Virtually everyone who has a BCG vaccination will develop a raised blister at the site of the injection immediately afterwards. This is normal and is nothing to worry about.
BCG injection scar
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. It usually leaves a small scar.
Rarely, some people may have a more severe skin reaction. This should heal within a few weeks.
If you are worried that you or your child's skin reaction is abnormal or that the spot may have become infected, contact your GP.