evidence: Randomized controlled trials are commonly held up as the gold standard for determining the safety and efficacy of a medical intervention, like a vaccine or a drug. Saline placebo-controlled trials are sometimes used to test a new vaccine, but such trials aren’t always practical or ethical to do. For instance, when testing a new vaccine against a disease for which there are existing, effective vaccines, such a trial would leave the placebo group unprotected from the disease. In such cases, it is considered acceptable to compare the new vaccine with an existing one.