The epidemiological contribution to the preparation of field trials for HIV and STI vaccines: objectives and methods of feasibility studies.

Feasibility studies are an important component of preparations for efficacy field trials of vaccine candidates, but represent a neglected area in clinical trial literature. These studies are designed to identify cohorts of higher-risk individuals with sufficiently high incidence to support a vaccine trial; to determine the readiness of these individuals to participate in a trial; to develop cost-effective recruitment and retention strategies, educational methods that ensure ethical informed consent, and instruments that accurately assess risk behaviours; to measure incidence rates over time and the effect of risk reduction interventions on incidence; to contribute estimates of incidence and loss to follow-up rates to sample size calculations; to assess the level of acceptability of a future efficacious vaccine in the target population and, in the case of several viruses, to identify circulating subtypes. Feasibility study methods have been largely developed in the context of HIV, but may also be used in the preparation of preventive intervention trials for other STI.