The scar on the left arm is usually a scar from the BCG vaccination, also known as a “card scar”. BCG is a vaccine against tuberculosis. The National Immunization Program and Instructions for Children (2021 Edition) issued by China’s Health Commission states that a single dose of BCG vaccine should be administered at birth, with the site of vaccination being the upper arm. About two weeks after BCG vaccination, redness, swelling and pus may appear at the vaccination site, and then slowly absorb, scab and heal. It is not mandatory for BCG vaccination to be given in the left or right arm, but because both Hepatitis B and BCG vaccines are required at birth, BCG is given in the left upper arm and Hepatitis B is given in the right upper arm in order to observe the reaction to the vaccine. Not everyone will have “stuck scars”, therefore, “stuck scars” should not be regarded as evidence of BCG vaccination or the success of vaccination, but should be judged in conjunction with the history of vaccination or the results of the tuberculin test, and consult with the immunization physician for specific guidance.