LONDON - The Bacillus-Calmette-Guerin vaccine against tuberculosis, developed in the early 1900s, seems to train the immune system to respond to infectious diseases, including viruses, bacteria and parasites. As new threats like monkeypox and polio re-emerge and the coronavirus continues to evolve, it has gained renewed interest among scientists.

The results of clinical trials on the vaccine conducted during the pandemic are coming in, and the findings, while mixed, are encouraging. One such trial of 144 participants found that people with Type 1 diabetes who had received several B.C.G. injections were far less likely to develop Covid-19 compared with those who had received dummy shots.

Although the trial was relatively small, “the results are as dramatic as for the Moderna and Pfizer mRNA vaccines,” said Dr. Denise Faustman, the study’s lead author. Participants generally experienced fewer bouts of illness, she added. The vaccine “seems to be resetting the immune response of the host to be more alert, to be more primed, not as sluggish.”

Caution: Other trials have had more disappointing results. A Dutch study of 1,500 health care workers who were vaccinated with B.C.G. found no reduction in Covid infections, and a South African study of 1,000 health care workers found no effect from B.C.G. on Covid incidence or severity.