The gel, spiked with the AIDS drug tenofovir, cut the risk of HIV infection by 50 percent after one year of use and 39 percent after two-and-half years, compared to a gel that contained no medicine.

To be licensed in the US, a gel or cream to prevent HIV infection may need to be at least 80 percent effective, Fauci said. That might be achieved by adding more tenofovir or getting women to use it more consistently. In the study, women used the gel only 60 percent of the time; those who used it more often had higher rates of protection.

The gel also cut in half the chances of getting HSV-2, the virus that causes genital herpes. That's important because other sexually spread diseases raise the risk of catching HIV.

Even partial protection is a huge victory that could be a boon not just in poor countries but for couples anywhere when one partner has HIV and the other does not, said Dr Salim Abdool Karim, the South African researcher who led the study. In the US, nearly a third of new infections each year are among heterosexuals, he noted.

Countries may come to different decisions about whether a gel that offers this amount of protection should be licensed. In South Africa, where one in three girls is infected with HIV by age 20, this gel could prevent 1.3 million infections and 826,000 deaths over the next two decades, he calculated.

He will present results of the study on Tuesday at the International AIDS Conference in Vienna. The research was published online on Monday by the journal Science.

"We now have a product that potentially can alter the epidemic trends ... and save millions of lives," said Dr Quarraisha Abdool Karim, the lead researcher's wife and associate director of the South African program that led the testing.

It's the second big advance in less than a year on the prevention front. Last fall, scientists reported that an experimental vaccine cut the risk of HIV infection by about 30 percent. Research is under way to try to improve it.

If further study shows the gel to be safe and effective, WHO will work to speed access to it, said its director-general, Dr Margaret Chan.

The gel is in limited supply; it's not a commercial product, and was made for this and another ongoing study from drug donated by California-based Gilead Sciences Inc., which sells tenofovir in pill form as Viread. If further study proves the gel effective, a full-scale production system would need to be geared up to make it.

The study tested the gel in 889 heterosexual women in and near Durban, South Africa. Researchers had no information on the women's partners, but the women were heterosexual and, in general, not in a high-risk group, such as prostitutes.

Half of the women were given the microbicide and the others, a dummy gel. Women were told to use it 12 hours before sex and as soon as possible within 12 hours afterward.

At the study's end, there were 38 HIV infections among the microbicide group versus 60 in the others.

The gel seemed safe -- only mild diarrhea was slightly more common among those using it. Surveys showed that the vast majority of women found it easy to use and said their partners didn't mind it. And 99 percent of the women said they would use the gel if they knew for sure that it prevented HIV.

This shows that new studies testing the gel's effectiveness without a placebo group should immediately be launched, said Salim Abdool Karim. The only other study testing the gel now compares it to placebo and will take a couple more years to complete.

The study was sponsored by the Centre for the AIDS Programme of Research in South Africa, or CAPRISA; Family Health International; CONRAD, an AIDS research effort based at Eastern Virginia Medical School; and the U.S. Agency for International Development, or USAID.

Gilead has licensed the rights to produce the gel, royalty-free, to CONRAD and the International Partnership on Microbicides for the 95 poorest countries in the world, said Dr. Howard Jaffe, president of the Gilead Foundation, the company's philanthropic arm.

The biggest cost of the gel is the plastic applicator -- about 32 cents, which hopefully would be lower when mass-produced, researchers said.

Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition, a nonprofit group that works on HIV prevention tools, said the study shows a preventive gel is possible.

"We can now say with great certainty that the concept has been proved. And that in itself is a day for celebration," he said.