To the Editor. —Dr Colditz and colleagues1have reviewed and digested an enormous amount of data in producing their meta-analysis of the efficacy of BCG vaccine in the prevention of TB. However, there are major considerations that cast doubt on their conclusions. Among these considerations is the implication in the title and the text that there is only one BCG vaccine. Numerous authors have called attention to the fact that the BCG vaccines produced in different laboratories vary in almost every measurable characteristic.2,3Variations might be expected to occur in a vaccine that had to be subcultured as often as every 2 weeks from its release in 1922 until freeze-dried strains were developed in the mid 1960s.4Strains have been found to differ in their survival on culture media, colony formation, viability, drug resistance, DNA restriction fragments, virulence in animals, anti-TB and antitumor protection in animals, production