“This is the only vaccine in the world that can be given to combat covid-19 right now,” said Jeffrey D. Cirillo, a professor of microbial pathogenesis and immunology at Texas A&M Health Science Center, who is leading a trial of the tuberculosis vaccine, called bacillus Calmette-Guérin and known by the shorthand BCG.

At more than 25 universities and clinical centers around the world, researchers have begun clinical trials, primarily in health care workers, to test whether a live tuberculosis vaccine that has been in use for 99 years called the bacillus Calmette-Guérin, or B.C.G., vaccine, could reduce the risks associated with the coronavirus.

Bacillus Calmette-Guérin tuberculosis vaccine that induces a broad innate immune-system response, which has been shown to protect against infection or severe illness with other respiratory pathogens.

The Bacillus Calmette-Guerin vaccine is still widely used in the developing world, where scientists have found that it does more than prevent TB.

To make things worse, currently there is a worldwide shortage of Bacillus Calmette-Guerin, a bacterium used to make the preferred immunotherapy for decreasing bladder cancer recurrence after surgery.