HIV treatment reduces the risk of transmission by reducing the quantity of HIV circulating in the body. When there is so little HIV in a person’s blood that their viral load is ‘undetectable’, the risk of sexual transmission is minimal.

The HPTN 052 randomised controlled trial has conclusively demonstrated that HIV treatment significantly reduces the risk of sexual transmission. The trial recruited 1763 couples in which an HIV-positive person had a CD4 cell count between 350 and 550 cells/mm³ and had an HIV-negative partner. Almost all the couples were heterosexual and most were living in African or Asian countries.

The HIV-positive participants were randomised either to start treatment immediately, or to defer treatment until their CD4 count fell below 250 cells/mm³.

A total of 28 individuals acquired HIV from their primary partner during the trial, one in the immediate-treatment arm and 27 in the deferred-treatment arm. This amounts to 96% fewer transmissions occurring.

The single transmission in the immediate-treatment arm took place a few days either before or after the person started HIV treatment, in other words before full viral suppression had been achieved.

In considering whether HIV treatment will reduce the transmission risk by 96% in all circumstances, it is worth remembering that HPTN 052 was a clinical trial, conducted under optimum conditions – participants received adherence and safer sex counselling as well as frequent testing for viral load and sexually transmitted infections. Moreover, the participants were couples in stable relationships in which each partner was aware of the other’s HIV status; only 5% of participants reported having unprotected sex.

A large observational study (the PARTNER study) has provided preliminary data that support the findings of HPTN 052. The researchers have recruited couples in which an HIV-positive partner is taking HIV treatment and has an HIV-negative partner. In contrast to the randomised trial, all couples report using condoms inconsistently or not at all.

Importantly, approximately half of the participants are men who have sex with men. The study is being conducted in 14 European countries.

An interim analysis, with data on almost 800 couples who reported just under 45,000 acts of penetrative sex, found that there had been no transmissions from a partner with an undetectable viral load. This applies to both anal and vaginal sex.

The researchers are collecting more data, and recruiting more gay couples, so that they can provide precise estimates of the transmission risk during different sexual acts. Final results are due in 2017.

A number of smaller observational studies have been conducted with heterosexual couples only. Their results have varied somewhat, but do broadly confirm the findings of HPTN 052. These studies have consistently shown that HIV transmission occurs very infrequently when the HIV-positive partner is taking treatment or has a low viral load.

Some concerns have been raised about circumstances in which transmission could still occur, despite the person with HIV taking treatment. Transmission might occur during the first few months of a person taking treatment – studies suggest that viral load is most likely to remain consistently undetectable after six months or more of treatment. If adherence is poor (i.e. drug doses are missed or taken late), viral load may rise. Temporary rises (‘blips’) in the viral load in sexual fluids, caused by sexually transmitted infections or menstruation, could possibly have an impact on the risk of transmission.

Different antiretroviral drugs may have different abilities to penetrate into the male genital tract, female genital tract and rectal tissue. Studies have sometimes found that individuals have had an undetectable viral load in blood, but not in other body fluids. Nonetheless most people who have an undetectable viral load in blood are also undetectable in their semen or vaginal fluids, as well as in their rectal mucosa and (in the case of women) vaginal mucosa.

Despite these uncertainties it remains clear that, overall, effective HIV treatment has a profound impact on infectiousness and sexual transmission. People taking HIV treatment who have an undetectable HIV viral load are much less likely to pass on HIV than people not on treatment.