It looks like you're using an Ad Blocker.
Please white-list or disable AboveTopSecret.com in your ad-blocking tool.
Thank you.
Some features of ATS will be disabled while you continue to use an ad-blocker.
Phase 3 Clinical Trial
An even larger study, often including thousands of volunteers, allows scientists to compare people who receive the vaccine to those who did not. They can better determine if the vaccine is safe and effective in a larger population of individuals. Volunteers are randomly selected to receive either the vaccine or an alternative "control" medication. The control medication is usually a placebo or another vaccine that is already FDA-approved.