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.

Dr Jared Baeten, a Professor of Medicine and Global Health at the University of Washington in Seattle, said on Tuesday in Washington that the results of the study, known as ASPIRE, were presented at the Conference on Retroviruses and Opportunistic Infections in Boston.

He said the result, which was contained in a study in four African countries where the risk of AIDS was high, has also been published online in the New England Journal of Medicine.

"Use of the product was enough to demonstrate HIV protection of 27 per cent over placebo. 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,’’ he said.

Baeten noted that such silicone rings, infused with a different drug, are already used for birth control.

He said a new release from the International Partnership for Microbicides (IPM), the non-profit organisation that developed the ring, showed that a second test of the device, known as "The Ring Study’’, were also to be presented, because it has found a 37 per cent reduction in women over 21.

Baeten said in sub-Saharan Africa, where the HIV epidemic was 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,’’ he said.

He said during a follow-up period, 71 women with the drug therapy became infected versus 97 in the placebo group, for a 27 per cent reduction.

"The rate is cut by 37 per cent when the researchers excluded two sites with poor compliance. When women over 21 were considered separately, 56 per cent 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," he said.

Baeten said the rings are easily inserted by the woman, like a diaphragm, it is easy to do, and doesn't interfere with sex nor the menstruation.

"The rings were given out for free, and the volunteers also received condoms and HIV prevention counselling. Only six per cent had exchanged sex for money in the previous year.

"New rings were withheld if the woman became pregnant and until she stopped lactating,’’ he said.

Baeten said the cost of the therapy has not been determined. He, however, said in terms of costs of production, the cost of the active ingredient was really small.

"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,’’ he said.IPM spokeswoman said the costs could vary from country to country and depend on factors such as the scale of manufacturing, delivery, packaging and volume.