The HPTN 052 study demonstrated the preventative benefit of ART, showing a dramatic 96% reduction in HIV transmission in HIV+ participants randomized to early ART initiation compared with the group that deferred treatment.  This is very encouraging.  But from the perspective of a gay person considering the risk of engaging in condomless sex with a long-term HIV+ partner, these results do not provide an adequate basis on which to make a decision.  For a start, HPTN 052, like other such studies, focuses largely on heterosexual couples engaging in vaginal sex – which is recognized to carry only a fraction (c. 0.1) of the risk of HIV transmission of anal sex.  Besides, the study reports high levels (93%) of condom use.  Much remains obscure, therefore, as to the level of protection that a gay person could reasonably expect from ART against HIV transmission through condomless anal sex with a long-term partner.

This question is squarely addressed, however, in a recently reported prospective observational study – PARTNER (Partners of People on ART – A New Evaluation of the Risks) – involving 888 sero-different MSM (330) and heterosexual (548) couples reporting condomless sex who contributed a total of 1,238 eligible couple-years.  This study did not limit itself to establishing cases of transmission, but conducted phylogenetic analysis in those cases in order to determine whether or not the transmission had resulted from sex with the long-term partner.  Of the 11 transmissions that took place in the course of the 1,238 couple-years (10 amongst heterosexual, one amongst the MSM, couples), none were found to be phylogenetically linked.  The paper also examined the association between the HIV transmission which did take place (i.e. not from primary partner) and the sexual behaviours reported by HIV- partners.  Not surprisingly, this was found to be elevated in heterosexual couples where anal sex was reported, and in MSM where anal sex was receptive and receptive with ejaculation (1.68 and 2.70 per 100 couple-years, respectively).

With the couple-years accrued hitherto, appreciable levels of risk over the long-term, especially with anal sex, cannot yet be excluded: a rate of 2.2 per 100 couple-years remains the upper limit (20% over ten years) – though, of course, the risks could prove to be considerably lower than this.   With a view to arriving at a more precise estimation, the MSM side of the PARTNERS study remains ongoing.  So far, however, the news seems to be good.

Aside from its implications for personal decision-making about condom use, the question of the preventative effectiveness of ART could presumably also have relevance for health policy decisions affecting resource allocation that involve determining the relative priority to be accorded to interventions promoting engagement and retention in treatment as against other interventions (e.g. PrEP) (Punyacharoensin & White/STIs/blog; NHS kicks PrEP into the long grass/STIs/blogs). The greater the effectiveness of viral suppression through ART as an HIV prevention tool, the better the case for prioritizing interventions to achieve higher targets for engagement and retention in treatment.