How are vaccines tested?

Short Answer

The Food and Drug Administration (FDA) meticulously regulates clinical development, safety, testing, and licensing of vaccines through a multiphase process. Tests involve computerized predictive modeling, animal testing, and human clinical trials. Development of vaccines can take between 8 and 17 years before they are available on the market. While drug manufacturers themselves are responsible for testing, the FDA monitors the process carefully and makes recommendations based on the data collected.


Before vaccines are licensed by the FDA, they are tested extensively in the laboratory and with human subjects to ensure their safety. First, computers are used to predict how the vaccine will interact with the immune system. Then researchers test the vaccine on animals including mice, guinea pigs, rabbits, and monkeys. After the vaccine completes these laboratory tests successfully, the FDA approves its use in clinical studies on human subjects. Participation in these studies is completely voluntary. Many individuals choose to contribute their time and energy for the advancement of science. Informed consent is obtained from all participants before they become involved in research. This ensures that they understand the purpose of the study and its potential risks, and are willing to participate. Volunteers agree to receive the vaccine and undergo any medical testing necessary to assess its safety and efficacy. [6]

Vaccine licensing is a lengthy process that may take 10 years or longer. The FDA requires that vaccines undergo three phases of clinical trials with human subjects before they can be licensed for use in the general public. Phase-one trials are small, involving only 20 to 100 volunteers, and last only a few months. The purpose of phase-one trials is to evaluate basic safety and identify very common side effects. Phase-two trials are larger and involve several hundred participants. These studies last anywhere from several months to two years, and collect additional information on safety and efficacy. Data gained from phase-two trials can be used to determine the composition of the vaccine, how many doses are necessary, and a profile of common side effects. Unless the vaccine is completely ineffective or causes serious side effects, the trials are expanded to phase three, which involves several hundred to several thousand volunteers. Typically these trials last several years. Because the vaccinated group can be compared to those who have not received the vaccine, researchers are able to identify true side effects. [13678]

If the clinical trials demonstrate that the vaccine is safe and effective, the manufacturer applies to the FDA for two licenses: one for the vaccine (product license), and one for the production plant (establishment license). During the application process, the FDA reviews the clinical trial data and proposed product labeling. In addition, the FDA inspects the plant and goes over manufacturing protocols to ensure vaccines are produced in a safe and consistent manner. Only after the FDA is satisfied that the vaccine is safe is it licensed for use in the general population.

Vaccine safety is estimated by the number of “adverse events” reported. An adverse event is “…a medical incident that takes place after an immunization…and is believed to be caused by the immunization.”[1] Adverse events include:

  • True reactions to the vaccine
  • Events that would have occurred even if the person had not yet been vaccinated (unrelated consequences)
  • Reactions related to mistakes in vaccine preparation, handling, or administration
  • Events that cannot be related directly to the vaccine; their cause is unknown


A formal scientific study usually is required to distinguish between coincidences and true reactions. It is rarely possible to say for sure whether a vaccine caused a specific adverse event. Almost all national immunization programs have a system for reporting adverse events.

The United States Vaccine Adverse Event Reporting System (VAERS)[2] and the Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) are examples. People who report a vaccine reaction to VAERS or CAEFISS are asked if the reaction led to hospitalization, life-threatening illness, disability, or death. These events are classified as “serious,” and are often subject to further study that yields important information.

For example, reports to CAEFISS identified a common illness among people who received flu vaccines from one Canadian manufacturer in one season.[3] Equally important, such systems have supported the safety of new vaccines, such as the new meningococcal B and C vaccines in New Zealand and the United Kingdom.

Other monitoring programs include the Vaccine Safety Datalink (VSD) project, the Clinical Immunization Safety Assessment (CISA) Network, and the Brighton Collaboration. The VSD project is a collaboration between CDC’s Immunization Safety Office and eight large managed-care organizations that allows for planned vaccine safety studies as well as quick investigation of possible issues. The CISA Network of six medical research centers conducts clinical research on vaccine-associated health risks, and the Brighton Collaboration develops standard case definitions for problems following immunization as well as guidelines for data collection, analysis, and presentation.

Vaccine safety monitoring becomes more important with new vaccines, expanded vaccine recommendations, and new global immunization initiatives. Reporting systems like VAERS will continue to be used to monitor adverse events, so vaccines can continue to be held to very high standards of safety.

Citations for this page

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