How are vaccines tested for safety and effectiveness?

Vaccines must go through a detailed scientific evaluation before they can be submitted to the FDA for approval. Each phase of the evaluation includes three different clinical research studies. In the clinical research study or trial, people volunteer to be part of the study. Each clinical trial emphasizes safety of the vaccine on people. As the research moves through to the next phase, the group of volunteers becomes bigger to include more diversity in people and circumstances.

  • Phase 1 involves 20 to 100 healthy volunteers to evaluate safety and common side effects of the vaccine.
  • Phase 2 involves several hundred volunteers to gather information on safety, vaccine dosing, and ability to stimulate an immune response.
  • Phase 3 involves several thousand volunteers and a longer time frame than the earlier studies. Along with safety and side effects, most Phase 3 studies focus on efficacy -- how well the vaccine works in clinical trials -- and compare people who have received the vaccine to those who receive a placebo (a shot without the real vaccine).  During these studies, neither the participants nor the study managers know who received the vaccine and who received the placebo until the end of the study. This phase provides the most firm scientific evidence possible showing the difference between people who have been vaccinated and people who have not been vaccinated for both safety and effectiveness.

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