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This study followed 1655 HIV infected male employees of a South African gold mining company (median age 37 years) for a median duration of 22.1 months. Isoniazid preventive therapy, 300 mg daily, was offered at clinic enrolment to 679 subjects who attended with no evidence of active tuberculosis (TB).
A total of 254 TB episodes met the study criteria. Before enrolment at the clinic the incidence of TB was 11.9 per 100 person-years, which reduced to 9.0 per 100 person-years following intervention. The adjusted incidence rate ratio for the effect of clinic enrolment on the incidence of TB was 0.68 (95% CI 0.48 to 0.96; p = 0.03). The overall reduction in the incidence of TB was 38%, and 42% in those with no prior history of TB. Failure to attain results as high as in previous studies may have been due to the fact that all the subjects who attended the clinic were assessed, irrespective of whether they received isoniazid.
Despite the reduction, the incidence rate is still high at 9 per 100 person-years. As the authors point out, long term prevention of TB needs to be considered as well as the emergence of isoniazid resistant strains. The follow up period in the study was short, and evidence suggests that the protective strategies of TB preventive therapy, particularly those without rifampicin, wane over time. Further studies to assess secondary prevention of TB—although not currently recommended—would be interesting, particularly in view of the prolonged survival now offered by antiretroviral therapy.