The Centers for Disease Control and Prevention presents information about how vaccines are developed and tested before they are ever used on people.
Scientists have to ensure that vaccines work, which usually takes years of effort in the lab. After testing there shows that it’s a good vaccine that is effective and appears safe, it moves into testing. There are several phases of testing, which are meant to verify its safety and assess efficacy. That testing can also take many years before it’s complete. Learn more about those phases, which start with small groups and increase to larger groups of volunteers, from the video.
Only after that testing is performed is a vaccine considered for wider distribution; a committee has to review all the data and approve it. Then, the vaccine has to be manufactured, which also involves rigorous monitoring and testing.