ART significantly reduces TB risk in SA

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Starting antiretroviral therapy (ART) significantly reduces the risk of tuberculosis (TB) for patients in South Africa, investigators report. Overall, ART initiation was associated with a 45% reduction in TB risk, which was lower than the effectiveness observed in other research. The authors believe this difference is because of their rigorous methodology, which unlike earlier studies, took into account patients’ pre-treatment CD4 counts.

“The study shows that ART initiation was associated with a substantially reduced risk of TB in HIV-infected patients with CD4+ cell counts of 350 cells/mm or less,” write the investigators. “The results differ from previous studies in high TB burden countries including South Africa, in which effectiveness estimates were greater and in which effectiveness was not modified by CD4+ count.”

TB is the commonest Aids-defining illness in Africa. However, there is some evidence that ART rollout is reducing TB incidence among HIV-infected patients in this region. South Africa has the worst HIV epidemic in sub-Saharan Africa with an estimated 6.3m infected patients. A national ART programme was introduced in 2004, and by 2010 approximately 2.4m patients had started HIV therapy. Data from government health services suggest that increasing access to ART has been accompanied by a fall in TB incidence. Moreover, a meta-analysis of eleven studies in low- and middle-income countries estimated that ART reduced TB risk or incidence by 65%. However, this study had an important limitation in that it did not take into account patients’ pre-treatment CD4 counts.

To get a better understanding of the true impact of ART initiation of TB risk, an international team of investigators designed an observational cohort study involving approximately 74,000 individuals who received HIV care in Free State, South Africa, between 2004 and 2010. TB risk and incidence was compared between patients who started ART and those who did not. Results were modified to take into account factors known to be associated with TB risk, especially CD4 count.

Patients were eligible to start ART if they had a CD4 count below 200 cells/mm3, had stage IV HIV disease (AIDS), or were pregnant with a CD4 count below 350 cells/mm3. Approximately 44,000 patients (62%) started ART and 30,000 (38%) did not (or initiated HIV therapy after an episode of TB). Factors associated with starting ART were gender (female vs. male, p < 0.001), CD4 count below 200 cells/mm3 (p < 0.001), higher body weight (p < 0.001), previous TB (p < 0.001) and enrollment in the later years of the study (p < 0.001).

Patients were followed for up to 6.5 years (median, 1.3 years). Individuals who started ART contributed 78,202 person-years of follow-up and during this time, 3858 first TB cases were recorded (incidence, 4.9 per 100 person years). There were 5669 first TB cases during approximately 63,000 person-years of follow-up among patients who did not start ART (incidence, 9.0 per 100 person years). The investigators therefore calculated that, overall, starting ART reduced TB risk by 45% (crude incidence rate ratio, 0.55; 95% CI, 0.52-57).

Factors associated with reduced TB risk included ART (p < 0.001), higher CD4 count (p < 0.001), female sex (p < 0.001), older age (p < 0.001) and enrollment in later study years (p < 0.001). Further analysis showed that ART reduced TB risk at all CD4 counts below 350 cells/mm3 (28-56% reduction in risk). The biggest risk reduction associated with ART initiation was observed in patients with CD4 counts below 100 cells/mm3. Starting treatment after 2006 was also associated with a significant reduction in TB risk.

“The study shows that ART was effective in preventing TB, although less effective than in previous studies, and that effectiveness increased with time,” conclude the authors. “The study supports further expansion of ART as a foundation of TB control in South Africa and other high TB burden countries.”

Objective: The objective of this study is to estimate the effectiveness of antiretroviral treatment (ART) in preventing tuberculosis (TB) in HIV-infected people during the first 6 years of ART programme expansion.
Design: A cohort study comparing TB risk without ART and after ART initiation.
Setting: Public sector HIV programme of the Free State province, South Africa.
Participants: Seventy-four thousand and seventy-four HIV-infected people enrolled from 2004 until 2010, of whom 43 898 received ART and 30 176 did not.
Intervention: Combination ART.
Main outcome measures: Time to first TB diagnosis, adjusted for CD4+ cell count, weight, age, sex, previous TB, district and year, with ART, CD4+ cell count and weight as time-varying covariates and with death as a competing risk.
Results: Three thousand eight hundred and fifty-eight first TB episodes occurred during 78 202 person-years at risk with ART and 5669 episodes occurred during 62 801 person-years without ART [incidence rates 4.9 and 9.0 per 100 person-years, crude incidence rate ratio 0.55 (95% confidence interval 0.52-0.57)]. The adjusted subhazard ratio (SHR) of time to first TB episode after starting ART, compared with follow-up without ART, was 0.67 (0.64-0.70). Within CD4+ cell count subgroups (<50, 50-199, 100-199, 200-349 and >350 cells/[mu]l), the respective SHRs were 0.64 (0.57-0.71), 0.63 (0.57-0.70), 0.66 (0.61-0.72), 0.67 (0.62-0.72), 0.72 (0.63-0.83) and 0.97 (0.60-1.59). Adjusted SHRs for ART decreased with each year of enrolment, from 0.90 (0.77-1.04) in 2004 to 0.54 (0.43-0.67) in 2010.
Conclusion: ART was effective in preventing TB in HIV-infected patients with CD4+ cell counts below 350 cells/[mu]l, but less so than previously estimated. Effectiveness increased each year.

Aidsmap material
AIDS abstract

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