A flexible and inexpensive ring that is inserted into the vagina, where it slowly releases an antiviral drug, helped protect African women against contracting H.I.V. from their sexual partners, researchers said Monday in reports on two major studies that included more than 4,500 women.

The protection was not complete: Overall infection rates were reduced by only 27 percent and 31 percent, though women who were over 21 fared better. But researchers said that the device was still a major advance and that the results were the most promising to date in H.I.V. prevention for African women. They said they would press ahead to get the ring approved and widely distributed as quickly as possible.

“The hope was to find something that could be usable enough by women that it would provide H.I.V. protection, and that’s what we got,” said Dr. Jared M. Baeten, from the University of Washington, who led one of the studies, called Aspire. “It gives me tremendous optimism.”

Globally, almost 37 million people are infected with H.I.V., more than half of them women. The majority of those women live in sub-Saharan Africa, which has some of the highest H.I.V. rates in the world. Three earlier H.I.V. prevention studies in African women failed, largely because participants did not consistently use the methods offered, which included antiviral pills and microbicidal vaginal gels.