Generational patterns of asthma incidence among immigrants to Canada over two decades: a population-based cohort study

Rationale — Canada, an industrialized country with high endemic asthma rates is characterized by a large immigrant population.

Objectives — Comparing asthma rates among recent immigrants relative to long term residents of Canada may provide insight into the relative contribution of environmental exposure to asthma risk.

Methods — This was a population-based retrospective cohort study performed using provincial health administrative data for all residents of Ontario, Canada. Residents with and without a diagnosis of asthma from fiscal years 1996 – 2012 were included. Individuals were categorized as being immigrants (landed in Canada after 1985) or long-term residents of Ontario by linkage with Immigration, Refugees and Citizenship Canada’s Permanent Resident Database. We calculated the age- and sex- standardized incidence of asthma among residents of Ontario, and compared the incidence of asthma among immigrants and long-term residents using incidence rate ratios.

Results — Analysis of approximately 11.7 million records showed that 2.2 immigrants arrived to Canada during the study period, with >50% from East and South Asia and the Pacific. We found that asthma incidence was lower among immigrants compared to long-term residents (Incidence Rate Ratio (IRR) 0.30 95% CI: 0.30, 0.30, p<0.001). However, Ontario-born children of immigrants from most world regions had significantly higher asthma incidence compared to children of long-term residents (IRR 1.44, 95% CI: 1.43, 1.45, p<0.001). The overall incidence of asthma in Ontario decreased between 1996 and 2012 (ptrend<0.001). Immigrants contributed to only a small proportion of the asthma incidence in Ontario, and changes within this group did not significantly affect trends in the overall Ontario population asthma incidence.

Conclusions — The higher asthma incidence seen among children of immigrants but not in their parents suggests that being born in Canada was critical for determining asthma risk. These findings support the importance of in-utero and/or early life exposures on asthma development.