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The goal is to see whether using the ring lowers women’s risk of HIV infection by at least 60 percent.

The first women in Uganda were enrolled Tuesday, Hillier said.

A smaller ring study of 1,650 women got under way last month in South Africa and aims to enroll women in Rwanda and Malawi, too.

Vaginal-based protection should cause fewer side effects than pills, and early-stage studies of the ring found no problems, said Zeda Rosenberg, chief executive of the International Partnership for Microbicides. Also, animal studies show no sign that the ring would harm a fetus if a woman became pregnant while using it, she added.

Rosenberg’s group licensed dapivirine from a Johnson & Johnson subsidiary. If the new research pans out, the partnership hopes to seek approval to sell the ring in 2015.

The first study to find even partial success with a microbicide used a vaginal gel made with the widely used AIDS drug tenofovir. Earlier-stage research is under way to develop a variety of gels, vaginal films and vaginal rings containing that and other drugs, Rosenberg said. Eventually, she said, a goal is for a combination ring that offers both HIV protection and birth control.

Also Tuesday, researchers reported more evidence that male circumcision is an important HIV-prevention tool in Africa, where it helps protect men from becoming infected by female partners.

In Orange Farm, South Africa, just over half of the 52,000 men had been circumcised by last year, up from 17 percent in 2008. Circumcised men had half the rate of HIV infection as the uncircumcised, said Bertran Auvert of France’s University of Versailles. He estimated that 1,000 new infections were avoided last year as a result.