Trouble is with the BCG immunisation is that it doesn't work in the same way as a straight viral / bacterial imm.
Say the flu vaccine - it contains a combination of the commonest marker particles (antigens) from the the types of flu circulating and has stuff added to make it particularly enticing to the immune system, which "sees" the antigen, starts developing antibodies from B cells and "remembers" the pattern. When the person is then exposed to the real virus, the body starts rapidly developing more of the antibody as it has seen it before - the antibody then leads to the destruction of the virus -
***ta ra, a victory for the immune system over the beastly virus and minimal damage to the body ***
HOWEVER ... (bet you knew that was coming) - much of the damage of TB is related to the body's immune response - also more problems can occur decades after an initial infection. The BCG is an attenuated weak version of ONE of the TB mycobacterium (irritatingly not a straightforward bacterium) - it is attempting to stimulate the T cells to recognise the TB if it appears .
However, there are many many types of mycobacterium about, and not all are like the BCG one - the kids in the outbreak I mentioned had one from the subcontinent which is not sensitive to the BCG.
BCG is thought to prevent about 50% of cases (but very difficult to know definitely), it does seem to reduce the risk of the more serious and rare complications of widespread TB and TB meningitis.
Hence the reason why it is still used in some risk groups.
I have worked in some pretty high incidence areas where we have some eye watering anxt amongst people who would not even drive through the city
- I had BCG aged 13 (but don't think it actually reacted at all) - I have weak immunity on a heaf test, possibly just from general exposure as a kid in Birmingham in the 60's.