An experimental drug-infused ring inserted in the vagina once a month cut the odds of becoming infected with HIV by more than half among women who used the device consistently, in a study in four African countries where the risk of AIDS is high.

"Use of the product was enough to demonstrate HIV protection of 27 percent" over placebo, chief author Dr. Jared Baeten, a professor of medicine and global health at the University of Washington in Seattle, told Reuters Health by phone. "And in some groups of women who appeared to use it better, such as women over age 21, the risk of HIV was reduced by more than half."

Such silicone rings, infused with a different drug, are already used for birth control.

Results of the study, known as ASPIRE, were presented Monday at the Conference on Retroviruses and Opportunistic Infections in Boston. Results were also published online in the New England Journal of Medicine.

Findings from a second test of the device, known as The Ring Study, were also to be presented; it found a 37 percent reduction in women over 21, according to a news release from the International Partnership for Microbicides (IPM), the nonprofit organization that developed the ring.

In sub-Saharan Africa, where the HIV epidemic is primarily driven by unprotected heterosexual sex, women ages 15 to 24 are twice as likely as men to be infected.

ASPIRE enrolled 2,629 volunteers from Malawi, Uganda, Zimbabwe and South Africa who received placebo rings or rings containing the antiretroviral medicine dapivirine. They were followed at 15 sites for an average of 18 months.

During that follow-up period, 71 women with the drug therapy became infected versus 97 in the placebo group, for a 27 percent reduction. The rate was cut by 37 percent when the researchers excluded two sites with poor compliance. When women over 21 were considered separately, 56 percent of infections were prevented.

"A third of infections were prevented overall and more than half (were prevented) in those who used the product better. That's a real cause for optimism," said Baeten.

The rings are easily inserted by the woman, like a diaphragm. "It's easy to do, doesn't interfere with sex, doesn't interfere with menses," he said.

The rings were given out for free, and the volunteers also received condoms and HIV prevention counseling. Only 6 percent had exchanged sex for money in the previous year. New rings were withheld if the woman became pregnant and until she stopped lactating.

The cost of the therapy has not been determined. However, Baeten said, in terms of costs of production, the cost of the active ingredient is really small. Also, he said, "The amount of drug in this ring is quite small . . . which is good in terms of safety because the amount of the drug the woman is exposed to is really quite low. So the idea is that these can be manufactured at scale for a fraction of what HIV treatment would cost."

An IPM spokeswoman said in an email, "Costs could vary from country to country and depend on factors such as the scale of manufacturing, delivery and packaging, and volume. Currently, the expected initial cost of the dapivirine ring - US$5 per ring - compares favorably" to other prevention methods.

The study was financed by the National Institutes of Health.

SOURCE: http://bit.ly/24mtdGD The New England Journal of Medicine, online February 22, 2016.