Nowadays, almost everyone has a visible scar on their arm, which is not something we are born with, but is present on most people’s arms. Some look like a flower, while others have a small raised bumps. This is the scar that remains after the BCG vaccination, which we call the “card scar”. What is BCG vaccination? Why does it leave a scar after vaccination? I. What is BCG vaccine? BCG is a vaccination used to prevent tuberculosis in children. The vaccination gives children a special resistance to tuberculosis. Since the vaccine was invented by two French scholars, Kamel and Gérin, the vaccine was named “BCG” in honor of the inventors. Currently, BCG vaccine has been included as one of the vaccines required for immunization in most countries around the world. BCG vaccination is mainly given to newborn infants and children, and it can prevent the occurrence of childhood tuberculosis, especially serious types of tuberculosis, such as tuberculous meningitis. Why does BCG vaccination leave scars after vaccination? The local reaction after BCG vaccination is different from that of other vaccines. Usually, about 3 weeks after vaccination, redness and swelling will appear at the vaccination site, which will gradually soften in the middle and form small white pustules, and after the pustules break down, the pus will be discharged, and it will take 1-2 weeks for the scab to form, and a round scar will remain after healing. The above process usually lasts for about 2 months. The swollen lymph nodes near the vaccination site (mostly the axillary lymph nodes) are often caused after BCG vaccination, and the swollen lymph nodes will subside on their own as the vaccination site heals. This is a normal reaction after BCG vaccination and is a sign of effective vaccination. If this reaction does not occur, it indicates that the vaccination has failed, and a supplemental vaccination should be considered. Many children will develop a fever after BCG vaccination, and the area where the injection was given will be red, swollen, and ulcerated, after which it will slowly crust over and leave a round scar. These symptoms are normal after vaccination, because this kind of vaccine with immunity usually contains a small amount of germs, and the main principle is to make a small infection appear in the place where we injected, so that it can stimulate the body’s own immune function, and then it will not be infected with this disease again. The scars after the vaccination may be larger or smaller for some people depending on their body type, and may be darker for those who have scars, but they will generally fade with age.