Researchers at the International AIDS Conference in Vienna are claiming a huge victory, saying they have developed a vaginal gel that can significantly reduce a woman's risk of contracting HIV.

Women in South Africa who volunteered to test the gel cut their chances of contracting the virus by 50 per cent after one year of use and 39 per cent after 2 1/2 years, compared to a gel that contained no medicine.

The researchers also discovered that the gel cut in half the chances of getting HSV-2, the virus that causes genital herpes.

Scientists call the gel a breakthrough in the search for a way to help women whose partners refuse to use condoms.

"We are giving hope to women," Michel Sidibe, the executive director of the World Health Organization's UNAIDS program, said in a statement. A gel could "help us break the trajectory of the AIDS epidemic," he said.

Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, called it a "historic day for HIV prevention research."

"We congratulate the trial sponsors, scientific collaborators, and partners who conducted this trial, and especially want to thank the nearly 900 South African women whose altruism and commitment as trial volunteers made this effort possible," Warren said in a news release.

The gel is colourless and odourless and spiked with the AIDS antiretroviral drug, tenofovir. It's inserted into the vagina using an applicator that resembles a tampon applicator before and after intercourse.

The trial of 889 women in both an urban community and a rural community showed that the women largely used the gel as directed, suggesting the product should work in the real world.

Women who used the gel more consistently were much less likely to be infected. A survey revealed that 99 per cent of the women said they would use the gel consistently if they knew for sure that it prevented HIV.

The gel also seemed safe; the only observed side effect was mild diarrhea. What's more, researchers say the gel is cheap: The microbicide is cheap to produce, and the gel itself cost only pennies. Even the applicator is inexpensive, at just 32 cents each.

Nearly 20 years of research have gone into development of a vaginal gel that could be controlled by a woman. Researchers made their breakthrough after finding that in studies in monkeys, tenofovir appeared to protect against both vaginal and rectal HIV infection.

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

"We look forward to seeing these results confirmed. Once they have been shown to be safe and effective, the WHO will work with countries and partners to accelerate access to these products," she said in a statement to the Vienna conference.

About half the people living with HIV in the world are women. In sub-Saharan Africa, more women are infected than men.

The researchers say they are optimistic that with further work, they can improve the microbicide gel's efficacy even further. Researchers are already working on another trial larger of the gel which will involve 5,000 women in South Africa, Malawi, Uganda and Zimbabwe, to further test the gel's safety and efficacy.

With reports from the Associated Press