A vaginal ring that dispenses an HIV-preventative drug blocked about a quarter of HIV infections in a trial involving 2,600 women in Africa, according to a report published in The New England Journal of Medicine. When the numbers were broken down by age, researchers found that the monthly ring blocked more than half of HIV infections among HIV-negative women over the age of 21 who used the ring most consistently.

Given that the vaginal ring can be replaced once a month and doesn’t require a partner’s cooperation — unlike the condom — that’s encouraging news. This is the first study to show that a device that involves the sustained release of an HIV medication can work to block the virus in women, the researchers say.

"This ring could provide a prevention option that a woman can use discretely and which is under her control — which is incredibly empowering," says Jared Baeten, an epidemiologist at the University of Washington and a co-author of the study.

Of the 35 million people living with HIV today, women make up more than half. And in some parts of Africa, one in four or even one in three women has HIV, Baeten says. Unfortunately, women don’t always have the social capital to impose common STD preventatives, like condoms, on male sexual partners. That’s why researchers have been working on HIV-prevention methods that women can use on their own. But trials involving the use of anti-HIV medicines in women haven’t always been successful.

Last year, for instance, researchers announced that a trial for PrEP — also known as the HIV prevention pill — was a dud because most women didn’t take their daily doses. This may have occurred because participants didn’t know if they were taking a drug or a placebo, researchers suggested at the time. And that does appear to have had an impact; a second trial in Botswana showed that women were more likely to take the pill when they knew it contained medicine. Still, these trials suggest that perhaps a method that doesn’t require taking a drug every day could be more effective. And that’s why today’s study stands out. Even though the women in this trial didn’t know if they had been given a placebo ring, the researchers noted a reduction in the risk of HIV infection among the women who were given the anti-HIV ring.

The study took place in Malawi, South Africa, Uganda, and Zimbabwe over the course of almost three years. Of the 2,600 women enrolled in the trial, half received a monthly ring with dapivirine — an anti-HIV medicine that stops the virus from replicating its genetic material inside a healthy cell. Overall, the researchers found that the ring with dapivirine lowered women’s chances of getting HIV by 27 percent. In geographic areas where women used the ring more consistently, that risk was lowered by 37 percent. And when results in those locations were broken down by age, the scientists found the ring decreased chances of getting HIV by 56 percent in women over the age of 21.

The trial "was not a slam-dunk outcome," says Stefan Baral, an epidemiologist at Johns Hopkins University who didn’t work on the trial. Even though the researchers recorded a meaningful reduction in the incidence of HIV overall, the drug didn’t work well in adolescents and vulnerable women. That may have to do with lower adherence rates among those populations, but the authors of the study say it’s also possible that the genital tract of younger women could be more susceptible to HIV infection. Finding answers to this question is critical, Baral says, because if it turns out that reduced effectiveness women under 21 is linked to adherence, then there’s an opportunity to improve those numbers. If the problem is biology-based, however, that’s something researchers can’t change; "that would necessitate a different product," he says.

In addition, the largest reduction in risk was just 56 percent, which means that the ring will probably have to be used in combination with other preventatives, rather than on its own, Baral says. So, even though the device is promising, today's results weren’t "as powerful as some of us had hoped," he says.

Now that the trial has been completed, the National Institute of Allergy and Infectious Diseases — the US institute that funded the trial — plans to convene a panel of experts in March to decide the future of dapivirine ring research. Depending on the outcome of those deliberations, two other government-funded trials could take place. Those would involve providing the dapivirine ring to former participants of today’s study, and trying to better understand the HIV prevention needs and desires of adolescent girls and young women. Finding an HIV preventative that works for everyone may be impossible, but developing a drug that’s delivered in a way that works for the people who need it the most could still be within reach.