Two years into a second major study examining the chances of transmitting HIV with an undetectable viral load, there have been no transmissions between either gay or heterosexual mixed-HIV status partners. The study, called PARTNER, includes couples in which the HIV-positive partner is on HIV treatment. To enter the study, all couples had to report having intercourse without condoms during the four weeks before the study. A total of 767 couples were included in this two-year interim analysis.

There were no transmissions within couples in which the HIV-positive partner had a viral load below 200. An estimated 50 to 100 transmissions would have taken place if no one living with HIV in the study had been taking antiretrovirals.

The researchers estimated that the overall 10-year risk of transmitting HIV in these circumstances was zero percent to 4 percent. As for how low the risk of transmitting a fully suppressed virus may ultimately prove to be, Alison Rodger, MD, a senior lecturer at University College London, and a member of the PARTNER study group, says her “best estimate is zero,” but she “cannot exclude that it may be higher.”