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This is a plain English summary of an original research article. The views expressed are those of the author(s) and reviewer(s) at the time of publication.

The risk of transmission of HIV between gay couples when the HIV-positive partner is taking antiretroviral treatment that successfully suppresses the viral load is ‘effectively zero.’

A study of men from 14 European countries, including the UK, found no cases of transmission of HIV from an HIV-positive partner taking antiretroviral therapy to an HIV-negative partner, as long as the viral load of the HIV-positive partner remained undetectable or very low.

The study recorded an average of two years during which the partners reported condomless anal sex. Although there were 15 cases where men who were initially HIV-negative became infected with HIV, viral genetic testing showed that in no case was this transmission from the partner taking antiretroviral therapy.

The researchers emphasised that adherence to treatment, regular testing of viral load and of HIV status are important to ensure HIV transmission does not occur.

Why was this study needed?

Without the suppression of HIV through antiretroviral therapy, the virus can be transmitted through blood and semen during sexual activity. While condoms reduce the risk of infection, they are not always used consistently, and people are not always in a position to insist on their use.

Previous studies have already found that the rate of transmission of HIV between partners where one was HIV-negative and the other was HIV-positive was very low - if the viral load was suppressed to undetectable levels in the HIV-positive partner. However, these earlier studies had included mostly heterosexual couples, and this research sought to clarify if the same applied for men who have sex with men.

This study was an extension to a previous mixed study of 340 male couples and 548 heterosexual couples. It recruited additional male couples, to provide more precise estimates of the risk of HIV transmission between partners in gay relationships.

What did this study do?

PARTNER2 was an observational study. Researchers recruited 972 male couples aged 18 or over where:

  • one partner was HIV-positive and one HIV-negative,
  • the HIV-positive partner was taking antiretroviral therapy and expected to continue,
  • the couple reported having condomless penetrative sex in the month before enrolment and were expected to continue.

The antiretroviral regime, CD4 count and HIV-1 RNA load were recorded for HIV-positive partners at each visit, while the HIV-negative partners were tested for HIV every 6 to 12 months.

Questionnaires every 4 to 6 months recorded adherence to antiretroviral therapy, sexual behaviour, other sexually transmitted infections, use of pre- or post-exposure prophylaxis and injecting drug use.

This was a well-conducted study in a large number of men so the results should be reliable.

What did it find?

  • From the 972 couples recruited, 782 couples contributed 1,593 couple-years of follow-up, with an average of two years of follow-up per couple. In 96% of the couple-years available, the HIV-positive partner had an undetectable viral load (less than 50 copies/ml plasma HIV-1 RNA), and in the remaining 4%, viral load was very low (between 50 and 200 copies/ml).
  • The median frequency for couples having condomless sex during the study was 43 times a year. There were no within-couple HIV transmissions recorded. The estimated transmission rate was 0.0 (upper limit of 95% confidence interval (CI) 0.23). This was equivalent to one transmission per 435 years of condomless sex. Without antiretroviral therapy, based on previous figures, the researchers say they would have expected to see around 472 HIV transmissions (95% CI 83 to 714).
  • Although 15 men initially HIV-negative did become HIV-positive during the study, RNA sequencing showed the virus had not been transmitted by the partner taking virally suppressive antiretroviral therapy. Condomless sex with other partners was reported by 37% of the HIV-negative men.
  • Only eight couple-years of follow-up were recorded during the first six months of antiretroviral therapy for the HIV-positive partner. So we can’t be sure that the results apply to men in the early months of antiretroviral therapy, when viral load may not yet be fully suppressed. Most couples reported having condomless sex for a median one year before being enrolled in the study.
  • Sexually transmitted infections were common, occurring in 24% of HIV-negative men and 27% of HIV-positive men. Condomless sex was reported 6,090 times while an infection was present.

What does current guidance say on this issue?

The NICE clinical knowledge summary for treatment of people with established HIV infection states: “Advise using condoms with water-based lubricant for vaginal and anal sex, and condoms or dams (latex sheets) for oral sex.”

The summary, last revised in 2018 and due to be updated in 2020, does not touch on transmission risk between partners when viral load is suppressed.

What are the implications?

The study demonstrates that HIV treatment can reduce viral load to levels where the risk of transmission is very low to non-existent.

As such, it shows the importance to their partners of HIV testing and treatment for people at risk of HIV, and of good adherence to antiretroviral treatment and regular testing of viral load for people who are HIV-positive. The same benefits would not be seen in practice if adherence to treatment were not as good as in the men in this study

It does also highlight the high risk of other sexually transmitted infections when condoms are not used.

Citation and Funding

A Rodger, V Cambiano, T Bruun et al. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational study. Lancet. 2019; May 2. doi: 10.1016/S0140-6736(19)30418-0. [Epub ahead of print].

This project was funded by the NIHR Research for Patient Benefit Programme (project number PB-PG-1013-32069).



Cohen M S. Successful treatment of HIV eliminates sexual transmission. Lancet. 2019: May 2. doi: 10.1016/S0140-6736(19)30701-9. [Epub ahead of print].

NICE. HIV infection and AIDS. Scenario: established HIV infection. Clinical Knowledge Summary. London: National Institute for Health and Care Excellence; last updated 2018.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre


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