Tuberculosis is a chronic infectious disease caused by Mycobacterium bovis that can affect many organs and systems throughout the body, with the most common part affected being the lungs, hence the name “tuberculosis. Mycobacterium tuberculosis can enter the body not only through the respiratory tract, but also through the digestive tract, broken skin mucous membranes, etc. Therefore, tuberculosis technically includes intestinal tuberculosis, bone tuberculosis, tuberculous meningitis, etc., of which pulmonary tuberculosis is one, but it accounts for 80-90% of the total number of tuberculosis in all organs. Intradermal BCG (BCG for short) is the longest used and the only vaccine against TB in the world, also called TB vaccine, which is a live vaccine that is not pathogenic but immunogenic to humans. According to China’s immunization planning procedures, newborns should receive BCG vaccine promptly within 24 hours of birth if there are no special circumstances. Why? Because TB is still one of the most dangerous infectious diseases in China, and WHO has assessed that China ranks second in the world after India in terms of the number of TB cases. BCG vaccine is to prevent tuberculosis. So, after BCG vaccination, you will be 100% free from TB? First, BCG vaccination can stop the spread of Mycobacterium tuberculosis in the bloodstream, but it cannot prevent the occurrence of infection, and BCG vaccination does not necessarily mean that you will not get tuberculosis. Second, the durability of BCG vaccine itself is difficult to determine. BCG vaccination has been effective in reducing morbidity and mortality from TB meningitis and disseminated TB disease, but not everyone who is vaccinated will successfully develop antibodies, and a very small number of people will not be immunized (this is also the case with other vaccines such as the hepatitis B vaccine). On the other hand, even if vaccination is successful, immunity declines over time and there is still a risk of infection with Mycobacterium tuberculosis. The effectiveness of BCG vaccination of newborns has been observed for more than 20 years and found that the protection rate was 82% in children <15 years old; 67% in 15-24 years old; and 20% in 25-34 years old. And therefore, it is speculated that BCG vaccine may not have protective power after 10-20 years of vaccination. Thirdly, the effect of BCG vaccination is different in different regions. The difference is mainly due to the influence of environmental mycobacteria, and researchers have considered a number of reasons, one of which is that the pre-existing immunity against environmental mycobacteria can interfere with the viability of BCG vaccine in humans, thus weakening the immune response to the vaccine. Therefore, BCG vaccination is not a permanent solution and it is indispensable to avoid contact with infectious TB patients and to take the right preventive measures.