If you mean how often does one need to immunize against Tuberculosis, that would depend on where you live. In the United States TB immunization is not recommended for all children. In cases where children are continuously exposed to someone who has TB and cannot be removed from that person's company or where the child is exposed to a form of TB resistant to drugs, the vaccination is administered.
Also in the case of health workers who are exposed to patients with drug resistant strains of TB, vaccines may be given. In several countries children receive the BCG vaccine as part of the routine vaccination schedule, but there is some debate as to whether it is really effective. One you have had the BCG shot, you are not required to take it again.
If by the TB shot, you mean the test that you take to determine whether or not you have TB, that would depend on whether your health care giver suspects that you have Tuberculosis. The test only determines whether your body has developed an immune response to TB. A positive test need not necessarily mean that you are currently suffering from tuberculosis. It could also mean that you were exposed to the infection at some previous time and so developed an immune response to it.
The TB skin test involves injecting a small quantity of tuberculin under the surface layers of the skin. The test is interpreted 48 to 72 hours later by skilled personnel. A positive test shows up as a raised bump at the point where the skin was pricked. The size of the bump will determine if your test result is significant.
Hope that answers your query.
answered by S E