clinical trial vaccine clinical trial YH Most vaccines against a pathogen (e.g., HIV) are not tested by vaccinating people and then deliberately exposing them to virulent pathogen (e.g., HIV). This strategy is never for a vaccine against a disease as serious as HIV. The best way to determine if a vaccine 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 a vaccine. A processual entity by which a vaccine is tested clinically for safety and effectiveness. Clinical trials are conducted in phases. Classically, clinical trials unfold in three phases in order to gather data and information about a vaccine and its performance. This will form the basis of a dossier submitted to regulatory authorities by way of an application for licensure. After a vaccine is licensed and is being used by large numbers of people, a Phase IV study may or may not be taken for vaccine clinical evaluation.