(P-411) Effectiveness of a Six-month Isoniazid as Tuberculosis Preventive Therapy in People Living with HIV in Eritrea: A Retrospective Cohort Study

Background: A six-month Isoniazid Preventive Therapy (IPT) has been used in Eritrea, since 2014, to reduce the risk of tuberculosis (TB) in People Living with HIV (PLHIV). However, no study has been conducted yet in the country to assess the effectiveness of IPT on preventing active TB and the duration of its protection.

Objectives: To evaluate the impact of IPT on prevention of TB and all-cause mortality and its duration of protection in PLHIV.

Methods: This was an observational retrospective cohort study which enrolled all eligible PLHIV attending all national and regional referral hospitals in Eritrea, including private, between November 1, 2014 and November 1, 2020. Data was collected using a structured questionnaire from patients’ clinical cards. Both exposed and unexposed groups were taking antiretroviral therapy and the main exposure of interest was a six-month isoniazid 300mg daily. The primary outcome measures were development of incident TB, probability of survival without TB, and duration of protection of IPT. All-cause mortality was considered as secondary outcome measure. The association between use of IPT and the outcomes of interest was assessed by fitting Cox proportional hazard regression models and Kaplan Meier curve. For all statistical tests, a two-sided alpha level of 0.05 was used to determine statistical significance.

Results: This study enrolled 6,803 patients which accounts over 75% of all PLHIV in Eritrea. About 75% were exposed to IPT while the rest 25% were unexposed. The mean follow-up time was 4.9 years (SD: 1.4). The incidence rate of TB was 1.7 cases per 1000 person-years in the exposed and 10 cases per 1000 person-years in the unexposed. Those unexposed to IPT had higher risk of developing incident TB (adjusted harzard ratio [aHR]: 3.75, 95% confidence interval [CI]: 2.89, 6.13) and all-cause mortality (HR: 2.37, 95% CI: 1.84, 3.02, p-value < 0.001) compared to the exposed. Kalpan Meier estimate showed that the exposed group had better survival probability without TB (98.75%) compared to the unexposed (95%) at 65 months follow-up (p < 0.001). A few patients developed incident TB soon after the completion of the six-month IPT and its protection capability started to diminish rapidly after one year following initiation of IPT.

Conclusions: Use of a six-month IPT, even in the absence of tuberculin skin test, was found to be effective in the reduction of incident TB and all-cause mortality in PLHIV. The duration of protection however appeared to diminish soon after 12-month following initiation of the intervention that warrants immediate attention from policy makers.