ABSTRACT
Currently, Bacille Calmette-Guerin(BCG) is still the only admitted vaccine to prevent tuberculosis around the world. The target population is infants and children, but its protective efficacy is limited. As more and more studies have shown that re-vaccination with BCG protects against tuberculosis in adults, BCG can also induce non-specific immunity against other respiratory diseases and some chronic diseases by training immunity, especially the immune effects against COVID-19. At present, the epidemic of COVID-19 has not been effectively contained, and it is worth considering whether BCG vaccine can be used as an intervention to prevent COVID-19. The WHO and China do not have a policy to support BCG revaccination, and as more and more BCG vaccines are discovered, whether selective revaccination can be carried out in some high-risk populations and whether the vaccine can be used more widely have led to intense discussions. This article reviewed the effects of specific immunity and non-specific immunity of BCG on tuberculosis and non-tuberculous diseases.