A vaginal gel containing a prescription drug can sharply reduce HIV infections in women. It is the first time such an approach has protected against sexual transmission of the virus.

The gel reduced HIV infections in women by 39 percent over two and a half years.

The findings, presented at an international AIDS conference in Vienna, were described as "groundbreaking" by the World Health Organisation and the United Nation's AIDS group (UNAIDS).

The South African findings show it may be possible to slow the spread of the disease by giving women a way to protect themselves.

"Boy, have we been doing the happy dance," Dr. Salim Abdool Karim, from the University of KwaZulu-Natal in Durban.

The trial of 889 women in the coastal city of Durban and a remote rural village showed women largely used the gel as directed, Karim said.

Margaret Chan, director general of the WHO, described the findings as "exciting".

"Once they have been shown to be safe and effective, the WHO will work with countries and partners to accelerate access to these products," Margaret Chan, director general of the WHO said.

UNAIDS director Michel Sidibe said it could become "a powerful option for the prevention revolution and help us break the trajectory of the AIDS epidemic."

Trial and results
The Caprisa trial was a classic medical study, with half the women using the gel before and after sex, and half being given a placebo. No one knew who got the real drug.

The women kept track of the applicators, which resemble applicators used to insert tampons, and gave them to researchers so they could be sure when the gel was actually used.

All the women were also given condoms and advice about sexually transmitted diseases, and tested for HIV once a month.

After 30 months, 98 women became infected with HIV - 38 in the group that got tenofovir in the gel and 60 in the group that got placebos. "We showed a 39 percent lower incidence of HIV in the tenofovir group," Karim said.

When they checked the data, it turned out that tenofovir lowered the risk of infection by 50 percent at 12 months but then the efficacy declined. Women who used the gel more consistently were much less likely to be infected.

If women understand the gel will protect them Karim said, he believes they will use it more consistently.

He added he did not know how much each dose would cost but said the applicators and gel cost just pennies.