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The availability of effective Pre-Exposure Prophylaxis (PrEP) for HIV introduces new challenges for testing novel on-demand, user-controlled HIV prevention products, including lower placebo arm incidence and increased between-participant variability in HIV risk. In this paper, we discuss how low HIV incidence may result in longer trials in which the variability in participants' risk may impact the estimate of risk reduction. We introduce a measure of per-exposure efficacy that may be more relevant than the population level reduction in incidence for on demand products and explore alternatives to the parallel arm design that could target better this parameter of interest: the crossover and the re-randomization designs. We propose three different ways in which crossover and re-randomization of intervention assignments could be implemented in event-driven trials. We evaluate the performance of these designs through a simulation study, finding that they allow for better estimation and higher power than the traditional event-driven parallel arm design. We conclude by discussing future work, practical challenges and ethical considerations that need to be addressed to take these designs closer to implementation.

Crossover and Repeated Randomization in Event Driven Trials for HIV Prevention: Addressing the Impact of Heterogeneity in Risk in the Trial Design