EXPERIMENTAL studies of vaccination, allergy, and immunity in tuberculosis. II. Effect of varying the dose of BCG.

Results are given for one of a series of projects designed to investigate the relation between observable post-vaccination responses and acquired resistance to tuberculosis. Controlled variations in the dose of BCG vaccine have previously been shown to cause systematic variations in the degree of skin sensitivity to tuberculin and the size of vaccinal lesions in human beings: the purpose of the present project was to see if similar variations would be produced in guinea-pigs and then, by infecting the animals with virulent tubercle bacilli, to see how survival time correlates with tuberculin allergy and vaccinal lesions.Four doses of freshly prepared BCG vaccine, ranging from 1/100 to 10 times the dose ordinarily used for intradermal vaccination of humans, and one dose of heat-killed BCG 100 times that strength, were used to vaccinate five groups of guinea-pigs, each containing 120 animals. A sixth group of 120 animals was not vaccinated. All animals were tuberculin-tested just before and five weeks after vaccination, challenged with a strong dose of H-37 Rv, and then allowed to die, so that survival time could be used as a measure of resistance.As the dose of living BCG was increased, groups of guinea-pigs showed a progressive increase in the average degree of post-vaccination tuberculin allergy, size of vaccinal lesion, and length of survival after virulent infection. The heat-killed BCG resulted in weak allergy and a short survival time, yet the vaccinal lesions averaged about as large as would be expected from a corresponding dose of living BCG. These results (excluding studies of survival time) correspond closely to those found in human studies.The implications of the results with respect to practical BCG vaccination programmes, while no more than speculative at present, point toward possible advantages in inducing high degrees of tuberculin allergy and toward the dubious significance of the vaccinal lesion as an index of a vaccine's immunizing potency.