• Dalia L Rotstein, Ruth Ann Marrie, Colleen Maxwell, Sima Gandhi, Susan E Schultz, Kinwah Fung, and Karen Tu.
• From the Department of Medicine (D.L.R.) and Departments of Family and Community Medicine and Institute for Health Policy, Management and Evaluation (K.T.), University of Toronto; St. Michael's Hospital (D.L.R.), Toronto, Ontario; Departments of Internal Medicine and Community Health Sciences (R.A.M.), Max Rady College of Medicine, Rady Faculty of Health Sciences, University of Manitoba, Winnipeg; Schools of Pharmacy and Public Health and Health Systems (C.M.), University of Waterloo; Institute for Clinical Evaluative Sciences (C.M., S.G., S.E.S., K.F.); and Toronto Western Hospital University Health Network (K.T.), Ontario, Canada. dalia.rotstein@unityhealth.to.
• Neurology. 2019 Dec 10; 93 (24): e2203-e2215.

ObjectiveTo determine risk factors for multiple sclerosis (MS) in immigrants and to compare MS risk in immigrants and long-term residents in Ontario, Canada.MethodsWe applied a validated algorithm to linked, population-based immigration and health claims data to identify incident cases of MS in immigrants and long-term residents between 1994 and 2016. We conducted 2 multivariable Cox proportional hazards regression analyses: 1 analysis limited to the immigrant cohort assessing potential risk factors for developing MS, and 1 analysis comparing MS risk between immigrants and matched long-term residents (1:3 match).ResultsWe identified 2,304,302 immigrants for the immigrant-only analysis, of whom 1,526 (0.066%) developed MS. Risk was greatest in those <15 years old at landing (referent <15 years; 16-30 years: hazard ratio [HR] 0.73, 95% confidence interval [CI] 0.63-0.85; 31-45 years: HR 0.55, 95% CI 0.47-0.64). Immigrants from the Middle East (HR 1.22, 95% CI 1.06-1.40) were at greater MS risk than immigrants from Western nations; all other regions had lower risk (p < 0.0001). The matched analysis included 2,207,751 immigrants and 6,362,169 long-term residents. Immigrants were less likely to develop MS than long-term residents (p < 0.0001), although this lower risk was attenuated with longer residence in Canada.ConclusionsMS incidence in immigrants to Ontario, Canada, varied widely by region of origin, with greatest risk seen in those from the Middle East. Longer residence in Canada was associated with increased risk, even with migration in adulthood, suggesting that environmental exposures into adulthood contribute to MS risk.© 2019 American Academy of Neurology.

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