Estimating long-term tuberculosis reactivation rates in Australian migrants

BACKGROUND: The risk of progression to tuberculosis (TB) disease is greatest soon after infection, yet disease may occur many years or decades later. However, rates of TB reactivation long after infection remain poorly quantified. Australia is a low-TB incidence setting and most cases occur among migrants. We explored how TB rates in Australian migrants varied with time from migration, age and gender. METHODS: We combined TB notifications in census years 2006, 2011 and 2016 with time and country-specific estimates oflatent TB prevalence in migrant cohorts to quantifypost-migration reactivation rates. RESULTS: During the census years 3,246 TB cases occurred among an estimated 2,084,000 migrants with latent-TB. There were consistent trends in post-migration reactivation rates, which appeared to be dependent on both time from migration and age. Rates were lower in cohorts with increasing time until at least twenty years from migration, and on this background there also appeared to be increasing rates during youth (15-24 years of age), and in those aged 70 years and above. Within five years of migration, annual reactivation rates were approximately 400 per 100,000 (uncertainty interval [UI]: 320-480), dropping to 170 (UI: 130-220) and 110 (UI: 70-160) from five-to-ten and ten-to-twenty, then sustaining at 60-70 per 100,000 up to sixty years from migration. Rates varied depending on age at migration. CONCLUSIONS: Post-migration reactivation rates appeared to show dependency on both time from migration and age. This approach to quantifying reactivation risk will enable evaluation of the potential impact of TB control and elimination strategies.