Vaccine development entails a rigorous system from identifying candidate molecules, testing in the laboratory, controlled testing in a few human volunteers, controlled testing in a big population and routine adverse event monitoring when vaccines are licensed for wider use. If the vaccine triggers an immune response, then it is tested in human clinical trials in three phases. Phase 1: small number of volunteers to assess its safety; phase 2: several hundreds of volunteers to further assess safety and ability to generate an immune response. Participants in this phase have the same characteristics (age, sex); Phase 3: thousands of volunteers are compared to a similar group that received a comparator product. Often, this trial is conducted in multiple countries and multiple sites.
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