Heterosexual HIV-positive people who are taking antiretroviral (ARV) therapy were 92 percent less likely to pass on the virus to their HIV-negative sex partners, according to a study published online May 27 in The Lancet.

A number of studies over the past decade have suggested that HIV transmission is cut dramatically in mixed HIV status couples—called serodiscordance—if the partner with HIV is taking ARVs and has an undetectable viral load.

These data were used by the Swiss Federal Commission for HIV/AIDS in its hotly debated February 2008 declaration concluding, “an HIV-infected person on antiretroviral therapy with completely suppressed viremia (‘effective ART’) is not sexually infectious, i.e. cannot transmit HIV through sexual contact.”

Further controversy has erupted in the past year over moves to initiate testing and early HIV treatment in order to lower HIV transmission rates at a citywide level. These programs, called testing and linkage to care and treatment, or TLC+, are being studied in New York City and Washington, DC. In addition, San Francisco’s Department of Health recently took the aggressive step of recommending that all of its HIV-positive residents should start ARV treatment regardless of their CD4 count, in large part to reduce HIV transmission rates in the city.

No one knows, however, to what degree TLC+ will be effective and whether rolling it out might have unintended consequences, including higher rates of side effects and an increase in the number of drug-resistant HIV cases. One trial examining the feasibility of TLC+ is the HIV Prevention Trials Network (HPTN) 065 study. A separate, large-scale controlled trial looking at the effect of ARVs on transmission, HPTN 052, is ongoing, but results are not expected for several years. In the meantime, researchers and advocates have been eagerly awaiting smaller studies on transmission risk between partners.

One such study has now concluded. A team of researchers led by Deborah Donnell, PhD, from the Fred Hutchinson Cancer Research Center in Seattle, studied the effect of treatment on transmission within 3,381 discordant couples in seven African countries. During the six-year study, 349 of the HIV-positive participants started ARV therapy. Of the 103 inter-couple transmissions—those where they could genetically link the strain of HIV from one partner to the other—only one occurred in a couple where the HIV-positive partner was on treatment. Moreover, in the couples where the HIV-positive partner was not on ARVs, high CD4 counts and low viral loads among the partners with HIV also appeared to reduce transmission rates.

“These results are an important finding in the search for effective HIV prevention strategies and the strongest evidence to date that ART might decrease HIV transmission risk,” concluded Connie Celum, MD, from the University of Washington in Seattle, and the paper’s senior author.