Does BCG vaccination protect against COVID-19? 'Trained immunity' against viral infections

A number of clinical trials are currently underway worldwide to see whether BCG, the long-standing vaccine against tuberculosis, can protect against covid-19. In this article we briefly outline the background, the immunological mechanisms (in particular induction of 'innate immune memory' or 'trained immunity'), and the further perspectives for the use of BCG against viral and other infections.