Building immunity against virus is a very complex biological process. We are all different; as are organisms' reactions to medications and vaccines. Few children may acquire strong immunity after just five or six doses of the polio vaccine, while the most vulnerable children need more than ten. The immune system of underweight, malnourished and children suffering from diarrhoea responds to the polio vaccine in a different way than of healthy children. Therefore, to remain protected, all children under five years of age should receive polio vaccine every immunization round.