A scar on the arm is usually from the BCG vaccination. BCG vaccine is the first vaccination within 24 hours after the birth of a newborn, the reason why the scar occurs after BCG vaccination is because BCG vaccine is a weakly toxic bovine tubercle bacillus suspension, which can lead to pus, redness and swelling of the tissue after local injection, and the pus bag will break down and heal in about 8 to 12 weeks, and a scar will be formed after the scabs fall off. Therefore, the vast majority of BCG vaccinations will result in scars, the size of which is related to body type but does not affect the effectiveness of the vaccination. BCG is a vaccine against Mycobacterium tuberculosis infection and is best administered within 24 hours of birth. There is no need to worry about a scar on your arm, it is mostly due to the BCG vaccination and is a sign of successful vaccination and does not require any treatment.