In assessing the state of knowledge of any subject at a particular point in time, it is as important to define what is not known as it is to appreciate what is known. It is for this reason that I would like to discuss some of the problems still before us, with respect to vaccination against poliomyelitis, as well as to review knowledge gained in the course of recent work. "Can durable immunity be induced by a noninfectious poliomyelitis vaccine?" is one of the principal questions still to be answered. Since the ultimate answer to this question cannot be obtained until time has passed, we must content ourselves only with an examination of facts now known, from which we can make some tentative interpretations from the trends observed.
The hypothesis that the presence of neutralizing antibody in the circulating blood is an effective barrier in reducing the likelihood of paralysis
Salk JE. Considerations in the Preparation and Use of Poliomyelitis Virus Vaccine. JAMA. 1984;251(20):2700–2709. doi:10.1001/jama.1984.03340440058031