A study of the immunogenicity of smallpox vaccines.

The authors report on a series of tests to determine the immunogenic properties of smallpox vaccines of varying degrees of infectivity. Rabbits were immunized with serial dilutions of the vaccines and challenged twenty days later with intradermal neurovaccine or testicular vaccine, with injection of testicular vaccine into the testes, or with intracerebral neurovaccine. It was possible to demonstrate a correlation between the infectivity of the immunizing vaccine and the degree of immunity conferred, as shown by the serum antibody level and by the reaction to the challenge inoculation. It also appears that the immunogenicity of the vaccines is determined not only by the number of active virus particles they contain but also by their virulence, the more virulent giving greater immunity in smaller doses. Unduly virulent vaccines, however, may give rise in humans to severe and painful reactions.