Liz Szabo

USA TODAY

A vaginal ring that releases an anti-AIDS drug has been found to modestly reduce the risk of HIV in women, according to two new studies presented Monday.

The rings contain an experimental drug called dapirivine, which is released over time. The rings are modeled after the rings already available to deliver birth control or hormone replacement therapy. Women change the rings every four weeks.

In a two studies with a total of 4,588 women, those who used the ring were 27% to 31% less likely to contract HIV than women given a placebo ring, according to the International Partnership for Microbicides, which presented the results at the Conference on Retroviruses and Opportunistic Infections in Boston.

The rings were far more effective among women older than 25: The rings reduced their risk of HIV infection by 61% in one study, and by 37% in the second study, largely because older women were more likely to use the devices consistently. The rings offered little to no protection in women younger than 25, according to the studies, which included women in Malawi, South Africa, Uganda and Zimbabwe. The women's average age in the studies, which lasted from 2012 to 2015, was 26.

Researchers have been eager to give women a way to reduce their risk of HIV. Many women, especially those in developing countries with the highest HIV rates, are unable to persuade men to use condoms, said Zeda Rosenberg, founding chief executive officer of the International Partnership for Microbicides.

About 37 million people worldwide are living with HIV. More than half are women, according to the National Institutes of Health, which funded one of the trials.

"Women need a discreet, long-acting form of HIV prevention that they control and want to use," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, part of the NIH.

The youngest women in the study had the highest risk. Among women receiving a placebo, up to 8.2% of women ages 18 to 21 contracted HIV, compared with 4.3% of those over 25.

AIDS researcher Carlos del Rio said it was disappointing that young women didn't use the products consistently. "The people who need it the most didn’t use it," said del Rio, co-director of the Emory Center for AIDS Research in Atlanta, who was not involved in the research. "It's a huge challenge we have ahead of us."

Researchers have struggled to find ways to prevent HIV that are both effective and acceptable.

Doctors hoped that women might be able to protect themselves by using vaginal gels that contain anti-HIV medications. Studies haven't found them be effective, mainly because women didn't use them consistently.

Health officials have known for more than 30 years that condoms can prevent HIV infection. Yet many people fail to use them consistently and correctly, said Mitchell Warren, executive director of AVAC, formerly the AIDS Vaccine Advocacy Coalition, a non-profit that focuses on HIV prevention.

Another prevention strategy, called PrEP, involves giving daily medications to healthy people with the aim of preventing infection.

International studies have found that PrEP reduced HIV infections 44% in men but didn't work at all in women, Rosenberg said. Many people in these studies didn't take the pills consistently. Later studies in men, conducted after the benefits of PrEP were clear, found that the pills reduced new HIV infections by 96%.

Researchers are trying to develop an injectable form of PrEP to make the medications even easier to use, Warren said.

The microbicide partnership plans to continue studying the rings to see whether women are more likely to use them now that the benefits are clear, Rosenberg said. The group intends to apply to get the rings approved in South Africa and Uganda.

A daily pill can prevent HIV infection, but few take it