In medicine, a booster dose is an extra administration of a vaccine after an earlier (prime) dose. After initial immunization, a booster injection or booster dose is a re-exposure to the immunizing antigen cell. It is intended to increase immunity against that antigen back to protective levels after it has been shown to have decreased or after a specified period. For example, tetanus shot boosters are often recommended every 10 years. The need for a booster dose is evaluated in several ways, including the immune response after a booster dose is administered, active B and T cell activity after a certain amount of time that the primary vaccine was given, and prevalence of the disease in vaccinated populations.
If a patient receives a booster dose but already has a high level of antibody, then a reaction called an Arthus reaction could develop, a localized form of Type III hypersensitivity induced by high levels of IgG antibodies causing inflammation. The inflammation is often are self-resolved over the course of of a few days, but could be avoided altogether by increasing the length of time between the primary vaccine and the booster dose.
It is not yet clear why some vaccines such as Hepatitis A and B are effective for life, and some such as tetanus need boosters. In some cases, the booster serves as a 'reminder' to the immune system 
Polio Booster Doses
In the case of the Polio vaccine, the oral polio vaccine (OPV) decreases in strength after only six months of consumption. Booster doses of the OPV were found ineffective, as they, too, resulted in decreased immune response six months after consumption. However, when the inactive polio vaccine (IPV) was used as a booster dose, it was found to increase the test subjects' antibody count by 39%-75%. This is in contrast to when IPV is used as the primary immunity, during which the immune response is low, allowing the virus to incubate in the gut and potentially attenuate to its fully reproductive form. In this instance, neither OPV nor IPV was effective enough to not require a booster dose, but instead OPV is administrated as a primary vaccine and IPV as a booster vaccine.
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