Men and women infected with the AIDS virus who take antiretroviral drugs immediately rather than waiting to become more ill dramatically cut the risk of infecting a sexual partner, U.S. researchers said on Thursday.

The findings in the study conducted in nine countries are expected to energize global efforts to slow the AIDS virus. The study showed that taking these drugs does not only slow the virus in people already infected, but makes these people far less infectious and far less likely to spread HIV to others.

The landmark study, mostly involving heterosexual couples, found a 96 percent reduction in HIV transmission to an uninfected sexual partner when antiretroviral drug treatment began early, before a person's immune system begins to weaken under the onslaught of the virus.

"The findings of this study strongly indicate that treating an individual with antiretroviral drugs sooner rather than later can have a significant impact on reducing the risk of transmitting HIV to their sexual partner," Dr. Anthony Fauci of the National Institute of Allergy and Infectious Diseases, told reporters in a telephone briefing.

His institute is part of the U.S. government's National Institutes of Health (NIH), which funded the study.

"This is a crucial development, because we know that sexual transmission accounts for about 80 percent of all new infections," Dr. Margaret Chan, director-general of the U.N. World Health Organization, said in a statement.

"This breakthrough is a serious game changer and will drive the prevention revolution forward. It makes HIV treatment a new priority prevention option," added Michel Sidibe, executive director of the Joint United Nations Program on HIV/AIDS.

"Now we need to make sure that couples have the option to choose treatment for prevention and have access to it."

Dr. Myron Cohen of the University of North Carolina, who led the study, said it was the first randomized clinical trial to definitively indicate that an HIV-infected individual can reduce sexual transmission of HIV to an uninfected partner by beginning antiretroviral therapy sooner.

It is part of a new effort to see if the drugs can slow the spread of the human immunodeficiency virus, or HIV, which infects 33.3 million people globally, according to the United Nations. HIV causes AIDS, a disease that has killed more than 25 million people globally.

No Cure for AIDS
There is no cure for AIDS, but cocktails of antiretroviral drugs can keep the disease at bay for many years and allow many infected people to live normal lives. The drugs work by suppressing HIV, which is classified as a retrovirus.

The study involved 1,763 couples from 13 sites in Botswana, Brazil, India, Kenya, Malawi, South Africa, Thailand, the United States and Zimbabwe. The vast majority of the couples (97 percent) were heterosexual.

The research team randomly assigned the couples to either one of two study groups. In the first group, the HIV-infected partner immediately began taking a combination of three antiretroviral drugs. In the second, the HIV-infected partners began antiretroviral therapy when their CD4 counts -- a measure of immune system health -- fell below 250 cells per cubic millimeter, or an AIDS-related event such as pneumonia occurred.

An independent panel looking at the interim results found 28 cases in which uninfected partners acquired an infection that was genetically linked to their partner.

But 27 of these occurred among the 877 couples in which the infected partner waited to begin antiretroviral therapy. Only one case of HIV infection occurred among couples in which the infected partner started taking drugs right away.

The findings were so convincing that the panel stopped the trial, which had been set to end in 2015.

Researchers said they will now begin offering treatment to all HIV-positive partners in the study, and all couples will continue to be followed through the end of the trial period.

The NIH said the antiretroviral drugs used in the study were made available by Abbott Labor