Early initiation of antiretroviral treatment in people infected with HIV prevents them from transmitting the virus to their partners, according to findings released Thursday from a large-scale international clinical study.

Led by Myron Cohen, of the University of North Carolina, the research found that treating HIV-infected individuals with antiretroviral therapy (ART) when their immune systems are still relatively healthy led to a 96 percent reduction in HIV transmission to their partners.

This critical new finding convincingly demonstrates that early treatment of infected individuals can have a major impact on the spread of the epidemic.

Study investigators enrolled 1,763 couples who were not eligible for ART by WHO standards in nine countries around the world. All couples were at least 18 years of age. The vast majority of the couples (97 percent) were heterosexual, which precludes any definitive conclusions about effectiveness in men who have sex with men.

Couples were randomly assigned to one of two study groups. In the first group, the HIV-infected partner initiated antiretroviral therapy (ART) as soon as the couple enrolled in the study (the immediate treatment group); in the second group, infected partners did not begin ART until their CD4+ counts fell to between 200 and 250 cells/mm3 or they developed an AIDS-related illness (the delayed treatment group). Couples in both groups received HIV primary care, counseling and condoms.

Results of this research study, which was scheduled to conclude in 2015, are being released early, after the independent data and safety monitoring board (DSMB) determined that the benefits of early treatment were clear.

At the time the study was evaluated by the DSMB on April 28, 2011, the available data show that among all couples enrolled in the study, 28 new cases of HIV infection occurred which were linked through genetic analysis to the infected partner enrolled in the study. Of those 28 cases, only one new infection occurred among couples in the immediate treatment group.

The results also demonstrated a clinical benefit of early ART, especially in the prevention of a type of tuberculosis infection outside the lungs (extrapulmonary tuberculosis).

"We want to thank the study participants for making such an important contribution in the fight against HIV/AIDS. We think that these results will be important to help improve both HIV treatment and prevention," Cohen said in a statement.

Study participants are being informed of the results, and HIV- infected individuals in the delayed treatment arm are being offered ART. The study investigators will continue to monitor participants for at least one more year.

The research was conducted by the HIV Prevention Trials Network, which is largely funded by the U.S. National Institute of Allergy and Infectious Diseases.

"Previous data about the potential value of antiretrovirals in making HIV-infected individuals less infectious to their sexual partners came largely from observational and epidemiological studies," said NIAID Director Anthony S. Fauci in a statement. " This new finding convincingly demonstrates that treating the infected individual -- and doing so sooner rather than later -- can have a major impact on reducing HIV transmission."