One dose of a vaccine is not enough to protect a child against some diseases. Depending on the vaccine and the disease, more than one dose is needed to build high enough immunity to prevent the disease, boost immunity that fades over time, make sure people who did not get immunity from a first dose are protected, or protect against germs that change over time, such as flu. This is because what’s in most childhood vaccines is inactive (i.e. dead) viruses or bacteria, which is why vaccines can’t get you sick. But because the viruses or bacteria are inactive, the immune system needs to respond to it multiple times for full protection. Scientists and medical providers study the best combination of timing and number of doses needed for full protection against diseases.

Learn more about this concept, known as waning immunity.

Source: CDC: The Childhood Immunization Schedule