Why bcg vaccine cause blister?

Body reacts with it: The bcg is not used in the U.S. But is used in many countries. Our immune cells react with the substance. The U.K. Website says: two to six weeks after the injection, a small spot may appear at the site of the injection. It can grow into a circle up to 7mm in diameter and may become crusty where fluid has dried on the surface. It can be painful and bruised for a few days, but will eventually heal.

Normal Reaction: It is normal reaction to the vaccine.You get bluish red papule which may look like a blister 2to3 weeks after the vaccination.After about 6 weeks the pustule ulcerates forming a lesion approx. 5 mm in diamneter. Scab forms and heals usually withn 3 months after vaccination and results in a permanent scar at the puncture site