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Nakul Shekhawat, Beatriz Munoz, Harran Mkocha, Sheila West; Cohort- and age-specific effects of annual mass drug administration on prevalence of trachoma: a longitudinal study in rural Tanzania. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4547.
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© ARVO (1962-2015); The Authors (2016-present)
Mass drug administration (MDA) in trachoma-endemic areas is part of the World Health Organization (WHO) SAFE strategy for elimination. The aim of this study was to determine the effect of three rounds of annual MDA on prevalence of active trachoma among thirteen longitudinal birth cohorts of Tanzanian children.
Complete census of four villages collected information on known trachoma risk factors (hygiene status, number of children in household, educational level of head of household, distance to primary water source, presence/absence of household latrine). All children ages nine years & under were followed for 36 months, with trachoma status re-assessed every 6 months. Annual MDA consisted of topical tetracycline for children <6 months old and oral azithromycin for those >6 months old. Thirteen birth cohorts were created, with newborns assigned to new cohorts over time. The prevalence of trachoma was compared across age groups & birth cohorts at each survey. Differences were assessed using tests of trend.
MDA coverage was consistently >80% for all groups. All age groups showed decreased prevalence of active trachoma after the first MDA, with further decreases after each additional MDA for most age groups (see figure). Children <1 year old showed sequential reductions in prevalence after each MDA (7.14% baseline, 3.35%, 2.99% and 0% after repeat MDA; p <0.001), suggesting a protective herd effect from previous rounds of MDA. Birth cohorts aged 1 year or older at baseline had persistent reductions in prevalence after each MDA, while younger cohorts had increased prevalence every year in spite of MDA. Other than starting prevalence of trachoma, there were no differences in trachoma risk factors corresponding to differences in younger versus older cohorts. The birth cohort that was 5 years old at baseline exhibited a cohort effect persisting across two annual surveys, with higher rates of trachoma than would have been predicted by older or younger cohorts. Excluding recently migrated children from the cohort analysis yielded similar results.
This study shows the benefits of multiple rounds of MDA, with lower prevalence of trachoma after each round and a protective herd effect for children born into communities that have previously undergone MDA. It also highlights the possibility of persistent cohort effects that should be explored.
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