As a live attenuated vaccine, BCG vaccine generally leaves scars after vaccination, and if there are no scars, it is possible that they were not injected. However, 90% of people will have reactions such as pus and scarring after vaccination, although 10% of people will not have scarring, and the size of the scarring is not an indication of the strength of the immune response. 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 pustules to scab over and leave a round scar 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 replacement vaccination should be considered. Is it a problem if the BCG vaccine does not leave a scar? The BCG vaccine usually leaves a scar. If the vaccination does not show a scar, it is possible that the vaccination was not successful. If you are concerned that the vaccination was not successful, you can do a tuberculin test (PPD skin test) to see if your child was successfully vaccinated; the PPD skin test requires 48-72 hours to observe the results. If the result is negative, the vaccination was unsuccessful and a follow-up BCG vaccination will be required. If the result is positive, the vaccination is successful. If the result is strongly positive, further tests will be needed. It is important to confirm the negative skin test before the BCG vaccination can be replenished. Symptoms of successful vaccination Nowadays, the success rate of BCG vaccination is about 95%. Most children will have local reactions 3-4 weeks or 1-2 weeks after BCG vaccination, through a process of redness, swelling, infiltration, pustule formation, breakage, and crusting, and usually the crusts will be removed and local scars will be formed in about 3 months. This is a normal reaction process after BCG vaccination and is a sign of effective vaccination. If this does not occur, it should be considered that the vaccination has not been successful, and this is a situation that needs to be replanted. Therefore, it is generally necessary to look at the post-vaccination marks to see whether the BCG vaccination has been successful or not.