Immunity to vaccinia induced by small doses of active virus

Abstract

Minimal quantities of active vaccinia virus in the form of a purified elementary body suspension were injected subcutaneously into rabbits. The amount administered was about 10 minimal infective doses as measured by intracutaneous titration and tissue culture assay: this is equivalent to about 1·3± 10−⁵μg of virus protein. The immune response to this minimal quantity of virus was determined by complement-fixation and virus neutralization tests and also by skin sensitivity tests. The results indicated that viral proliferation occurred to an extent sufficient to sensitize the tissues to the virus protein. Measurable amounts of antibody could also be detected in about one half of the experimental animals. A second inoculation of virus produced a considerable augmentation of the immune response. Viral proliferation must have taken place in order to evoke this immunity, but infection always remained at the subclinical level. The results were much improved when the infecting does of virus was incorporated in an adjuvant. DPT-polio vaccine functioned well as adjuvant but equally good results were obtained with a solution of polyvinylpyrrolidone. The latter substance also has valuable properties as a protective colloid and is thus able to maintain the viability of the virus over long periods of time.On the evidence provided by these experiments it is suggested that an adequate immunity to smallpox might be achieved in man by similar methods. A primary sensitization could be achieved by incorporating a very small amount of active virus in one of the prophylactics given as a routine in the early months of life. This could be followed by normal jennerian vaccination a few months later; or the second inoculation of virus could also be incorporated in amultivalent prophylactic. For adults, a somewhat similar two-stage immunization would be required.It is hoped that the publication of these preliminary experiments will encourage other investigators to continue what promises to be an important line of research.All assays involving the use of monkey kidney tissue cultures were carried out by my former assistant Mr Edmund Bischoff. His help is gratefully acknowledged.