Patients with H.I.V. were 96 percent less likely to pass on the infection if they were taking antiretroviral drugs - a finding that is so overwhelming that it is likely to change the way American AIDS doctors treat patients and what treatment policies are adopted by the World Health Organization and other countries, said Dr. Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases, which paid for the trial.

The data was so convincing that the trial, scheduled to last until 2015, is effectively being ended early.

There have been previous studies, notably among drug abusers in San Francisco and Vancouver, British Columbia, that concluded that starting patients on drugs immediately would stop them from infecting others.

Those studies led Unaids, the United Nations AIDS-fighting agency, to adopt “test and treat” as its goal last year; the policy encourages doctors to start people on treatment as soon as they test positive for H.I.V. However, this is the first evidence from a randomized clinical trial, the gold standard in medical research.

In a large randomized trial involving more than 1,700 heterosexual couples - in which one person was HIV-positive and the other was not - infected people who took the anti-HIV drugs reduced their risk of transmitting the virus to their partners by 96%, compared with those who did not immediately start treatment.

The results were so stark that Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), which sponsored the trial, elected to release them early and stop the global study four years before its scheduled end. All study participants are now being offered antiretroviral therapy.

These results come not long after another large-scale landmark trial (read about it here) that found evidence that using antiretroviral drugs could be an effective prevention measure. That trial included nearly 2,500 HIV-negative men in six countries, who were at high risk of contracting HIV. Those who took a combination anti-HIV pill called Truvada (a combination of the drugs tenofovir and emtricitabine) were anywhere from 44% to 73% less likely to acquire HIV, depending on how faithfully they took their medication, during the study’s three-year follow-up than participants who took a placebo.

The findings add weight to the “treatment as prevention” strategy that some AIDS scientists increasingly believe, if broadly implemented, can help slow the spread of HIV and AIDS.

The latest study was carried out in 13 countries including Botswana, Brazil, India, Kenya, Malawi, South Africa, Thailand and the U.S., and mostly involved heterosexual couples. Infected partners were randomly assigned to begin receiving antiretroviral drugs immediately at the start of the study, or to wait to start treatment until their disease had progressed (signaled by a drop in the infected partner’s immune cell count below a certain threshold). All participants were also counseled on how to protect against HIV transmission and were given free condoms.