Genome-based transmission modeling separates imported tuberculosis from recent transmission within an immigrant population

Background In many countries tuberculosis incidence is low and largely shaped by immigrant populations from high-burden countries. This is the case in Norway, where more than 80 per cent of TB cases are found among immigrants from high-incidence countries. A variable latent period, low rates of evolution and structured social networks make separating import from within-border transmission a major conundrum to TB-control efforts in many low-incidence countries. Methods Clinical Mycobacterium tuberculosis isolates belonging to an unusually large genotype cluster associated with people born in the Horn of Africa, have been identified in Norway over the last two decades. We applied modeled transmission based on whole-genome sequence data to estimate infection times for individual patients. By contrasting these estimates with time of arrival in Norway, we estimate on a case-by-case basis whether patients were likely to have been infected before or after arrival. Results Independent import was responsible for the majority of cases, but we estimate that about a quarter of the patients had contracted TB in Norway. Conclusions This study illuminates the transmission dynamics within an immigrant community. Our approach is broadly applicable to many settings where TB control programs can benefit from understanding when and where patients acquired tuberculosis.