Notes on the frequency of routinely collected and self-reported behavioral data in HIV prevention trials.

HIV prevention trials typically randomize thousands of participants to active or control intervention arms, with regular (e.g. monthly) clinic visits over one or more years of follow-up. Because HIV infection rates are often lower than 3 per 100 person-years even in high prevalence settings, tens of thousands of clinic visits may take place before the number of infections required to achieve adequate study power has been observed. In addition to clinical outcomes, the multitude of study visits provides an opportunity to assess adherence and related participant behaviors in great detail. These data may be used to refine counseling messages, gain insight into patterns of behavior, and perform supporting analyses in an attempt to obtain more precise estimates of treatment efficacy. Exploratory analyses were performed to assess how our understanding of participant behaviors and their relationships to biological outcomes in two recent prevention trials might have been impacted had the frequency of routine behavioral data collection been reduced from monthly to just months 1, 3, 6, 9, and 12. Results were comparably informative in the reduced case, suggesting that unnecessarily extensive amounts of routine behavioral data may be collected in these trials.