A vaccine trial is a clinical trial that aims at establishing the safety and efficacy of a vaccine prior to it being licensed. A vaccine candidate drug is first identified through preclinical evaluations that could involve high throughput screening and selecting the proper antigen to invoke an immune response. Some vaccine trials may take months or years to complete, depending on the time required for the subjects to react to the vaccine and develop the required antibodies. Preclinical stage Preclinical development stages are necessary to determine the immunogenicity potential and safety profile for a vaccine candidate. This is also the stage in which the drug candidate may be first tested in laboratory animals prior to moving to the Phase I trials. Vaccines such as the oral polio vaccine have been first tested for adverse effects and immunogenicity in monkeys as well as non-human primates and lab mice. Recent scientific advances have helped to use transgenic animals as a part of vaccine preclinical protocol in hopes to more accurately determine drug reactions in humans. Understanding vaccine safety and the immunological response to the vaccine, such as toxicity, are necessary components of the preclinical stage. Other drug trials focus on the pharmacodynamics and pharmacokinetics; however, in vaccine studies it is essential to understand toxic effects at all possible dosage levels and the interactions with the immune system. Phase I The Phase I study consists of introducing the vaccine candidate to assess its safety in healthy people. A vaccine Phase I trial involves normal healthy subjects, each tested with either the candidate vaccine or a "control" treatment, typically a placebo or an adjuvant-containing cocktail, or an established vaccine (which might be intended to protect against a different pathogen). The primary observation is for detection of safety (absence of an adverse event) and evidence of an immune response. After the administration of the vaccine or placebo, the researchers collect data on antibody production, on health outcomes (such as illness due to the targeted infection or to another infection). Following the trial protocol, the specified statistical test is performed to gauge the statistical significance of the observed differences in the outcomes between the treatment and control groups. Side effects of the vaccine are also noted, and these contribute to the decision on whether to advance the candidate vaccine to a Phase II trial. One typical version of Phase I studies in vaccines involves an escalation