WASHINGTON (AP) -- Thousands of women in Africa can volunteer for major new research to see if inserting a vaginal ring coated with an anti-AIDS drug could protect them from HIV infection, U.S. scientists announced Tuesday.

Giving women tools to protect themselves when their partners won't use a condom is crucial for battling the epidemic. Women already make up half of the 34.2 million people worldwide living with HIV, even more -- 60 percent -- in hard-hit Africa.

But developing these so-called microbicides has proved a hurdle. Previous research found an anti-AIDS vaginal gel offered partial protection but women may have a hard time using it every time they have sex. The vaginal ring, in contrast, would have to inserted just once a month for ongoing protection -- prompting hope that it will prove more effective.

The work marks an attempt at "the next generation of women-focused prevention tools," Dr. Carl Dieffenbach of the U.S. National Institutes of Health said Tuesday in announcing the new research at the International AIDS Conference.

"We need options that fit readily into women's lives," added Dr. Sharon Hillier of the University of Pittsburgh and the Microbide Trials Network, which is conducting the new NIH-funded study.

Developed by the nonprofit International Partnership for Microbicides, the silicone ring contains an anti-AIDS drug named dapivirine, which slowly oozes out into the surrounding vaginal tissue. Unlike vaginal rings sold today in the U.S., the experimental ring doesn't contain birth control -- for now, the focus only is on HIV prevention.

Early-stage studies suggested the ring could work, and women said they liked using it better than a gel, said Dr. Saidi Kapiga of the London School of Hygiene and Tropical Medicine.

Now come the large studies needed to prove it.

The NIH-funded study, named ASPIRE, will enroll nearly 3,500 women in Malawi, South Africa, Uganda, Zambia and Zimbabwe. They'll receive either a dapivirine-containing vaginal ring or an identical-looking drug-free ring, to be inserted once a month for a year.

The goal is to see if 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 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 International Partnership for Microbicides chief executive Zeda Rosenberg. 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.

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