Polio is a disease caused by the polio virus, and usually spread directly from the stool of an infected person to the mouth of another person, probably from contaminated hands.
In the majority of cases, the virus produces a mild and harmless infection. However, it may cause meningitis, paralytic illness or encephalitis. The paralysis occurs when the virus invades nerve cells of the brainstem and spinal cord, a condition that may lead to permanent muscle weakness and disability.
Wild polio virus occurs naturally and causes paralytic polio, and the infection gives lifelong immunity, but only to the particular subtype involved (either types 1, 2 or 3). In rare cases, the oral polio vaccine (OPV) can cause a polio type associated with vaccines, called Vaccine-induced paralytic polio where the live attenuated vaccine virus in OPV may cause paralysis in the vaccinated child, or in a close contact, especially if one suffers from an immune weakening condition like HIV and Aids.
Alternatively, in Vaccine-derived polioviruses, a strain of poliovirus in OPV may genetically change so that it can both cause paralysis and circulate among a population.
OPV is also called the Sabin vaccine, and comprises of live attenuated strains of poliovirus 1-3. It is cheap; orally administered and promotes antibody formation in the gut; this protects more effectively against wild polio infection and reduces transmission of the wild virus.
It also provides community benefit because the vaccine virus is excreted; therefore the contacts of recently immunised children may, in effect, get a second-hand dose of the vaccine. It is therefore used to contain outbreaks, and for eradication where polio is endemic, like in Sub Saharan Africa.
Inactivated polio vaccine (IPV) also known as the Salk vaccine, contains inactivated strains of polioviruses 1-3. It does not have the risk of causing vaccine-related polio. The disadvantage is that it does not stimulate antibody in the gut, so less effective against wild poliovirus. It protects only the immunised person; no community benefits. In Kenya children receive the polio vaccine at birth, then at six, ten and 14 weeks of age.