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Experimental vaccines are first tested in laboratory and animal studies for safety and immunogenicity. If the results are favorable, a vaccine must successfully complete several phases of testing in people before it can gain Food and Drug Administration (FDA) approval for licensing and marketing to the public. All the phases conduct randomized, controlled, and double-blinded studies. These studies compare the safety and effectiveness of the vaccine to an inactive placebo or a control that may be another licensed vaccine of potential benefit to the study population. *
A Phase I study is the first setting in which an experimental vaccine is given to people. The trial, which can last up to 2 years, may enroll between 20 to 100 volunteers. A Phase I study primarily seeks information on safety, particularly looking for any vaccine-related side effects. The study can also provide data on the dose and administration schedule needed to achieve the optimal immune responses.
Once Phase I studies show the experimental vaccine is safe, well tolerated, and appears promising, it can advance into Phase II. These studies, which can last longer than 2 years, enroll between 100 to 300 volunteers. In these studies researchers gather more data on safety and immunogenicity.
Investigators supported by the National Institute of Allergy and Infectious Diseases (NIAID) are also developing designs for intermediate studies that can provide preliminary answers about a candidate vaccine's efficacy, and determine if the vaccine candidate should move forward into large Phase III, or efficacy, studies. A Phase IIb study, also known as a "proof-of-concept" study, is smaller, usually requiring 2,000 to 9,000 volunteers, and may provide clues about the immune responses that can protect against disease.
The most promising vaccine candidates move into Phase III, enrolling 10,000 or more people. A Phase III study, which can last up to 4 years, is typically designed to ensure enough data are collected on safety and effectiveness to support a license application to FDA. *
ClinicalTrials.gov has additional Clinical Trials Resource Information.
* Courtesy National Institute of Allergy and Infectious Diseases
Current Clinical Studies
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Last Updated May 14, 2008