 All contacts will undergo clinical evaluation at enrolment and review. The development of active TB will trigger withdrawal and full treatment. Fortnightly clinical review of group B during the deferred phase will be undertaken. ELISPOT analysis will be performed on all subjects at 0, 1, 4, 13, 26 and 40 weeks in groups A and C, and at 0, 13, 14, 17, 26, 40 and 54 weeks in groups B and D. A subset of 10 patients per group will be sampled for expression analysis at 0, 4, 26 and 40 weeks groups A and C and at 0, 13, 17 and 40 weeks in groups B and D 160 hybridizations in total. 