When a vaccine dose is repeated, it is called a booster dose. A booster dose is used to enhance the body’s immune response to a given disease. Even though the memory B and T cells are long lived, they will slowly disappear over time. This might be months or years depending on the disease, but over time, the immunity against that infection fades.
However, when the body sees a vaccine a second time, the memory B and T cells do two things. First, they make the necessary plasma cells and helper and killer T cells to restimulate the immune system. Second, they make extra copies of the memory B and T cells. Extra copies mean that it will take longer for all the memory cells for a certain infection to die off and it will take longer for immunity to that disease to fade away.
Another reason a booster dose is sometimes required is that studies have shown that for certain vaccines, a single dose is not enough to provide protection to the majority of recipients. For example, after one dose of hepatitis B vaccine, only about 60 percent of recipients produce sufficient antibodies to be protected. After two doses, the number of protected recipients increases to over 85 percent, and by the third dose, over 98 percent of recipients are protected. Because different people react differently, some people need an additional stimulus to provide a protective response. That additional stimulus comes in the form of a booster dose.