VIENNA - A woman's risk of infection with HIV can be significantly cut by the use of a vaginal gel, a study has found. The research marks the first success in a 15-year search for a way women can independently protect themselves from contracting HIV infection through sex.

Short of a vaccine, an effective vaginal microbicide has been the most elusive goal in the epidemic.

The research, which was conducted in South Africa and will be presented Tuesday at the 18th International AIDS Conference in Vienna, tested a gel containing the antiretroviral drug tenofovir. While far from perfect, it was unambiguously helpful, reducing the risk of HIV infection by 39 percent in a group of women who used it for about three-quarters of their sexual encounters. Those who used it more consistently experienced 54 percent fewer infections.

If development follows the expected course, more-potent formulations, combined with campaigns to make the product appealing (or even sexy), could result in vastly better protection.

Of the 33 million people worldwide infected with the AIDS virus, 16 million are women. In Africa, 60 percent of people with HIV infection are women, nearly all of whom acquired the virus through sex. For many, the proven methods of preventing infection, such as abstinence, being faithful and using condoms, are either not an option or out of their control. A vaginal microbicide that could be used with or without a man's knowledge is considered essential, missing until now.

News of the results of the Caprisa-004 study, which leaked out a day before they were to be presented, sent a wave of optimism through the AIDS research community.

"We have never had any kind of tool that has effectively allowed women to protect themselves," said Bruce Walker, an AIDS researcher at Harvard Medical School. "This is really a game-changer."

"It's groundbreaking," said Catherine Hankins, chief scientific officer of the United Nations' AIDS agency, UNAIDS. "This in combination with [male] circumcision in places where the epidemic is generalized could really turn the tide."

"Everyone is just delighted. There were a lot of skeptics that the concept would work at all," said Zeda Rosenberg, head of the International Partnership for Microbicides in Silver Spring.

Researchers would need to show that the microbicide is effective in at least one other group of women before it could be licensed for commercial use, several people said Monday. That effort now climbs to the top of the international research agenda, although at a minimum the work will take several years.

A larger study testing tenofovir gel and antiretroviral drugs in pill form as a way to protect women against sexual transmission of HIV is underway in four African countries but will not be finished until 2013. Several other experiments, including ones in which the drug is in a long-acting vaginal ring, are in earlier stages. A microbicide might also be useful in protecting men who acquire the virus through anal sex.

"I think the big challenge is to get confirmatory studies done quickly," Hankins said.