Around 41 million women worldwide use the injectable form of hormonal contraception. According to a new study, however, this widely used form of birth control may increase women’s risk of becoming infected with HIV.

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Depot medroxyprogesterone is used by around 41 million women around the world.

A team of researchers from the University of California at Berkeley conducted a meta-analysis of 12 studies from sub-Saharan Africa involving a total of 39,560 women. Their findings are published in The Lancet Infectious Diseases.

The association between hormonal contraceptives and an increased risk of HIV acquisition has been debated for the past 2 decades. A lot of uncertainty around the subject still remains.

As well as the shot, women can also receive hormonal contraception in the form of a pill taken orally. Around 103 million women worldwide receive birth control in this manner.

These forms of contraception work by preventing ovulation. Injectable hormonal contraception – depot medroxyprogesterone acetate (DMPA) – also alters the lining of the uterus so that pregnancy cannot occur. Women receive the birth control shot once every 3 months, whereas the pill must be taken daily.

“Use of these hormonal contraceptives prevents unintended pregnancies, reduces the rate of maternal and infant morbidity and mortality, and enables women to achieve other life goals,” write the authors of the study.

If an increased risk of HIV infection was associated with hormonal contraception, there would be implications for both contraceptive counseling and policy implementation. The authors state that some countries in sub-Saharan Africa are considering withdrawing DMPA.

The researchers analyzed 12 observational studies, and their findings suggest that using DMPA increases the risk of HIV infection for women by 40% in comparison with other methods of contraception and not using contraception at all.

Statistically, 40% is a significant figure. However, in relative terms it only represents a moderate increase in risk. According to lead author Lauren Ralph, an epidemiologist at the University of California at Berkeley, this increase in risk is not enough to justify a complete withdrawal of DMPA:

Banning DMPA would leave many women without immediate access to alternative, effective contraceptive options. This is likely to lead to more unintended pregnancies, and because childbirth remains life-threatening in many developing countries, could increase overall deaths among women.”

Ralph adds that more evidence specifically pertaining to high-risk women, “such as commercial sex workers and women in serodiscordant partnerships (where one partner is HIV-positive and the other is not),” is urgently required.

The authors state that efforts are now underway to fund a randomized trial to investigate the association between hormonal contraception and HIV further.

Published alongside the study is a linked comment by Christopher Colvin from the University of Cape Town, South Africa, and Abigail Harrison from Brown University School of Public Health in the US.

“Ralph and colleagues’ findings add an important element to the longstanding debates about the HIV and [DMPA] relation, and growing calls for further evidence about the magnitude, mechanisms and health effects of this link,” they write.

The debate centers around whether a large randomized controlled trial should be conducted in order to better understand the association, with opinions polarized due to concerns about existing evidence, policies and financial costs.

“Ralph and colleagues’ signature contribution is their nuanced discussion of what their research adds and what is possible with current and future evidence,” conclude Colvin and Harrison. “They describe an approach to evidence, policy and practice rooted in an ‘ecology of evidence’ as the foundation for thinking through the next steps.”

Recently, Medical News Today reported on a study suggesting an explanation for why HIV vaccines can “backfire” and lead to increased rates of infection.