News & Perspective

PARTNER2 study confirms undetectable viral load equals untransmittable HIV

Infectious Diseases
2 years ago, OP Editor

The first phase of an observational study, PARTNER1 had provided statistical certainty of zero HIV transmission rate between serodiscordant heterosexual couples through condomless sex when the positive partner is virally suppressed. The irrefutable evidence presented by the second phase of the study, PARTNER2 now extends a similar level of confidence about zero HIV transmission rate through condomless sex for serodiscordant gay couples when the positive partner has undetectable viral load.1 The results from the PARTNER2 study were presented at 22nd International AIDS Conference (AIDS 2018) held in Amsterdam.1 The new evidence would help combat decades of HIV-related stigma and discrimination which has been earnestly desired by HIV-infected patients.1,2

PARTNER is a multicenter observational study focused on studying the absolute risk of HIV transmission through condomless sex amongst human immunodeficiency virus (HIV) serodiscordant couples (the HIV positive partner is on suppressive antiretroviral therapy [ART] with an undetectable viral load).3 Although the first phase of the study, PARTNER1, had already provided statistical certainty of the “Undetectable equals Untransmittable” (U=U) concept, the results did not hold much promise for gay couples or for anal sex.1

The higher bound of the 95% CI was 0.45% for all patients in PARTNER1 (including heterosexual couples), however, it was 0.84% for any condomless sex between gay men and 4% for receptive anal sex with ejaculation.1 Therefore, PARTNER2, a continuation of PARTNER1, was conducted at 75 clinical sites in 14 European countries to narrow the CI for anal sex between men.1

PARTNER2 prospectively recruited new gay couples, plus the original gay couples from PARTNER1, forming a cohort of 972 gay couples for the study.1 They reported having had condomless sex with each other for roughly one year prior to joining the study, while the HIV positive partners had already been receiving ART for about 4 years. Couples provided couple-years of follow up (CYFUs) throughout their time in the study.1 However, they contributed to the data only if they had had condomless sex since the last data were collected, if the positive partner had maintained a viral load below 200 copies/ml throughout, and if the negative partner reported no use of pre- or post-exposure prophylaxis.1 Owing to these conditions, only 783 gay couples ended up contributing to 1,596 CYFUs including an estimated total of 76,991 individual condomless sex acts.1 Every 6 to 12 months, both partners completed sexual behavioral surveys, the negative partner was tested for HIV, and the positive partner received a viral load test.4

The 1,596 couple-years of data reported zero viral transmission between the partners. “We looked so hard for transmissions. And we didn’t find any,” the principal researcher of the PARTNER study, Dr. Alison Rodger said. During the course of PARTNER2, 15 new HIV infections were reported, but genotyping of the HIV found that none of the new infections came from the primary partner.1,3

These new data have now produced a narrower CI for the whole PARTNER study, with the upper limit of the 95% CI being 0.57% for receptive anal sex with ejaculation and 0.23% for all condomless sex.1

In 2017, the Opposites Attract study also reported zero HIV transmissions in roughly 17,000 occasions of condomless anal sex between serodiscordant gay partners.1 If the results from this study were to be combined with those from the whole PARTNER study, zero transmission has been observed in nearly 126,000 sex acts.1

When enquired about the clinicians who are still not accepting this message and withhold this information from their HIV patients, Dr. Rodger replied, “I think the time for excuses are over. I think it’s very, very clear that the risk is zero.” The time PARTNER2 data became public, the instant response from the gay community was immensely positive.1 The co-founder of THRIVE SS in Atlanta, Daniel Driffin, who attended AIDS 2018 Congress said, “We still have to continue the efforts to get key populations – especially black people in the U.S., gay and bisexual men and trans women.”3


  1. Zero transmissions mean zero risk-PARTNER 2 study results announced. NAM. 2018 (Accessed August 8, 2018, at
  3. HIV transmission risk through condomless sex if the HIV positive partner is on suppressive ART: PARTNER study. CROI. 2014 (Accessed on August 8, 2018,
  4. Gay Men Cannot Get HIV From Partners Who Are Virally Suppressed, New Study Proves. THE BODY PRO. 2018 (Accessed on August 8, 2018,


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