Women who use an injectable hormone contraceptive may be twice as likely to become infected with HIV as women who do not use contraception at all, according to a large study conducted in Africa. What’s more, the male partners of HIV-positive women who used the contraceptive were also twice as likely to be infected with the virus than men whose partners did not use the hormone shot.
The new study by University of Washington epidemiologists involved about 3,800 couples in Botswana, Kenya, Rwanda, South Africa, Tanzania, Uganda and Zambia. Each couple had one HIV-positive partner at the start of the study, and researchers tracked them for two years to record their contraceptive methods and HIV status.
The findings: women who used the contraceptive shot and had an HIV-positive partner were about twice as likely to acquire HIV, compared with women who used any other contraceptive method (including none). Specifically, women using hormonal contraception had an infection rate of 6.61 per 100 person-years (a measurement commonly used by epidemiologists to determine incidence of disease in a population over time), compared with 3.78 among women who used any other method.
PHOTOS: The Prevention of HIV Transmission in Zimbabwe
The male partners of HIV-positive women who used the injectable contraceptive had an infection rate of 2.61 per 100 person-years, compared with a rate of 1.51 per 100 person-years in men whose HIV-positive partners used no contraception.
The findings are particularly troubling since unintended pregnancy and HIV/AIDS are both dire health problems in sub-Saharan Africa. And the hormone shot is one of the most popular birth control methods used by women in the region.
“The best contraception today is injectable hormonal contraception because you don’t need a doctor, it’s long-lasting, it enables women to control timing and spacing of birth without a lot of fuss and travel,” Isobel Coleman, director of the women and foreign policy program at the Council on Foreign Relations, told the New York Times. “If it is now proven that these contraceptions are helping spread the AIDS epidemic, we have a major health crisis on our hands.”
It’s unclear how the medication, commonly known by its brand name Depo-Provera in the U.S., could increase the risk of virus transmission. The researchers took vaginal fluid samples from the HIV-positive female participants in the study and found a higher viral load of HIV in the samples from women who used hormonal contraception than in women who didn’t, even though the women didn’t have any increased viral load in their blood.
LIST: AIDS at 30: Medical Milestones in the Battle Against a Modern Plague
Reported the Times:
The progestin in injectable contraceptives appears to have a physiological effect, scientists said. Renee Heffron, an epidemiologist and co-author of the study, said research examining whether the hormone changes genital tissue or vaginal mucous had been inconclusive. Studies in macaques found that progestin thins vaginal tissue, she said, “but studies among women didn’t show the same amount of thinning.”
It could be that progestin causes “immunologic changes in the vagina and cervix” or could increase the H.I.V.’s “ability to replicate,” Dr. Morrison said.
Impelled by the new findings, the World Health Organization will meet in January to discuss whether the mounting evidence associating hormonal birth control with a higher risk of HIV infection is strong enough to rethink WHO’s clinical recommendations on contraceptive use, the Times reported.
MORE: Online Gamers Solve a Tricky AIDS Puzzle
The study was published in the Lancet Infectious Disease.
Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.