Paediatrician Dr Paul Sinclair answers:
The BCG (or Bacille Calmette Guerin) vaccine is a live bacterial vaccine that is given to your baby shortly after birth. It is injected into the upper outer aspect of the right arm. It is an attenuated form of cow tuberculosis, so it does grow and multiply, forming a “cold” abscess. This abscess is the pimple-like spot to which you are referring. It can increase in size and generally looks like a small boil. This is the only vaccination that this happens with as a matter of course. The presence of the abscess tells you that the immunisation is working. It is not at all painful for your baby.
This should never be opened or drained (except by a medical professional) and is associated with big glands in the right armpit. The abscess generally opens spontaneously and will drain some caseous pus as a matter of course – so don’t be alarmed. It is also normal for there to be a scab or even peeling on the injection site. Don’t try to scratch or treat this either.
Wipe it with some saline water and leave it open to drain. There is no need for any antiseptic lotion or any creams at all, and don’t put a plaster on it either. It will heal by itself over time.
It may seem to be getting better and then come back again (what we call reaccumulating) and may take up to a year to settle completely. Unfortunately it will leave a small scar.
Remember that the BCG should not be given to infants with HIV, or if there is a family history or concern about primary immune deficiency. If you’re concerned,or large glands are felt under the right arm, let your doctor look and discuss further treatment.