comes, the power of a community trial will be lower than a trial of the same size employing randomization at the individual-level (i.e., the effective sample size will be between the number of participants and the number of sites). Increasing the number of communities will provide more power than increasing the number of participants in a single community. Increasing the number of communities in a single trial does increase logistics and cost, however, and care must be taken to provide adequate resources to enable local partners to carry out the study.

It would be important to select trial sites where the anticipated effect size (percent reduction in the incidence of HIV) would justify the effort. The trial would provide an opportunity to collect data on both biological and behavioral outcomes. One of the limitations of previous evaluations is the reliance on self-reported changes in behavior as a proxy for changes in risk HIV transmission. This study, in contrast, would have the opportunity to measure actual HIV incidence. The trial would also represent an opportunity to evaluate the impact of multi-component prevention programs on HCV transmission. Investigators could consider the feasibility of addressing this primary endpoint during the design phase of the trial.

Secondary outcomes might include subjective and objective measures of risk behavior, including drug-related behavior (such as self-reports of needle sharing and needle disinfection) and sexual behavior (such as self-reports of condom use). Secondary outcomes might also include potential harm at the individual and community level (such as an increase in discarded needles or recruitment of new users). Mediating variables (e.g., self-efficacy in not sharing, or knowledge about infectious disease transmission through sharing of cotton or water) could also be collected. Mediating and moderating variables are important for scrutinizing causal pathways, which distinguish successful interventions from less successful ones. A formative evaluation component could also provide information on the best implementation strategies for the prevention program. Data on program costs and cost-effectiveness could also be collected to help inform decisions on allocating resources.

DESIGN AND IMPLEMENTATION CHALLENGES

A trial of this size and complexity would present major operational and logistical challenges. For example, before the actual trial began, investigators would have to collect baseline measures of biological and behavioral outcomes at all study sites. Investigators would also have to conduct follow-up assessments periodically to measure outcomes. A study of this size would take several years and require extensive cooperation and monitoring among an array of organizations and stakeholders. However, while the proposed rigorous study would present significant challenges, that real-



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