A new piece in the Lancet by Heffron et al. finds that hormonal contraception (specifically the injection-based method most common in sub-Saharan Africa) roughly doubles the risk of HIV transmission between an infected partner and an uninfected one. My first instinct upon hearing the result was to question whether they were really measuring the biological effect of hormonal contraceptives, as opposed to behavior change. After all, why do people take up these methods in the first place? So they can stop using condoms, of course, which naturally increases their exposure to HIV. The answer, however, is that there almost surely is some biological effect going on: in their last section the authors report that women on hormonal birth control had higher levels of HIV RNA in their cervical canals. To me this is the lede, and the article totally buries it, leaving it out of the abstract.
On top of that, the study finds that controlling for condom use has little effect on . But the way they account for condom use is pretty problematic. While they report that they tried separate controls for the number of condom-protected and unprotected sex acts, those variables aren’t in any of their tables and their final regressions include only a dummy for “any unprotected sex”. We know from previous studies of HIV transmission in Africa (e.g. Wawer et al. 2005) that inconsistent condom use has no predictive power for transmission – and inconsistent use (meaning that people sometimes do and sometimes don’t use condoms with the same partner) is evidently pretty common in African countries: so common that Wawer et al. found that none of the couples in their study used condoms all the time. This means it’s credible that the effect on condom use was “inframarginal”, affecting how often they were used rather than whether they are used at all. Controlling for any use might not adequately adjust for the change in condom use that comes with uptake of hormonal contraceptives.
What can we we do about this? One cheap way to get at the biological effect of hormonal contraceptives would be to run the regressions from the paper with a control for the level of endocervical HIV RNA. If that’s a good proxy for the biological increase in HIV exposure, then in those regressions the effect of hormonal treatments should drop out unless there are also behavior changes going on. My take is that the authors should have stressed that the biological risks aren’t that interesting, and focused on regressions that don’t control for sexual behavior. Imagine a policy where you promote hormonal birth control. In terms of HIV transmission risks, you’d see not just the biological impact that Hefron et al. are trying to estimate, but the sum of that and behavior changes. The paper basically controls for an outcome of hormonal birth control use (it leads to less condom use) which is almost never a good idea.
Hat tip: haba na haba