Is the BCG vaccine a useful tool against COVID-19?

The Bacillus Calmette–Guérin (BCG) vaccine has been used since 1921 initially for protection against tuberculosis. BCG acts through stimulation of cell-mediated adaptive immunity with activation of the Th1 cells and production of interferon gamma. Additionally, it is able to stimulate the immune system in a nonspecific manner, which results in effectiveness of the BCG against non-mycobacterial infections and in some malignant, autoimmune, and inflammatory diseases. Recently, its potential use in the fight against the coronavirus disease 2019 (COVID-19) pandemic has been suggested. This is based upon the concept of BCG-induced trained innate immunity—a memory-like response of the innate immune system that can realize greater protection in case of re-infection. This hypothesis represents a milestone in the potential use of the BCG vaccine in the fight with the novel coronavirus.