A study has found an anti-HIV vaginal ring has been successful in partially protecting some women in hard-hit Africa from HIV infection.

Researchers say women who inserted a vaginal ring coated with an anti-HIV drug once a month were partially protected against HIV infection.

Two large studies in Africa found the effect was modest, reducing overall HIV infection by about a third.

But surprisingly, the ring worked far better in women 25 and older, leaving researchers wondering if younger women who got little to no benefit simply didn't use the device properly.

Scientists have long sought tools to help women protect themselves from HIV when their partners won't use a condom. While researchers debate the age disparity, the nonprofit International Partnership for Microbicides said it plans to seek regulatory approval for more use of the ring in parts of hard-hit Africa.

Study results are being presented at the Retrovirus Conference in Boston.