Using hormonal contraceptives – such as the birth-control pill or shots of Depo-Provera – boosts the risk of spreading HIV, the human immunodeficiency virus, according to a study conducted in seven countries in eastern and southern Africa.
The research involved almost 4,000 heterosexual couples in which one partner was infected with HIV, the virus that causes AIDS, while the other partner was HIV-free at the start of the trial.
The findings revealed that use of the hormonal contraceptives doubled an uninfected woman's chances of catching HIV, compared with women who did not use this form of birth control. The study, published in the journal The Lancet Infectious Diseases, also showed that HIV-infected women who used hormonal contraception were twice as likely to pass on the virus to their partners.
Condom use was recorded, but neither group seemed more or less likely to use them. So differences in condom use can't explain the different rates of infection.
Most of the women using hormonal birth control relied on injections that last for three months. Injectable contraception has become very popular in some parts of the developing world where family-planning measures have met with resistance, as women can get the injections without the husband's knowledge.
The researchers, led by Renee Heffron of the University of Washington, aren't sure why hormonal birth control appears to facilitate HIV transmission. In an e-mail from Kenya, Dr. Heffron noted that studies "have not been able to provide a conclusive answer." But she added that hormones may "thin the vaginal walls, allowing HIV to penetrate into the body easier." It's also possible that hormones alter "the way the immune system functions, which would also affect susceptibility."
Until researchers get a better handle on the situation, Dr. Heffron said, the results re-enforce the need for couples to use condoms in addition to other forms of contraception.