ATLANTA - In what's being called the first research of its kind, a study found that HIV-infected women in Africa are more likely to spread the AIDS virus if they use hormone-based birth control.
The women studied were about twice as likely to transmit HIV if they were on the pill or taking a hormone shot like Depo-Provera, compared to those not on the such birth control. The research is the first to look at this question, according to Renee Heffron of the University of Washington, one of the researchers.
Their research also found that uninfected women were about twice as likely to catch AIDS virus from their infected partners if they were on hormonal contraception, compared to those who were not. That finding echoed a phenomenon seen in earlier studies.
The researchers checked to make sure there were no significant differences in condom use, sexual behavior or other factors that would account for the differences.
The research was presented Wednesday at a meeting in Rome of the International AIDS Society.
The researchers said the findings need to be confirmed in follow-up studies, and should not cause women to immediately change birth control practices.
The increased risk of HIV infection also must be balanced against the consequences of unintended pregnancy, which in Africa can include maternal mortality and financial squalor, they explained.
"Contraception is incredibly important to economic and social development of women and children worldwide," said Dr. Jared Baeten, another University of Washington researcher on the study team.
Hormone shots release progestin, which keep a woman's ovaries from releasing eggs and also thins the lining of the uterus. Birth control pills contain progestin or progestin and estrogen and work the same way.
It's not clear exactly how the hormones may help spread the virus, but the theoretical risk has been known from earlier studies. A Kenya study found an increase in HIV-infected cells in cervical tissue after women started using various hormonal contraception.
The new study was done from 2004 to 2010 in seven African countries -- Kenya, Uganda, Rwanda, Botswana, Zambia, Tanzania and South Africa.
It included nearly 2,500 women with HIV whose male partners were not infected. About a third took hormonal contraception at least once. Most of them were on the shots, which are taken once every few months.
The men had a 2.61 percent chance of becoming infected in a year's time if their partner was on hormonal contraception. If not, their chances of infection were 1.51 percent.
The research team also looked at about 1,300 couples in which the men were infected but not the women. About 20 percent of the women were on hormonal contraception, mostly injections.
The study found those women had a 6.6 percent chance of becoming infected in a year, compared to 3.8 percent for woman not on that kind of birth control.