Vaccine Clinical Trials

The immune system is the body’s 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.

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.  

What happens in a clinical trial depends on the ‘protocol.’ A protocol is a detailed plan that explains the purpose of the clinical trial and how it will be run. If you consider joining a clinical trial, these details will be explained to you as part of the informed consent process. They include: 

• the length of the clinical trial 
• the study medicines, procedures, and tests in the clinical trial  
• the schedule of study activities 
• information about who can participate 
• how side effects will be tracked, managed, and reported 
• the rules that must be followed 

A clinical trial may also have aspects such as randomization, placebo, or blinding.

We encourage everybody to consider joining clinical trials. There are many types of clinical trials that study different aspects of health in people with a variety of backgrounds and conditions. Each clinical trial has its own criteria for who may participate. These are called eligibility criteria.  
 
Eligibility criteria include things like your age, sex, overall health, type of condition, and your medical treatment history. As part of the steps to join a clinical trial, the study doctor will confirm whether you meet all eligibility criteria and are able to participate.

Participation in a clinical trial is always voluntary. The decision to join is personal, and it is yours.
 
To learn more about Pfizer’s clinical trials, visit Our Research or search for trials here. 

We list all of our clinical trials that are recruiting participants or plan to recruit shortly in our Find a Trial search engine. We welcome you to explore our clinical trials and the information provided about each study. There is contact information available on each study’s webpage. 
 
For general inquiries about Pfizer clinical trials, you may call 1-800-887-7002 or email [email protected].

Our Find a Trial search engine only provides information about Pfizer clinical trials. To search for all registered clinical trials, regardless of the study sponsor, you may visit ClinicalTrials.gov.