The difference between the cowpox vaccine and the BCG vaccine is that they prevent completely different types of diseases. The cowpox vaccine is used to prevent smallpox; the BCG vaccine is used to prevent Mycobacterium tuberculosis infection. BCG and cowpox vaccines are two different vaccines. Cowpox vaccine is used to prevent the spread of smallpox, which is an acute infection caused by the smallpox virus in cattle, and with the help of cowpox vaccine inoculated on the human body, the human body can produce resistance to the smallpox virus. BCG vaccine is a live bacterial vaccine made from attenuated bovine tuberculosis mycobacteria suspension, and its main function is to activate T-lymphocytes, enhance the function of cellular immunity of the body, and it is mainly used to prevent tuberculosis mycobacteria infection. Therefore, BCG vaccine and cowpox vaccine are two completely different vaccines, and the diseases they prevent belong to two completely different diseases. At present, BCG vaccine is still being promoted, while the World Health Organization has declared smallpox to be extinct worldwide, so we no longer need to popularize cowpox vaccination. However, there are similarities between BCG and cowpox vaccination, that is, after vaccination, both vaccines will cause ulceration of the vaccination site on the left upper arm, resulting in scarring.