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PARTNER2 Study: Viral Suppression of HIV Prevents Sexual Transmission Between Gay Men


Findings from PARTNER2 show that HIV-positive individuals who were virally suppressed by antiretroviral therapy had no risk of transmitting the virus by having sex without condoms.

Findings from the landmark PARTNER2 study provided clear evidence that individuals with HIV who had achieved viral suppression through antiretroviral therapy (ART) had no risk of transmitting the virus to their partners through sexual contact, according to The Lancet.

“With no linked transmissions, the estimated rate for transmission through condomless anal sex when the positive partner on ART had HIV viral load less than 200 copies per milliliter was zero,” researchers said.

PARTNER2, the second phase of the PARTNER study, included gay male serodifferent couples, in which 1 partner was HIV-positive and receiving ART while the other partner was HIV-negative, who reported acts of condomless anal sex from 2010 to 2018. The PARTNER study included heterosexual couples while PARTNER2 was restricted to only gay couples.

The goal of PARTNER2 was to provide precise figures of transmission risk among gay serodifferent partnerships. Researchers conducted the study due to the fact that data regarding the risk of HIV transmission through condomless sex between serodifferent gay couples is limited compared to data collected concerning HIV transmission risk between heterosexual couples.

At baseline, couples had already been engaging in condomless anal sex for a median period of 1 year and HIV-positive partners were receiving ART with high adherence for a median of 4 years.

In PARTNER2, a total of 972 gay couples were recruited between September 2010 and July 2017. The study collected 1593 observation years of follow-up data from 782 couples. Data were collected every 4 to 6 months. Those that weren’t included in the analysis either reported no acts of condomless anal sex, involved a partner who reported using pre-or post-exposure prophylaxis at some point during the study, or were missing results from HIV or viral load tests.

Results from PARTNER2 found that no cases of HIV transmission occurred between couples over 8 years of follow up, albeit over 76,000 acts of condomless sex were reported during the study period. Researchers predicted that as many as 472 HIV transmissions were prevented by ART.

Couples frequently followed up every 6 to 12 months to complete confidential questionnaires concerning their sexual behavior, to test HIV-negative partners for HIV, and to measure plasma HIV-1 viral load in HIV-positive partners. If HIV-negative partners became infected with HIV, genetic testing was conducted to determine whether the virus was genetically similar to their study partner’s virus, or if it was acquired another way. While 15 HIV-negative individuals became infected with HIV over 8 years of follow up, genetic testing showed that none of the viruses newly infected partners contracted were identical to the virus their main partner had, proving HIV wasn’t transmitted through sex between study couples.

Throughout all couple-years of follow-up, only 5% of HIV-positive partners reported that they missed ART for more than 4 consecutive days. For 1461 of 1593 eligible couple-years of follow-up, HIV-positive partners reported their rate of adherence to ART was over 90%.

During the study, 37% of HIV-negative men reported having condomless anal sex with someone other than their main partner. Over 27% of HIV-positive and nearly 24% of HIV-negative men reported at least one sexually transmitted infection since their last visit.

The study’s findings support the international Undetectable Equals Untransmittable (U=U) campaign, endorsed by 780 HIV organizations throughout 96 countries, which raised awareness that HIV-positive individuals could have sex without the concern of transmitting the virus to others.

“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 criminalization laws that continue to affect HIV-positive people,” researchers said.

The first phase of the PARTNER study, which was conducted between September 2010 and May 2014 and was published in 2016, recruited and followed up 1116 serodifferent couples that engaged in condomless sex. A total of 888 couples, 548 heterosexual and 340 that were gay, met inclusion criteria provided 1238 eligible years of follow up data from 14 European countries.

Researchers stressed that HIV-positive individuals must maintain adherence to ART to ensure HIV remains undetectable in the blood and they have no risk of transmitting the virus.

“The HIV Care Continuum in the USA indicated that of the 1.1 million people living with HIV in 2014, 85% were diagnosed, but only 49% were virologically suppressed. A sustained effort is required to increase rates of testing and HIV diagnosis with early initiation of ART,” researchers concluded.


Rodger AJ, Cambiano V, Bruun T, 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 [published online May 2, 2019]. The Lancet. doi: 10.1016/S0140-6736(19)30418-0.

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