Background: In Hong Kong, neonatal bacillus Calmette-Guerin vaccination coverage has been around 99% since 1970. Children younger than 14 years of age appear to have a relatively low risk of tuberculosis (TB), but the risk of TB increases rapidly after 15 years of age to a secondary peak in young adulthood.
Methods: We followed prospectively 19,383 students who were 6 to 10 years of age participating in the 1999/2000 bacillus Calmette-Guerin revaccination program by cross-matching with the territory-wide TB registry until December 31, 2010, using the identity card number as a unique identifier.
Results: After 214,753 person-years of follow-up, 44 active TB cases (22 culture-confirmed) were detected for an overall incidence of 20.5/100,000 person-years. The incidence differed significantly by baseline tuberculin reaction sizes (13.0, 18.8, 22.5, 280.4 per 100,000 person-years for reaction size of 0-4, 5-9, 10-14, and ≥15 mm, respectively, P < 0.001). Consistent results were observed for culture-confirmed cases and after adjustment for gender and baseline age. For those with tuberculin reaction size ≥15 mm, the incidence of TB was significantly higher beyond the age of 15 years than for those less than 15 years (608.1 vs. 37.5 per 100,000 person-years, P < 0.001). Although older baseline age was associated with larger tuberculin reaction sizes, it did not independently predict subsequent development of disease.
Conclusion: Strong tuberculin reactions in primary school children predicted TB in adolescents after an initial quiescent period. Endogenous reactivation, possibly related to changes in host immunity, might account for the upsurge of TB in adolescence.