Scars on the arm may be the result of smallpox vaccination or BCG vaccination. Smallpox vaccine is mainly used to prevent a virulent infectious disease caused by smallpox virus, which is a kind of DNA virus with strong reproduction ability and can be spread throughout the body through blood circulation to cause viremia. Smallpox vaccine is mainly made by processing cowpox virus, which can stimulate the body’s immune response, resulting in localized skin infections, and more obvious scars will appear after healing. BCG vaccine is mainly used to prevent tuberculosis caused by Mycobacterium tuberculosis infection, is a kind of live attenuated tuberculosis vaccine, usually in infants within 24 hours of birth to start the first injection, the skin at the needle hole is prone to vaccine reaction, the vaccination site will be infected with a small pustule, after a period of time after the healing of the formation of round scars. If you experience prolonged discomfort after vaccination, it is recommended that you go to a specialized hospital to determine the cause of the disease and then under the guidance of a doctor, you can undergo the next step of examination and treatment.