Who will enroll? Predicting participation in a phase II AIDS vaccine trial

Background: The problems of underenrollment and selective enrollment may un dermine AIDS vaccine trials. If prospective study subjects' stated willingn ess to participate (WTP) in hypothetical vaccine trials predicts future enr ollment, then measuring WTP before recruitment may enhance the enrollment i n, and ethics of, such trials. Methods: We prospectively studied changes over an 18-month period in the st ated WTP in, and knowledge of, a hypothetical AIDS vaccine trial among 610 Philadelphia residents at high risk for HIV infection. Of these people, 499 were subsequently recruited to participate in an actual, phase II AIDS vac cine trial. We used multivariable logistic regression and the area under th e receiver-operating characteristic (ROC) curve to model predictors of actu al enrollment. Results: Actual enrollment rates were 8.3%, 6.8%, 15.8%, and 29.0% among th ose who had initially said they were "definitely not," "probably not." "pro bably," and "definitely" willing to participate, respectively (p = .006). T he area under the ROC curve was 0.65, indicating a modest ability of stated WTP to differentiate those who enroll from those who do not. Knowledge of basic vaccine trial concepts, though unrelated to enrollment, increased ove r an 18-month period with repeated education sessions (p < .0001), whereas stated WTP declined over this same period (p < .0001). Conclusion: Although other factors not captured by stated WTP may also infl uence future enrollment, prospectively assessing stated WTP may augment the validity of the informed consent process, help prevent underenrollment, an d clarify the population from which the study sample is drawn.