A vaginal ring containing an antiretroviral drug reduced the risk of HIV infection among African women, according to a study co-led by a University of Washington School of Public Health professor.

Researchers found the vaginal ring, which slowly and continuously released the drug dapivirine over four weeks, reduced the rate of HIV-1 infection among study participants by 27 percent. The results, published online in the New England Journal of Medicine, were hailed for giving women another tool to protect themselves. Sub-Saharan Africa has some of the world’s highest rates of HIV infection.

Results of the ASPIRE study, led by the Microbicide Trials Network and primarily funded by the National Institutes of Health, were also announced at the Conference on Retroviruses and Opportunistic Infections in Boston on Monday.

“To help bring about an end to the HIV/AIDS epidemic, women – especially those in sub-Saharan Africa – need multiple options for HIV prevention,” said study co-leader Dr. Jared Baeten, professor of global health, medicine and epidemiology at the University of Washington. “The ASPIRE study was an important step towards determining whether the dapivirine ring could become one such option.”

The study began in 2012 and recruited more than 2,600 women between 18 and 45 at 15 research sites in Malawi, South Africa, Uganda and Zimbabwe. Researches said 168 HIV-1 infections occurred by June 2015; 71 in the dapivirine group and 97 in the placebo group. All study participants received HIV risk-reduction counseling, including partner testing, treatment for sexually transmitted infections and free condoms.

When investigators excluded data from two sites where women were not using the ring consistently, the risk of HIV infection was reduced by 37 percent. Younger women appeared to use the ring less consistently, based on measurements of dapivirine in participants’ blood. For women 25 and older, the ring reduced HIV risk by 61 percent.

“Women need a discreet, long-acting form of HIV prevention that they control and want to use,” said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, part of the NIH. “This study found that a vaginal ring containing a sustained-release antiretroviral drug confers partial protection against HIV among women in sub-Saharan Africa. Further research is needed to understand the age-related disparities in the observed level of protection.”

An ongoing large trial called The Ring Study also tested the dapivirine ring and found an overall effectiveness of 31 percent. The results of that study were also announced at the Boston conference.