Crucial requirement for standardization during the development of novel recombinant BCG vaccines: Does the corresponding substrain background matter?

The Bacillus Calmette-Guerin (BCG) vaccine is not a single organism, but consists of substrains that vary in genotypes and phenotypes. Actually, BCG is the common name given to a family of vaccines created in 1921 by the in vitro attenuation of a virulent Mycobacterium bovis in France. Even nearly a century of use, the BCG vaccine lingers generating confusion and debate due to its diversity and failure to protect against tuberculosis (TB). That is probably owing to the enduring lack of standardization during production, distribution and administration procedures. Since the 1940s, substantial sequence modifications among the BCG substrains have been described. To increase the level of complexity, even though that the prolific generation of recombinant BCG vaccines has been promising, the relationships between those candidates used in current clinical trials and their parental substrains are either unsatisfactorily connected or have been never fully delineated. Consequently, the use of the most protective BCG substrain as the background or platform in the development of all recombinant BCG vaccine candidates has not been standardized. In order to schematize and to clarify the subject regarding substrains commonly used to generate those novel vaccines, a sequential emergence of the parental BCG vaccine substrains and their matching recombinant ones, if any, was built. Hence, for a total of 24 BCG substrains currently in circulation worldwide, 9 have been used to sustain one or more genetic modifications, resulting in around 21 novel recombinant BCG vaccines. Although this is a remarkable success, only 2 out of the 21 recombinant BCG substrains harbor a background representative of the most immunogenic group. Systematizing the novel BCG vaccines and their parental strains may facilitate our understanding of protection provided by BCG immunizations.