Local Transmission Plays no Important Role in the Occurrence of MDR-TB In Immigrants to Canada: A Molecular Epidemiologic Study of Clustering and Contact Tracing.

BACKGROUND Multidrug-resistant tuberculosis (MDR-TB) is increasing among migrants in Canada. To what extent this increase is attributable to local transmission, recent immigration, or changed proportion of MDR-TB in immigrants' country of birth, is unknown. METHODS We performed a retrospective cohort study in a Canadian province with substantially increased immigration between 1982-2001 and 2002-2019. The proportion of MDR-TB among migrants arriving from high MDR-TB burden (HMDR-TB) countries during these two periods was used to estimate the proportion of cases due to immigration versus change in proportion in country of birth. Epidemiologic, spatio-temporal, and drug resistance pattern data were used to confirm local transmission. RESULTS Fifty-two of 3,514 (1.48%) foreign-born culture-positive TB patients had MDR-TB; 8 (0.6%) in 1982-2001, 44 (2.0%) in 2002-2019. Between time periods, the proportion of MDR-TB among migrants with TB from HMDR-TB countries increased from 1.11% to 3.62%, p=0.003; 31.6% attributable to recent immigration, 68.4% to a higher proportion of MDR-TB in cases arrived from HMDR-TB countries. No cases of MDR-TB were attributable to local transmission. CONCLUSION In stark contrast to HMDR-TB countries, local transmission plays no important role in the occurrence of MDR-TB in Canada. Overseas investments to improve TB programming in HMDR-TB countries are urgently needed.