a week).


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During the study, over a third (37%; 288/777) of HIV-negative men reported having sex without a condom with other partners. Around a quarter of HIV-positive (214/779) and HIV-negative (185/779) men reported at least one STI such as syphilis and gonorrhoea (table 2) since their last visit. Only 5% (37/779) of HIV-positive men reported missing ART for more than 4 consecutive days.

The authors note several limitations, including that most HIV-negative gay men were white with an average age of 38 years—yet most HIV transmission occurs in young people aged 25 years or younger. Additionally, the majority of HIV-positive partners had been on suppressive ART for several years, so the authors had limited data on transmission risk during the initial months of ART.

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Commenting on the implications of the findings in a linked Comment, Dr Myron Cohen from the UNC Institute for Global Health and Infectious Diseases, USA writes: “These important results serve to inspire and challenge us. Timely identification of HIV-infected people and provision of effective treatment leads to near normal health and lifespan and virtual elimination of the risk of HIV transmission. Yet maximising the benefits of ART has proven daunting, especially for men who have sex with men It is not always easy for people to get tested for HIV or find access to care; in addition, fear, stigma, homophobia, and other adverse social forces continue to compromise HIV treatment. Furthermore, diagnosis of HIV infection is difficult in the early stages of infection when transmission is very efficient, and this limitation also compromises the treatment as prevention strategy…The results of the PARTNER2 study provide yet one more catalyst for a universal test-and-treat strategy to provide the full benefits of antiretroviral drugs. This and other strategies continue to push us toward the end of AIDS.”

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