Tuberculosis (TB) remains a significant global health challenge, with approximately 1.2 million deaths reported annually. Despite widespread public health efforts, TB continues to be a major cause of morbidity and mortality, especially in low- and middle-income countries. TB is caused by Mycobacterium tuberculosis and primarily transmitted through aerosolized droplets, leading to pulmonary and, in some cases, extrapulmonary disease. The pathogen's ability to evade immune responses, persist in latent forms, and reactivate under conditions of immune suppression underpins its pathogenesis.

The Bacillus Calmette-Guérin (BCG) vaccine, derived from an attenuated strain of Mycobacterium bovis, is the only licensed vaccine for TB and has been in use for over a century. Its mechanism of action involves the induction of cell-mediated immunity, particularly through activation of CD4+ and CD8+ T cells, as well as innate immune memory.

Multiple strains of the BCG vaccine have evolved due to genetic variations introduced during manufacturing processes. These strains differ in immunogenicity and efficacy. While traditionally administered via the intradermal route, alternative routes such as intranasal, subcutaneous, and intravenous administration are under investigation to enhance efficacy, particularly against pulmonary TB. Recent advances in TB research focus on the development of improved vaccines.