Further evidence reporting a lack of HIV transmission when viral load is undetectable on ART was presented at IAS 2017 from the prospective Opposites Attract study in gay men.
The study included 358 serodifferent gay male couples from Australia (n=157), Thailand (n=105) and Brazil (n=96) who were already not consistently using condoms. The final study results were presented in a late breaker oral abstract by Andrew Gulich from the University of New South Wales, Sydney. 
Undetectable viral load on ART was defined as <200 copies/mL and participants were asked to keep a diary in order to estimate approximate levels of exposure risk by sexual activity. Also, phylogenetic analysis (of pol and env genes) were used to determine whether any new infections were likely linked to the main partner.
At baseline, only 80% of HIV positive partners were on ART, of whom just under 80% had undetectable viral load and STI prevalence was 14.3% and 11.7% in the positive and negative partners respectively. Open relationships were common with almost 60% of couples reported sex outside the main partnership.
By the end of 2016, the study had 591 couple years of follow up (CYFU) from 343 couples with at least one follow-up visit, of which 318 CYFU included periods when condoms were not used.
From 16,889 times when condoms were not used, there were three new HIV diagnoses, but none of these were linked infections to the main partner, incidence 0/100 CYFU (95%CI: 0 to 0.62).
The study also reported risks by exposure category, see Table 1.
|Linked transmissions (n)||CYFU||CLAI acts (n)||Incidence per 100 CYFU (95% CI)|
|Any CLAI||0||318.0||16,889||0 (0-1.16)|
|Any CLAI, no daily PrEP||0||241.3||12,928||0 (0-1.53)|
|Insertive CLAI||0||210.0||8,389||0 (0-1.76)|
|Receptive CLAI||0||132.1||4,569||0 (0-2.79)|
|UVL (VL <200)||0||236.2||12,638||0 (0-1.56)|
|VL >200||0||5.17||290||0 (0-71.4)|
|STI diagnosed||0||23.2||1,007||0 (0-15.9)|
|First 6 months ART||0||10.0||341||0 (0-36.9)|
Key: ART: antiretroviral treatment; CI: confidence interval; CLAI: condom-less anal intercourse; CYFU: Couple years of follow up; PrEP: pre-exposure prophylaxis; STI: sexually transmitted infection; UVL: undetectable viral load.
These important results are useful to read in association with earlier results, including the PARTNER study. 
Similar to PARTNER, undetectable was defined using a <200 copies/mL threshold and diaries were used to categorise different risk activity. Unlike PARTNER, enrolment criteria did not include being on ART or having undetectable viral load.
Taken together, both studies provide further evidence showing the lack of HIV transmission when viral load is undetectable, even when condoms are not used.
The barrier of proof should now shift towards producing evidence for whether HIV transmission is actually possible. This is especially in the absence of any published cases in the ten years since the Swiss Statement first asserted that transmission could not take place. [3, 4]
This body of evidence makes it easy to understand why leading scientists, including from the IAS, the US NIH and BHIVA support the campaign to say that that Undetectable = Uninfectious (U=U). 
- Grulich A et al. HIV treatment prevents HIV transmission in male serodiscordant couples in Australia, Thailand and Brazil. IAS 2017, Paris. Oral abstract TUAC0506LB.
- Rodger AJ et al for the PARTNER study group. Sexual activity without condoms and risk of HIV transmission in serodifferent couples when the HIV-positive partner is using suppressive antiretroviral therapy. JAMA, 2016;316(2):1-11. DOI: 10.1001/jama.2016.5148. (12 July 2016). Full free access.
- HIV i-Base. The Swiss Statement.
- Vernazza P et al. HIV-positive individuals not suffering from any other STD and adhering to an effective antiretroviral treatment do not transmit HIV sexually. (Les personnes séropositives ne souffrant d’aucune autre MST et suivant un traitment antirétroviral efficace ne transmettent pas le VIH par voie sexuelle). Bulletin des médecins suisses 89 (5), 30 January 2008. Included with English translation.
Swiss Statement: English translation and original French publication (PDF)
- U=U Campaign. Undetectable=Uninfectious.