The beginning of the end for polio arrived in 1955 when the Salk vaccine was released for general use. The Salk vaccine is a concoction comprised of killed polio virus yet it still retains the capacity to stimulate the immune system to produce humoral immunity to live polio virus.
Humoral immunity means that the immunity is restricted to the blood.
A person with humoral immunity is protected from live polio virus migrating from the intestine, where it begins multiplying, to the Central Nervous System, where it does lasting damage. The Salk vaccine is produced by subjecting live virus to formaldehyde until it is damaged enough to prevent it from causing a polio infection but not damaged enough to prevent it from stimulating an antibody response in the blood. It is delivered by injection intramuscularly. During those early years of Salk, from 1955-1961, there were many cases of polio, a significant number of which occurred in individuals who had already been vaccinated with the Salk vaccine. The only other way to produce immunity in 1955 was to become infected with polio and recover from it.