The best way to determine if a vaccine or therapy is effective is to test it in a randomized, controlled, double-blind clinical trial. This type of trial is often referred to as the gold standard in medical research and provides the strongest evidence for the efficacy of an experimental product. A controlled clinical trial compares the vaccine candidate or therapy being tested to either the best available treatment for that disease or, in the case of a preventive technology like a vaccine, against an inactive substance known as placebo that has no biological effect. Whether a volunteer in a clinical trial receives the vaccine candidate or placebo is determined completely randomly by a computer program. Another factor in the design of clinical trials that adds credibility to the results is double-blinding, which requires that neither the volunteers nor the researchers know who is receiving the vaccine candidate or placebo.
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