For the first time, a vaginal gel has proved capable of blocking the AIDS virus: It cut in half a woman’s chances of getting HIV from an infected partner in a study in South Africa. Scientists called it a breakthrough in the long quest for a tool to help women whose partners won’t use condoms.

The results need to be confirmed in another study, and that level of protection is probably not enough to win approval of the microbicide gel in countries like the United States, researchers say. But they are optimistic it can be improved.

“We are giving hope to women,” who account for most new HIV infections, said Michel Sidibe in a statement. He is executive director of the World Health Organization’s UNAIDS program. A gel could “help us break the trajectory of the AIDS epidemic,” he said.

And Dr. Anthony Fauci of the U.S. National Institutes of Health said, “It’s the first time we’ve ever seen any microbicide give a positive result” that scientists agree is true evidence of protection.

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 2 1/2 years, compared with a gel that contained no medicine.

To be licensed in the U.S., 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 U.S., 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.