True about polioviruses is -

91-96% cases of polio virus infections results in inapparent or subclinical infection .There are no presenting complaints .Recognition only by virus isolation or rising antibody titre. In case of paralytic polio the predominant sign is asymmetrical flaccid paralysis. The primary course of immunisation consists of 4 inoculations. First 3 doses are given at interval of 1to 2 months and 4th dose 6 to 12 months after 3rd dose. IPV first dose is given when 6 weeks old.Additional doses are recommended prior to school entry and then every 5 years until 18 years Reference : Park's Textbook of preventive and social medicine, 24th edition.Pg no. 221