Another major study has seen no transmissions of HIV within mixed-HIV-status couples when the partner living with the virus was on antiretroviral (ARV) treatment and had a fully suppressed viral load.

“We think this is a really exciting result and adds to a body of research that HIV is not passed on in the context of undetectable viral load, even with high rates of sexually transmitted infections,” says Andrew Grulich, PhD, an HIV epidemiologist at the Kirby Institute at the University of New South Wales in Sydney and chief investigator of the study, called Opposites Attract. “There has never been a recorded case of HIV transmission from an HIV-positive person to their HIV-negative sexual partner when the HIV-positive partner had undetectable viral load.”

A total of 343 gay couples in Australia, Thailand and Brazil made at least one follow-up visit to the study, contributing 591 cumulative years of follow-up data.

Seventy-four percent of the couples reported engaging in condomless sex with one another during the study, including a total of 16,889 such acts.

For about 5,000 of those sex acts, the HIV-negative partner was taking Truvada (tenofovir disoproxil fumarate/emtricitabine) as pre-exposure prophylaxis (PrEP). (By the end of the study, 32 percent of the partners who did not have HIV had taken Truvada.) The researchers analyzed the 12,000 condomless sex acts that were protected only by effective ARV treatment in the HIV-positive partner.

Between Opposites Attract and a study called PARTNER, mixed-HIV-status gay couples reported about 35,000 condomless sex acts not otherwise covered by PrEP. None of the HIV-positive men have transmitted the virus to their partners in this context. A new phase of PARTNER is currently gathering even more data.

“This is life-changing news for couples of differing HIV status,” Grulich says of the Opposites Attract findings. “But it is important that the HIV-positive partners are under regular medical care and do not miss any of their antiretroviral medication in order to ensure they maintain an undetectable viral load.”

Given all the available data on both gay and heterosexual mixed-HIV-status partnerships, Anthony S. Fauci, MD, director of the National Institute of Allergy and Infectious Diseases (NIAID), characterizes the risk of HIV transmission through condomless sex among men or women with an undetectable viral load as “vanishingly small” and says the risk “may be so low as to be unmeasurable, and that’s equivalent to saying they are uninfectious.”