How does polio vaccine taken orally by the digestive system
Two types of polio vaccines are currently used to fight polio. The first devoloped and tested in 1952 consists of injected dose of inactivated (dead) poliovirus. The second one, an oral vaccine was devoloped in 1957[1]. Oral polio vaccine (OPV) is a live-attenuated vaccine, produced by the passage of the virus through non-human cells at a sub-physiological temperature, which produces spontaneous mutations in the viral genome. The attenuated polio virus given orally is an enterovirus. This means the entry of the virus into the body is through the gastrointestinal system.
         Viruses are of different types. There are some viruses such as influenza and HIV viruses which are enveloped in a lipid layer which can be destroyed in the digestive system by the the lipolytic agents.
         However there are other types of viruses which are not enveloped in such lipid layers, known as naked viruses[2]. Polio virus is an example of a naked virus, which means it cannot be destroyed by digestive enzymes present in the digestive tract[3].
        As a result the orally adminstered attenuated strain of polio virus is not destroyed by the digestive system. This attenuated or weakened polio virus induces an immune response, thereby producing antibodies in the intestine and preventing the disease in the future[4].


[1] http://en.wikipedia.org/wiki/Polio_vaccine
[2] http://www.biology-online.org/dictionary/Naked_virus
[3] http://www.hindu.com/seta/2006/05/25/stories/2006052500751500.htm
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4] http://www.chop.edu/service/vaccine-education-center/a-look-at-each-vaccine/polio-vaccine.html