How do Vaccines Work?
A vaccine contains a killed or weakened part of a germ that is responsible for infection. Because the germ has been killed or weakened before it is used to make the vaccine, it can not make the person sick. When a person receives a vaccine, the body reacts by making protective substances called "antibodies". The antibodies are the body's defenders because they help to kill off the germs that enter the body. In other words, vaccines expose people safely to germs, so that they can become protected from a disease but not come down with the disease.
- How Vaccines Prevent Disease - Centers for Disease Control and Prevention