OPV (Oral Polio Vaccine) is a vaccine taken by mouth,which colonizes the gut and induces immunity.
It is important to give OPV to children under five years of age even though they are vaccinated'because one can still harbor the disease causing virus in his intestine. This makes the chance for it, getting excreted in stool and also increases the risk for polio to get transmitted in the unvaccinated children of the community.
This virus cannot survive for a long time in the atmosphere when the oral polio vaccine is given as it does not allow virus to live in intestine and expel them out. And it has been observed that this virus only lives in the intestine of babies below five years of age, thus it is necessary to give OPV to children below five years to eradicate the disease totally.