Institute for Global Health


Partners of people on ART - a New Evaluation of the Risks (The PARTNER study)

Response from PARTNER study team to recent Thai media discussion (February 2020)

Results from the PARTNER study have led to a discussion in Thai media of “Undetectable = untransmissible (U=U)” for people with HIV.

Our study's author's have subsequently issued a re-statement of the scientific facts, addressing the discussion.  Likewise, the Ministry of public Health of Thailand, the World Health Organization (WHO) and UNAIDS have issued their own joint statement:

Ministry of Public Health of Thailand, WHO and UNAIDS joint statement

Project Summary

The PARTNER study was a study of HIV serodifferent couples (one HIV positive and one HIV negative) to precisely estimate the risk of within couple transmission through sex during periods where condoms not used, and the positive partner was on suppressive ART (HIV viral Load <200 copies/ml).

Phase 1 of the study (PARTNER1), recruited and followed both gay and heterosexual serodifferent couples from 2010 to 2014. The second phase (PARTNER2), recruited and followed gay serodifferent couples from 2014 to 2018.  The PARTNER studies were specifically designed to be large enough to make the upper 95% confidence interval risk negligible around the study finding of zero.

PARTNER1 results were published in JAMA in 2016. The results indicated that among 548 serodifferent heterosexual and 340 gay couples in which the HIV-positive partner was using suppressive ART and who reported condomless sex during eligible follow up there were no documented cases of within-couple HIV transmission (determined by genetic linkage of the viruses).

This was despite the couples reporting a significant number of sex acts where condoms were not used (58,000 acts). However, there was still not enough data from PARTNER1 to definitively establish the risk of condomless sexual exposure in in gay men with HIV viremia suppressed to <200 copies/ml and to be able to say with confidence that the risk was zero. 

The final results of the PARTNER 2 study were presented at the World AIDS conference in 2018 (IAS2018) and published in the Lancet in 2019 and demonstrated that the risk of HIV transmission through condomless anal sex when the HIV positive partner has undetectable viral load is zero, with no linked transmissions despite gay couples in the study reporting 76,000 sex acts without using condoms. The results of PARTNER demonstrated zero risk applies in both heterosexual serodifferent couples (PARTNER1) and also in gay men (PARTNER2). 

Links to other research

Other research from the UCL Centre for Clinical Research, Epidemiology, Modelling and Evaluation (CREME)

Other research from IGH on HIV, Sexually Transmitted Infection (STI), Epidemiology and Prevention

Other research from IGH in Europe