Phase I: Antibody response is evaluated after different doses of vaccine are administered to a small number of volunteers. In this phase, the aim is to determine the appropriate dose and to evaluate safety. If the safety of a vaccine is proven by phase I studies, phase II clinical trials start.
Phase II: Hundreds of volunteers participate in this phase. Volunteers are closely monitored for side effects. More data are obtained on the level of the immune response induced and the safety of the vaccine. If the results of phase II studies are promising, phase III studies can be started.
Phase III: This phase involves thousands of volunteers and evaluates the effects of vaccines on a much larger scale. In this phase, volunteers participating in the study are divided into two groups. While a group of volunteers is injected with the vaccine, the other group of volunteers is injected with a solution, called placebo, which does not contain the vaccine. The person administering the vaccine and the person receiving the vaccine are not informed of the group they are in. Using a placebo allows comparisons on the vaccine efficacy. Results obtained from vaccine and placebo groups are compared in terms of vaccine efficacy and safety. In this phase, it is essential to prove the efficacy and to monitor the side effects. Phase III studies allow data collection from thousands of volunteers which helps to decide for approval or refusal of the vaccine. If these studies show positive results, the vaccine is submitted for licensing.
Phase IV: After the vaccine is licensed and marketed, routine vaccination starts in the community.. During this phase, regulatory agencies still monitor safety, efficacy, side effects and long term immunity. Information obtained in this phase on the vaccine continues to be analyzed.