It wasn't a linear path but after a lot of trial and error, and some fitful starts (including deaths) microbiologists Albert Calmette and Camille Guérin introduced the Bacillus Calmette–Guérin (BCG) vaccine and it has been administered routinely to protect babies against tuberculosis since 1921.
Today only a few countries, where TB is rare such as the United States and the Holland don't use it. So perhaps it is fitting that Dutch health care workers have volunteered to be the first 1,000 in a clinical trial to see if it might help with coronavirus. It's not because TB has much in common with COVID-19 but rather that the vaccine may help because an observational study - so be skeptical accordingly as you should any epidemiological claim - concluded that it helped prevent 30 percent of infections from any pathogen. Basically, by giving infants the shot they were preventing lots of other diseases besides TB.
Though lots and lots of claims are coming out, many of them are based on similar epidemiological correlation and have not been validated scientifically so randomized clinical trials are the way to go. They take far more time, but give far more confidence in the results than throwing compounds at people and hoping they work.