Experimental studies of vaccination, allergy, and immunity in tuberculosis: 3. Effect of killed BCG vaccine.

The antigenic effects of living and killed BCG vaccine have been investigated by comparing the degree of tuberculin allergy, the size of vaccinal lesions, and the survival time after challenge infection in large numbers of guinea-pigs vaccinated with BCG suspensions containing various concentrations of living and killed organisms.The allergizing as well as the immunizing potency of BCG was markedly reduced-though not entirely abolished-when the bacilli were killed, yet the size of the vaccinal lesion was only slightly reduced. Results were about the same regardless of whether heat, light, or phenol had been used to kill the organisms. It would thus appear that viability of the organisms in BCG vaccine is important for immunity as well as for allergy, whereas the size of the vaccinal lesion is determined largely by the total mass of bacterial cells injected.The results of this experiment in laboratory animals are in agreement with results of studies in human beings, so far as allergy and lesions are concerned. Persons vaccinated with heat-killed or irradiated BCG have been shown to have very low degrees of post-vaccination tuberculin allergy compared with persons given a corresponding dose of living BCG, yet the size of the vaccinal lesion differs only slightly. The combination of a large vaccinal lesion and a low degree of post-vaccination allergy should therefore invite suspicion that a vaccine contained mainly killed bacilli and, perhaps, the presumption that it has produced only a low degree of immunity.