Today, several phases or stages are involved in a clinical trial conducted for marketing a new drug or vaccine in the United States. The suggestion that a new agent has a promising therapeutic action usually comes from a series of laboratory studies in vitro and in vivo among animals. This is referred to as the preclinical trial phase. Phase I trials consist of testing the agent among human volunteers, primarily for safety concern rather than for treatment efficacy. Phase II is concerned with testing treatment efficacy. At the conclusion of Phase III, much information would have been gathered about the agent’s overall protective efficacy. The evaluation of treatment or drug safety and efficacy continues through postlicensing surveillance.

Clinical trials are more complicated than animal controlled experiments, in that ethical and treatment crossover issues are always barriers to achieving a successful randomization design, in which the test subjects would otherwise be randomly assigned to the treatment or control conditions. At times, a treatment crossover for a patient is necessary during the course of a clinical trial, because the patient’s health condition may have been deteriorated to the point to require the same test or a special treatment. Ideally, clinical trials should be conducted with a randomized, double blind-design in order to minimize experimental biases.