AIDS drugs designed to treat HIV can also be used to reduce dramatically the risk of infection among heterosexual couples, two studies conducted in Africa showed for the first time on Wednesday.

The findings add to growing evidence that the type of medicines prescribed since the mid-1990s to treat people who are already sick may also hold the key to slowing or even halting the spread of the sexually transmitted disease.

The research involving couples in Kenya, Uganda and Botswana found that giving daily AIDS drugs reduced infection rates by an average of at least 62 percent when compared with placebo.

“Effective new HIV prevention tools are urgently needed and these studies could have enormous impact in preventing heterosexual transmission,” Margaret Chan, director-general of the World Health Organization (WHO), said in a statement.

In an indication of the importance of the latest evidence, Chan said the United Nations health agency would now work with countries to use the new findings to implement better protection strategies.

The larger of the two studies examined 4,758 couples in Kenya and Uganda in which one partner was HIV-positive and one was negative. Those negative partners taking Gilead Sciences Inc’s tenofovir, or Viread, had on average 62 percent fewer infections.

For couples on Truvada - another Gilead drug combining tenofovir and emtricitabine - the infection risk was cut by an estimated 73 percent in the clinical trial, which was led by researchers at the University of Washington.

The study was funded by the Bill & Melinda Gates Foundation, whose director of HIV and Tuberculosis, Stefano Bertozzi, said it marked “a significant milestone in the quest to develop new HIV prevention measures”.

The second study, involving just over 1,200 sexually active men and women in Botswana, found those on daily Truvada reduced their risk of HIV infection by 62.6 percent.

The idea of such “pre-exposure prophylaxis”, known as PrEP, has gained traction in the past year, following results of other research showing a fall in infection rates among gay men taking AIDS drugs.

However, PrEP took a knock earlier this year when another study failed to demonstrate a protective effect in high-risk women. The latest strong evidence is likely to restore confidence in the approach.

“TIPPING POINT” IN HIV FIGHT

Around 33 million people worldwide have the human immunodeficiency virus (HIV) that causes AIDS, most living in Africa and Asia. Only about half know their HIV status, and the WHO hopes that news of an effective approach to prevention will encourage more people to get tested.

Michel Sidibe, head of the UN’s program on HIV/AIDS, said the new studies “could help us to reach the tipping point in the HIV epidemic”.

The larger study, conducted in Kenya and Uganda, had been scheduled to run until late 2012 but it was stopped early because the evidence of efficacy was so strong.

Results of the Botswana study, led by the U.S. Centers for Disease Control and Prevention, had been due to be unveiled next week at an international AIDS congress in Rome but were released ahead of time to coincide with the University of Washington research.

When treating HIV/AIDS, antiretrovirals are typically given in cocktails of three or more medicines. The PrEP approach of using just one daily pill is much more convenient - and the drugs are available as generics in many poor countries at prices as low as 25 U.S. cents a tablet, according to the WHO.

Prices could fall further and supplies increase following an agreement by Gilead, the leading maker of HIV drugs, to share intellectual property rights on its medicines in a new patent pool. The California-based group on Tuesday became the first drugmaker to sign up to the Medicines Patent Pool.

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By Ben Hirschler

LONDON