My child has a PID and receives immunoglobulin (Ig) therapy should they have a pneumococcal immunisation and if so how much protection does it give against pneumonia?

Pneumococcal vaccine is not usually given whilst on Ig therapy.  This is because there is a good spread of anti-pneumococcal antibodies in Ig, and this is part of its value.  Pneumococcal vaccine can be “neutralised” by the antibodies present in the Ig.  There is a limited useful T-cell response that would help provide further protection to these organisms, so there is no great utility in vaccinating. This is unlike the killed flu vaccine, which can provide useful T-cell protection even if there is no antibody response.