Dr David Elliman, Consultant in Community Child Health, Royal College of Paediatrics and Child Health, and Great Ormond Street Hospital for Children, said:

"The way that BCG is given is different from most other vaccines. Usually, vaccines are distributed by manufacturers as single doses in either a vial or a syringe. However, for BCG each vial contains the equivalent of 10 standard doses of 0.1ml. A baby normally has half the standard dose given to a child (0.05ml rather than 0.1 ml), so there are effectively 20 doses for a baby in each vial.

"The BCG vaccine is for TB but doesn't contain human TB (Mycobacterium tuberculosis) itself. The vaccine contains a related germ that causes bovine tuberculosis (Mycobacterium bovis). This means a vaccinated child does not technically have tuberculosis. The vaccine, Bacillus Calmette-Guerin, is a live, but heavily toned down vaccine that has been modified to be much less severe.

"When a baby is administered the vaccine it is given just under the skin i.e. intra-dermally and usually produces a small hard bump. Ten times the dose would produce a bump ten times as big, so would normally be an indicator of an overdose, but if the injection was given too deep then the bump may be much smaller. Giving the vaccine too deeply can raise its own problems and normally if either too much vaccine is administered, or it is given too deeply then you would expect an abscess to develop. They are not like normal abscesses as they don't tend to get red and tender and they develop slowly.

"A specialist will decide on the treatment that needs to be given if an abscess develops. The two options are to puncture it and/or administer antibiotics as for tuberculosis. These are administered for months rather than days. Very, very, very occasionally, if a child has something wrong with their immune system for example, there may be more generalized spread which is more serious."