Comparison of adherence to pre-exposure prophylaxis during blinded and open-label phases of a randomized trial

Abstract

The Partners PrEP Study, a randomized trial of daily oral pre-exposure prophylaxis for HIV prevention, demonstrated efficacy at an interim analysis, prompting discontinuation of its placebo arm and early release of results. Active arm participants continued followup, and placebo arm participants optionally re-consented to additional followup on active drug. We compared PrEP adherence, by multiple measures, during the placebo-controlled and subsequent open-label phases of the study. Learning of PrEP efficacy led to a modest, although statistically significant, increase in adherence: the odds of refilling study medication at each monthly visit increased by 24% (p = 0.03), and the odds of detecting PrEP drug in plasma increased by 76% (p < 0.01), the latter returning to counterfactual levels within twelve months. Placebo arm participants who chose to continue followup on active drug were more likely to refill than those originally randomized to active drug (odds ratio, 1.32; p < 0.01). These results suggest that knowledge of PrEP efficacy had a small impact on PrEP adherence in a trial where adherence was already high.