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,
Inoculation Needham states that this information comes from the "Zhongdou xinfa" (種痘心法) written in 1808 by Zhu Yiliang, centuries after the alleged events. The first clear and credible reference to smallpox inoculation in China comes from Wan Quan's (1499–1582) "Douzhen xinfa" (痘疹心法) of 1549, which states that some women unexpectedly menstruate during the procedure, yet his text did not give details on techniques of inoculation. Inoculation was first vividly described by Yu Chang in his book "Yuyi cao" (寓意草), or "Notes on My Judgment", published in 1643. Inoculation was reportedly not widely practised in China until the reign of the Longqing