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In page Booster dose:

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In the case of the polio vaccine, the memory B and T cells produced in response to the vaccine persist only six months after consumption of the oral polio vaccine (OPV). Booster doses of the OPV were found ineffective, as they, too, resulted in decreased immune response every 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%.[1] Often in developing countries, OPV is used over IPV, because IPV is expensive and hard to transport. However, in places where polio is still present, following up an OPV primary dose with an IPV booster may help eradicate the disease.[2]