Most Cases (90% to 98%) of illness associated with poliovirus remain either inapparent or characterized by sore throat, headache, nausea, and abdominal pain. It is usually diagnosed as a cold or flu. Only 1% of infections result in paralytic disease. Only a small percentage of these cases have residual paralysis.
The virus is spread from stool to mouth (fecal-oral), but may also be spread by pharyngeal route (nose and mouth).
Oral, live-virus vaccine (OPV) can cause polio in vaccine recipients and contacts. It has caused Guillain-Barre syndrome (GBS) in recipients. It may also contain live monkey viruses that have been associated with human diseases. The oral vaccine is no longer recommended in the U.S.
Killed polio vaccine (IPV) has also caused significant effects, including cancers from a contaminated vaccine, but is not known to cause polio in recipients.
The Vaccine Guide: Risks and Benefits for Children and Adults, Randall Neustaedter, OMD
In regards to the mortality rate with polio, it had been decreasing since the 1920s, long before the introduction of the killed polio vaccine was introduced in 1955, followed by the live polio vaccine introduced in 1959.