The PARTNER Study, a multisite European study, evaluated rates of HIV transmissions within serodifferent heterosexual and MSM couples (n = 548 and n = 340, respectively, contributed eligible data). The HIV-positive partners were on ART with HIV RNA levels of <200 copies/mL, and the couples engaged in ongoing condomless sex (vaginal and/or anal) during a median of 1.3 years per couple (1,238 "couple years"). It was noted that 33% of HIV-negative MSM had outside partners, compared with 4% of HIV-negative heterosexual subjects). During follow-up, there were no linked HIV infections (there were 11 total infections, 10 in MSM, 1 in heterosexuals). The within-couple HIV transmission rate thus was zero but the upper 95% confidence limit was not zero (for MSM it was 0.84 per 100 couple years) and was the subject of some discussion in the paper; the study will continue to follow the MSM couples to try to define transmission risk more precisely. Evaluating this risk is of great import to our patients (heterosexual and MSM) in serodifferent sexual relationships in which condoms are not used. Note that this study is different from the well-known HPTN 052 not only in that it includes MSM, but also in that patients were sought out because they were not using condoms consistently and did not plan to do so (HPTN 052 encouraged participants to use condoms, and the reported condom use rate was high).
On a related note, the final results of HPTN 052 were published very recently. Authors concluded: "Early ART was associated with a 93% lower risk of linked partner infection than was delayed ART (hazard ratio: 0.07; 95% CI, 0.02 to 0.22). No linked infections were observed when HIV-1 infection was stably suppressed by ART in the index participant."