A monthly vaginal ring infused with a microbicide helped prevent HIV infection in about a third of women overall — and more than half who used the device faithfully, according to results of two new studies in Africa, including one led by a University of Washington researcher.

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A drug-laced vaginal ring that gives women in Africa more control over potential HIV infection can help safely prevent the disease, according to results announced Monday from two long-awaited studies — including one led by a University of Washington researcher.

The once-a-month ring, which contains the antiretroviral drug dapivirine, helped prevent infection in about a third of women overall — and in more than half of women ages 22 and older who used the devices faithfully.

That’s according to the first large, late-stage clinical trials to show that a long-acting microbicide can help halt HIV. The ASPIRE study, conducted through the National Institutes of Health’s Microbicide Trials Network, was led by Dr. Jared Baeten, vice chairman of global health in the UW’s School of Public Health. The Ring Study was led by the nonprofit International Partnership for Microbicides (IPM).

“I’m really optimistic about the results,” Baetten said. “To see statistically significant HIV protection is a great step forward.”

Results of both studies were presented Monday at the annual Conference on Retroviruses and Opportunistic Infections in Boston. They represent a promising advance in the ongoing effort to reduce new HIV infections in sub-Saharan Africa, where 25.8 million people were living with the disease as of 2014, according to the World Health Organization (WHO).

In the ASPIRE study, published online in the New England Journal of Medicine, use of the ring reduced risk of HIV infection by 27 percent overall. It reduced infection by about 56 percent in women older than 21, who appeared to use the device more consistently, the study showed. The Ring Study also showed a higher effect — 37 percent — for women older than 21, IPM officials reported.

However, both studies found little or no effect in younger women, those ages 18 to 21, who may not have used the devices consistently, IPM officials said.

That underscores the need for further study of multiple ways to prevent HIV and AIDS in sub-Saharan Africa. The burden of the disease falls disproportionately on women there, who make up nearly 60 percent of cases, according to amfAR, the Foundation for AIDS Research.

In Africa, women may not have control over certain HIV-prevention decisions, said Baeten, 42, who has been working on the issue in Africa since 1997. By contrast, the ring can be used privately with or without the partner’s consent.

“She may not be able to ask or require a male partner to use a condom,” Baeten said. “Developing protection tools that women can use and control is incredibly empowering.”

The ASPIRE trial, which began in 2012 and ended in 2015, enrolled more than 2,600 HIV-negative women ages 18 to 45 at seven sites in Malawi, South Africa, Uganda and Zimbabwe. The Ring Study, which also started in 2012, enrolled nearly 2,000 HIV-negative women at seven sites in South Africa and Uganda. IPM is reporting results early after an independent safety board recommended proceeding to final analysis.

The results are a significant step forward, noted Dr. Larry Corey, member of the Vaccine and Infectious Disease Division at Seattle’s Fred Hutchinson Cancer Research Center.

“I’m glad to see results from two such important studies,” said Corey, who was not involved in the research. “We continue to need additional tools for the HIV prevention toolbox to slow down the spread of new HIV infections in women most at risk in southern Africa.”

Based on the results, IPM officials said they plan to seek regulatory approval to license the ring.