Tuberculosis (TB) is a major cause of death worldwide, and the number one cause of death in persons with HIV infection. Drug-resistant TB, an increasingly global epidemic, requires prolonged treatment with toxic, expensive drugs. BCG, the only vaccine currently in use for prevention of TB, is very effective for the first few years of life, but loses efficacy after 10-15 years.

Booster immunization of adolescents or adults with a repeat dose of live BCG does not increase protection (presumably because the immunity induced by a priming dose of BCG reduces replication of a booster dose).