Bacille Calmette-Guérin (BCG) was introduced in 1921 after a virulent strain of M. bovis was apparently attenuated and had lost its virulence over 230 passages on potato bile media from 1908 onwards.

BCG vaccination must be given before acquisition of infection with M. tuberculosis to reduce the risk of progression from latent infection to overt clinical tuberculosis, should infection be acquired subsequent to vaccination.

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