Can colony-forming unit testing be used to extend the shelf life of BCG vaccines?

A recent shortage in supply of Bacille Calmette-Guérin (BCG), the live attenuated vaccine given to protect against tuberculosis (TB) caused major disruption to global vaccination programs. In this study, we assessed whether quantification of viable bacteria, could be used to inform the use of the BCG vaccine beyond its manufacturer-assigned expiration date. The viability of a single batch of BCG-Denmark was tested in three independent laboratories. There was high inter-vial and inter-laboratory variability in viability counts, however all three laboratories detected a decrease in BCG viability over time. Despite this, there was no difference in the rate of BCG scar formation in infants who were vaccinated with this batch of BCG before and after its manufacturer-assigned expiration date. This study demonstrates the potential for using BCG viability counts to guide the use of BCG vaccine beyond the manufacturer-assigned expiration date.