there are 2 reasons for the scar not to be formed.. one is that in many children the vaccine
gets taken up by the body and a good immune response is built up but outside scar formation on the skin may not occur.. the second reason being that vaccine may not be taken up at all.. the efficacy of this vaccine ranges widely between 0 - 80%!!
it is difficult to distinguish this on basis of scar so v can do a mantoux test
done.. if the response is shown well that means child already has been exposed to tubercular antigens and is mounting a response and there is no need for another round of vaccination
.. but in case his mantoux does not respond then it is better to get another dose of vaccination done.. and to be called a contact we usually take a period of 2 years to be living in the same house as with an active case of tuberculosis
..this visit to uncle should not be much of an issue..