Before recommending a vaccine for use, it should be tested in labs. This procedure can take many years. FDA uses the information from these tests to agree whether to test the vaccine with people or not.
Throughout a clinical trial, a vaccine should be tested on people who agree to get vaccinated. The Clinical trials start with 20 - 100 volunteers, but ultimately include thousands of volunteers. These tests take so many years and answer important questions like:
Adverse effect (medicine): An adverse effect is an unwanted harmful effect resulting from an intervention or other medication, such as surgery. An adverse effect also called as a "side effect", when judged to be secondary to a main or therapeutic effect.
Adverse drug reaction: An adverse drug reaction (ADR) can be defined as an unwanted or an unappreciated harmful reaction resulting from an intervention related to the use of a medicinal product. Adverse effects usually predict dangerous from future administration and warrant prevention, or alteration of the dosage regimen, or specific treatment or withdrawal of the product.
Artificial induction of immunity: The process of artificial induction of immunity enhances people immunity to a specific disorder by means other than waiting for them to catch the disease. The aim is to decrease the risk of death and suffering. Artificially acquired active immunity can be introduced to our body by a vaccine, a drug that contains the antigen. It will encourage the response against the antigen without causing symptoms of the disease.