The virulence of some strains of BCG for golden hamsters. Further studies

In principle, a strain of BCG used for the preparation of live vaccine should retain a moderate residual virulence. The inoculation of golden hamsters with large doses of BCG causes progressive, fatal disease but not all strains are equally active. In a previous paper, the present authors gave data from experiments with 9 BCG strains; in this report, 4 additional strains, three of which are in routine use in vaccine-production laboratories, are compared with one of the strains used in the previous experiments. The five strains differ widely in their ability to kill hamsters and were ranked in nearly the same order in two identically designed experiments. The differences in virulence found between strains derived relatively recently from a single mother strain were particularly noticeable. These differences were sometimes accompanied by striking changes in the growth characteristics of the strains with a lower virulence. The hypothesis that the lower virulence in such cases is a sign of genetic mutation is consistent with the general biological experience that virulence is often lost in vitro but that it practically never increases.