• Immunity to poliomyelitis comes about naturally and early in life in some unvaccinated populations. In others, high susceptibility prevails in all age groups so that severe epidemics involve children, parents, and grandparents. In the United States differences exist not only between states but even within cities, and in general the people in the poorest urban areas become immune early in life. It is a question whether vaccination with the killed antigen will prepare young people, for instance, to resist the risk young parents face when exposed to the infection by their own children. If it turns out that natural infection helps to keep up the individual's immunity, it may be necessary to have the feeding of live virus regularly follow the inoculation of killed vaccine. The invaluable start that has been made should not be regarded as the final technique of immunization against poliomyelitis.
Paul JR. INDICATIONS FOR VACCINATION AGAINST POLIOMYELITIS. JAMA. 1956;162(18):1585-1588. doi:10.1001/jama.1956.02970350001001