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How does a vaccine work?

The immune system is the bodys natural defense against infections. Vaccines help the immune system recognize bacteria or viruses that may invade the body in the future. They do this by introducing antigens, parts of the bacteria or viruses, to the immune system. The immune system processes these antigens in a similar way to a real infection, but without the sickness that goes with it. As a result, certain immune system cells remember how to recognize these antigens quickly and fight the real infection if it happens in the future.

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How do vaccine clinical trials work?

All vaccines that become available to the public are first tested in clinical trials to show that they are safe and effective. 

In phase 1 clinical trials, we give different doses of the study vaccine to a small group of healthy adult volunteers. We look at safety and use laboratory tests to check the participants immune responses to those doses. We focus on the immune response because vaccines train the immune system to fight off possible future infections. Based on what we learn in phase 1, we then decide whether to study the vaccine candidate in phase 2 clinical trials.

Phase 2 clinical trials are conducted with larger and more targeted populations — for instance, people who may be at risk for a specific infection. In these studies, we learn more about the safety and immune response. If the study vaccine is considered safe and the immune system responds as expected, then the study vaccine moves into phase 3.

In phase 3 clinical trials, we see how well the study vaccine decreases the number of infections in large groups of participants under normal daily living conditions. The number of infections in vaccinated participants is compared to the number among those who did not receive the study vaccine. The safety of the study vaccine also continues to be studied.

Phase 4 clinical trials are conducted once the vaccine has received approval for use by regulatory authorities such as the U.S. FDA. In these studies, we continue to study the safety and effectiveness of the vaccine in larger populations who receive the vaccine as part of their routine care.  

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