BCG vaccine bacilli that seemed less virulent, and changed the course of their research to see if repeated subculturing would produce a strain that was attenuated enough to be considered for use as a vaccine. The BCG strain was isolated after subculturing 239 times during 13 years from virulent strain on glycerine potato medium. The research continued throughout World War I until 1919, when the now avirulent bacilli were unable to cause tuberculosis disease in research animals. Calmette and Guerin transferred to the Paris Pasteur Institute in 1919. The BCG vaccine was first used in humans in 1921. Public acceptance was slow,
BCG vaccine potentially more severe in those with poor immune function. It is not safe for use during pregnancy. The vaccine was originally developed from "Mycobacterium bovis" which is commonly found in cows. While it has been weakened, it is still live. The BCG vaccine was first used medically in 1921. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. Between 2011 and 2014 the wholesale price was $0.16 to $1.11 USD a dose in the developing world. In the United States it costs $100 to $200 USD. As