Immigrants from outside Canada and interprovincial migrants have a mortality advantage, compared with long-term residents of Manitoba, new data show.
In a matched cohort study that included almost 360,000 participants, the all-cause mortality risk of international immigrants (2.3 per 1000 person-years) and interprovincial migrants (4.4 per 1000 person-years) was lower than that of long-term Manitobans (5.6 per 1000 person-years). Similar trends were observed for premature mortality.
“Our findings suggest that efforts to achieve gains in life expectancy in the province of Manitoba may be better focused on the local population,” wrote Marcelo Urquia, PhD, an associate professor of community health sciences at the University of Manitoba in Winnipeg, and colleagues. “Understanding the protective factors that underlie the migrant mortality advantage may help with devising prevention strategies for the local population and the migrant subgroups at higher risk, such as refugees and those from the USA.”
The article was published online December 27, 2022, in the Canadian Journal of Public Health.
Mortality Advantage
The investigators conducted a retrospective study to examine all-cause and premature mortality among 355,194 international immigrants, interprovincial migrants, and long-term Manitoba residents from 1985 to 2019. Each of the three cohorts included 118,398 participants, and cohorts were matched on birth year, sex, and place of residence.
The all-cause mortality rate per 1000 person-years was 2.30 for international immigrants, 4.38 for interprovincial migrants, and 5.57 for long-term Manitobans. The adjusted incidence rate ratio (aIRR) for all-cause mortality risk was 0.43 for international immigrants and 0.81 for interprovincial migrants, compared with long-term Manitobans.
International immigrants had a lower risk of death, compared with interprovincial migrants (aIRR, 0.50). Within the former group, however, a higher mortality risk was observed for refugees, those from North America and Oceania, and those with low educational attainment.
Compared with long-term Manitobans, international migrants had a substantially lower adjusted risk of death (aIRR, 0.43 for all-cause mortality; and aIRR, 0.35 for premature mortality).
Among interprovincial migrants, those from Eastern Canada had a lower mortality risk than those who migrated from Ontario and Western Canada. Furthermore, interprovincial migrants were less likely to experience all-cause and premature mortality, relative to Manitobans.
The mortality advantage of international immigrants was observed in all analyses stratified by age and by period of the start of follow-up, and associations did not differ with regard to sex.
“Migrants had a mortality advantage over non-migrants, being stronger for international immigrants than for interprovincial migrants,” write the authors. “Among the two migrant groups, there was heterogeneity in the mortality risk according to migrants’
characteristics.”
The authors acknowledge various study limitations. For example, sociodemographic data were collected only from permanent residents, not temporary residents such as students, work permit holders, and refugees awaiting a decision on their status. The study also could not distinguish between participants who left the province and returned to their place of origin because of ill health from those who remigrated to another destination and may have been healthier.
In addition, some migrant characteristics, such as education, were measured on arrival and could not be updated later, which may have contributed to residual confounding. No measure of migrant morbidity or medical history before migration was available, and an examination of mechanisms that might affect particular subgroups was beyond the scope of the study.
Salmon Bias?
Commenting on the study for Medscape Medical News, Sean Hennessy, PharmD, PhD, a professor of epidemiology at the University of Pennsylvania Perelman School of Medicine in Philadelphia, said, “The authors assumed in advance that any observed differences [in mortality] would be due mainly to the reasons that people migrate and the rules governing who is allowed to leave the source country and who is allowed to enter the host country (collectively known as ‘selective migration’), rather than to any effects of migration itself. Thus, the results don’t provide much information about the causes of the different mortality rates that were observed.”
Dr Sean Hennessy
Also, as the authors expected, “those who entered Manitoba as refugees had higher mortality than those who immigrated for other reasons (eg, economic). The results suggest that migrants are, on average, at least as healthy as long-term residents. I believe that this conclusion is sound,” said Hennessey.
“The main caveat, which I don’t put much stock in, is what has been called the ‘salmon bias,’ ” he noted. According to this hypothesis, “some migrants may move back to their homeland shortly before death, which would artificially make migrants look healthier than they are, since their death is not observed in the country to which they migrated. If the salmon bias were hiding what is a true higher mortality among migrants, it would need to be very large — probably implausibly large.”
Hennessey pointed to earlier research, cited in the current study, that found that the salmon bias could not sufficiently explain the migrant mortality advantage in England and Wales. “Therefore, I think that this caveat is not much of a concern,” Hennessey concluded.
The study was supported through funding from Manitoba Health, Seniors and Active Living and the Canadian Foundation of Innovation. Urquia and Hennessy have disclosed no relevant financial relationships.
Can J Public Health. Published December 27, 2022. Full text
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