Successful immunization against specific infections by oral administration of killed micro-organisms or microbic products would be a major therapeutic advance, since it might free the public mind from its main objections to vaccine therapy. Thus far, laboratory studies have not been encouraging. Apparently most of the suggested oral vaccines are so rapidly destroyed or denatured in the gastro-intestinal tract as to lose completely their antigenicity. Perhaps they take on new antigenicities not specifically protective. Oral immunization, therefore, will presumably be limited to the relatively few vaccines sufficiently resistant to gastro-intestinal denaturization to be absorbed practically unchanged in effective immunizing doses. The most resistant group of micro-organisms thus far studied are apparently the pneumococci. Recently, successful oral vaccines against pneumococci are reported to have been developed by Dr. Victor Ross of the Board of Health Laboratories in New York.
In earlier researches, Ross demonstrated that rats fed with tissues of animals
ORAL IMMUNIZATION AGAINST PNEUMOCOCCI. JAMA. 1930;95(14):1024-1025. doi:10.1001/jama.1930.02720140046014