Understanding the sources of efficacy dilution in a trial of a monthly dapivirine vaginal ring for HIV-1 prevention.

Introduction: Women-initiated HIV - 1 prevention products are key to reducing women's HIV-1 risk. Clinical trials of vaginal microbicides have shown limited to no efficacy in intention-to-treat (ITT) analyses. It is hypothesized that these negative results are partly due to efficacy dilution.

Methods: We developed a microsimulation model of MTN-020/ASPIRE, a phase 3 trial that evaluated monthly use of a dapivirine vaginal ring for HIV-1 prevention. We evaluated four sources of efficacy dilution: trial-level factors: (i) an imbalance in the number of monthly sex acts between study arms and (ii) heterogeneity in risk emergent over time; and individual-level factors: (iii) product non-adherence and (iv) receptive anal intercourse.

Results: Assuming 70% per-vaginal exposure efficacy (consistent with the ITT estimate of 27%), heterogeneity in risk accounted for the largest proportion of efficacy dilution, at 42% (90% CrI: 38, 45), followed by non-adherence (33%; 90% CrI: 27, 39), an imbalance in arms (18%; 90% CrI: 16, 21) and lastly, anal intercourse with less than 10% of efficacy dilution.

Conclusion: Our results suggest that heterogeneity in risk was the most important source of efficacy dilution in the ASPIRE trial. Future trials of HIV-1 prevention products for women should consider alternative trial designs and analytic approaches that minimize bias introduced by heterogeneity in risk.