The Heterologous Effects of Bacillus Calmette-Guérin (BCG) Vaccine and Trained Innate Immunity

Bacillus Calmette-Guerin (BCG) is a live-attenuated strain of Mycobacterium bovis that protects against mycobacterial diseases, including tuberculosis (TB) and leprosy. BCG has more than 80% protective efficacy against the severe forms of TB (such as meningitis and disseminated disease) that affect infants and children, but the vaccine has limited efficacy, ranging from 0% to 80% against pulmonary TB in adults. The World Health Organization (WHO) recommends BCG vaccination at birth for all infants in TB-endemic regions, with the aim of reducing TB deaths by 95% by 2035. Given to more than 120 million infants globally each year, BCG is one of the most widely used vaccines. Despite the use of BCG for almost a century, TB continues to cause more than 1 million deaths each year, and it is estimated that one-third of the population is infected with the causative pathogen Mycobacterium tuberculosis.