Are BCG-induced non-specific effects adequate to provide protection against COVID-19?

The world is experiencing a pandemic of Coronavirus Disease (COVID-19) caused by type-2 Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). Vaccination is the only option to prevent future surges of the disease. Efforts for developing an effective vaccine are underway, but the timeline for the widespread availability of safe and effective vaccines is unknown. Some ecological reports have linked regional universal use of the Bacillus Calmette-Guerin (BCG) vaccine with reduced morbidity and mortality of COVID-19. BCG protects from non-tuberculous diseases through 'non-specific' effects mediated by the modulation of innate immunity. This commentary provides details of the immunological mechanism of BCG-induced 'trained innate immunity' responsible for its nonspecific protective effects. A probable role of the BCG vaccine in the current pandemic is also examined.