International study of gay couples reports no transmissions from an HIV-positive partner on treatment

Image from Opposites Attract study:

A study of 343 gay couples, where one partner had HIV and the other did not, has not found a single case of HIV transmission in 16,889 acts of condomless anal sex, the 9th International AIDS Society Conference on HIV Science (IAS 2017) in Paris, France, was told today.

The Opposites Attract study looked at whether HIV is transmitted between gay male couples of different HIV status when the HIV-positive partner is on treatment that fully suppresses HIV. The HIV-positive partners in Opposites Attract had a so-called ‘undetectable viral load’ 98% of the time.

The study recruited and followed-up gay couples at clinics in Australia, in Bangkok and in Rio de Janeiro.



Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 

detectable viral load

When viral load is detectable, this indicates that HIV is replicating in the body. If the person is taking HIV treatment but their viral load is detectable, the treatment is not working properly. There may still be a risk of HIV transmission to sexual partners.

Undetectable = Untransmittable (U=U)

U=U stands for Undetectable = Untransmittable. It means that when a person living with HIV is on regular treatment that lowers the amount of virus in their body to undetectable levels, there is zero risk of passing on HIV to their partners. The low level of virus is described as an undetectable viral load. 

virological suppression

Halting of the function or replication of a virus. In HIV, optimal viral suppression is measured as the reduction of viral load (HIV RNA) to undetectable levels and is the goal of antiretroviral therapy.


The Joint United Nations Programme on HIV/AIDS (UNAIDS) brings together the resources of ten United Nations organisations in response to HIV and AIDS.

The evidence from Opposites Attract adds to the evidence from the PARTNER study that HIV-positive people on effective HIV treatment that fully suppresses their virus cannot transmit their infection through sex. Taken together, the two studies have not found a single case of HIV transmission in nearly 40,000 acts of condomless anal sex between gay men.

This adds further strength to the “U=U” (Undetectable equals Untransmittable) tagline of the Prevention Access Campaign, whose consensus statement has been signed by NAM and also by the International AIDS Society (IAS), organisers of this week’s Conference on HIV Science in Paris.

The studies also found that if the positive partner is on successful treatment, even having another sexually transmitted infection (STI) does not increase the risk of HIV being transmitted. In Opposites Attract, participants had an STI during 6% of anal sex acts and in PARTNER, 17.5% of participants had an STI at some point in the study.

Sexual position also made no difference even though when viral load is not suppressed, transmission is 10-20 times more likely if the HIV-positive partner is the insertive one; in Opposites Attract, the HIV-positive partner was ‘top’ over a third of the time.

During the Opposites Attract study, three men became infected with HIV, but genetic analysis showed that these infections came from a partner outside the main relationship who was not virally suppressed.

Only 0.9% of the total proportion of condomless anal sex acts happened where the HIV-positive partner had a detectable viral load, and only 1.7% during their first six months on antiretroviral therapy. There were no transmissions from men in these groups either. Presenter Andrew Grulich commented: "In our Thai site, 40% of the men who entered the study were not on antiretroviral therapy when they entered the study, but immediately started it and were rapidly virally suppressed. They were really good at using condoms and other strategies to avoid transmission while they were still detectable, so the number of condomless anal sex acts with a detectable partner was very small."

In PARTNER, despite not many more episodes of condomless sex, there were ten such ‘unlinked’ infections in gay men. The difference may be due to 24% of the HIV-negative partners in Opposites Attract taking pre-exposure prophylaxis (PrEP). Researcher Benjamin Bavinton told there was evidence that PrEP-takers in Opposites Attract were indeed mostly taking it to protect themselves from HIV infection from partners outside the main relationship.

At a press conference the previous day on viral load and infectiousness, Dr Anthony Fauci, director of the US National Institute for Allergies and Infectious Diseases, said: “Scientists never like to use the word “Never” of a possible risk.

“But I think in this case we can say that the risk of transmission from an HIV-positive person who takes treatment and has an undetectable viral load may be so low as to be unmeasurable, and that’s equivalent to saying they are uninfectious. It’s an unusual situation when the overwhelming evidence base in science allows us to be confident that what we are saying is fact.”

Dr Luiz Loures, Deputy Executive Director of UNAIDS, said that in terms of the public impact of treatment as prevention, the agency was seeing more and more cities where HIV incidence was falling as they reached a tipping point in terms of the number of people who are on therapy and non-infectious: he quoted San Francisco, Sāo Paulo and Nairobi as examples.

Bruce Richman, a Harvard-trained lawyer, is the prime mover behind the “U=U” campaign.

He said: “In 2003 when I was diagnosed, I was terrified of infecting someone I loved and was terrified of taking a pill that reminded me every day I was infectious. But in 2012 when I finally started therapy, my doctor told me that if I suppressed my viral load, I would become non-infectious.

“My initial elation as being told this turned to outrage, because every website I found was saying I was still a risk. The breakthrough science was not breaking through to communities that needed to know it. Doctors would tell people on a one-to-on basis while withholding the info from those they deemed irresponsible.

“So we collaborated with doctors to endorse the U=U consensus statement. This is demolishing HIV stigma and encouraging people to start treatment and bring an end to the epidemic. We need people like UNAIDS, as they did today, to confirm it’s true.”


Bavinton B et al. (presenter Grulich A) HIV treatment prevents HIV transmission in male serodiscordant couples in Australia, Thailand and Brazil. 9th International AIDS Society Conference on HIV Science, Paris, abstract no TUAC0506LB, July 2017.

View the abstract on the conference website.

Watch the webcast of this session on YouTube.