The effect of BCG vaccine in the era of COVID-19 pandemic.

Coronavirus Disease-2019 (COVID-19) has become a worldwide emergency and announced as a pandemic recently with variable incidence and mortality in different geographical areas. The relationship between COVID-19 and Bacille Calmette Guerin (BCG) vaccine is still under investigation, and we aim to clarify this matter. BCG is a live attenuated tuberculosis vaccine. BCG vaccination of adult humans induces strained circulating monocytes having an increase in their capacity to produce proinflammatory cytokines. This effect translates to non-specific protection against unrelated pathogens, an example of which is S. aureus and C. Albicans. Also, vaccination of BCG of healthy volunteers showed an increase in the capacity of natural killer (NK) cells to secrete proinflammatory cytokines, such as IL-1s and IL-6 after stimulation with M. tuberculosis or unrelated pathogens (S. aureus, C. Albicans).