Objective: To measure the tuberculosis (TB) incidence rate and assess the relative risk of TB disease in contacts based on the tuberculin skin test (TST) and sputum status of index cases.
Design: All contacts aged <20 years who were exposed to a TB case in 2005 were cross-matched using an electronic surveillance system to estimate TB incidence over a 24-month follow-up period.
Results: Among 6959 contacts there were 67 secondary cases (1%). The incidence was highest in the first year after exposure and decreased by half in the second year (P = 0.001). The relative risks of developing TB in contacts aged 0-4, 5-9, 10-14 and 15-19 years were respectively 325, 209, 337 and 53 times greater than for the general population. The hazard ratio of developing TB among contacts with a TST ≥ 15 mm induration was 12 times higher than for those with a TST < 5 mm (P = 0.003).
Conclusions: The relative risk of developing TB disease within 24 months of exposure was approximately 200-300 times greater for contacts aged <15 years. The majority developed TB within 12 months of exposure.