MS risk in immigrants in the McDonald era: A population-based study in Ontario, Canada.

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 p p 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.