Quantitative evaluation of the effectiveness of Connaught freeze-dried BCG vaccine in mice and in guineapigs.

Although the field trials carried out by the Medical Research Council of Great Britain demonstrated that BCG vaccination can confer a substantial degree of immunity against tuberculous infection, it does not follow that BCG substrains other than the one used for those trials will produce equally favourable results. In fact, there is increasing evidence that different BCG strains may differ widely in their protective potency. The experiments described here further confirm these differences. They also show how the determination of the minimum dose of a BCG vaccine capable of delaying the development of tuberculous infection in mice and in guinea-pigs can yield reproducible data that may help to characterize individual BCG strains.The main purpose of these experiments was to determine the protective potency of Connaught freeze-dried BCG vaccine, lot 140, and to compare it with that of three other BCG vaccines. Marked differences were found with respect to the minimum protective dose for mice or guinea-pigs and the degree of immunity and tuberculin allergy produced in guinea-pigs as shown by the dose-response relationships recorded over a wide dosage range. The results suggest that the Connaught vaccine equals or surpasses the other vaccines in effectiveness.Such tests require a reference BCG vaccine of high protective potency for both animals and man.