• Various studies, summarized here, have established beyond doubt that immunization of humans by the oral route of administration not only is possible but has been successfully accomplished. Since attenuated strains of poliovirus were found to vary greatly in the extent of their residual neurotropism for the most sensitive lower motor neurons as well as in the homogeneity of their populations, the crucial problem was to find strains that were so highly attenuated and homogeneous that one would be justifed in using them in increasingly larger numbers of humans in those stepwise tests that must precede any trial of such a vaccine on a large scale. The finding of such strains after tests on the progeny of large numbers of individual virus particles is here described.
Sabin AB. PRESENT STATUS OF ATTENUATED LIVE-VIRUS POLIOMYELITIS VACCINE. JAMA. 1956;162(18):1589-1596. doi:10.1001/jama.1956.02970350005002