In the early days of the COVID-19 pandemic, when prevention seemed light years away, several scientists launched trials to see whether a tuberculosis vaccine developed in the early 1900s might protect people by bolstering the immune system.
The Bacillus-Calmette-Guerin vaccine has long been known to have broad effects on the immune system, and is still given to infants in the developing world and in countries where TB is prevalent.
Scientists observed many years ago that the vaccine seems to train the immune system to respond to a variety of infectious diseases, including viruses, bacteria and parasites, and reduces infant mortality.