Vaccine can didates are tested many times in progressively larger clinical trials to ensure they are safe and effective.
The first clinical studies in humans are phase 1 clinical trials which primarily assess the initial safety of the vaccine and identify the correct dose. Phase I clinical trials typically enroll tens or hundreds of healthy adults.
Vaccine candidates that show promise then move on to phase 2 clinical trials. Phase 2 clinical trials are larger studies designed to assess safety and to determine how well the vaccine activates the immune system response in healthy individuals. Typically hundreds or thousands of people participate in phase 2 trials. Sometimes phase 1 and phase 2 trials are combined into phase 1/2 trials where the aims of both the phase 1 and phase 2 trials are achieved.
Phase 3 clinical trials are the final test of the vaccine prior to approval. In phase 3 trials, the vaccine is given to tens of thousands of healthy volunteers to determine whether the vaccine protects them from becoming sick. Typically, phase 3 trials enroll those at highest risk of the disease. If the vaccine proves to be safe and effective throughout these trials, all of the evidence will be reviewed by regulatory agencies who will determine whether to approve it for widespread use.
Once approved, vaccines continue to be monitored for safety and effectiveness. In addition, once approved, public health officials must implement detailed and transparent plans to deliver vaccines to individuals and communities who need them.