The prophylactic clinical trial as a epidemiologic resource.

We encourage investigators to use data collected in a clinical trial of a prophylactic agent or procedure to study the epidemiology of the disease or event the prophylactic was meant to prevent. Making additional use of previously collected data is economically attractive. Problems can arise, however, if the sample is not representative of the universe of all people at risk of the disorder, the data set is not adequate to the task, or the prophylactic and/or its correlates influence the risk of the disorder. Investigators should consider modifying data collection procedures in future prophylactic trials so that they are suitable for an epidemiologic study. They and readers of their reports, however, are advised to be cautious in drawing inferences from epidemiologic studies extracted from prophylactic trials.