A Systematic Review on the Development of Asthma and Allergic Diseases in Relation to International Immigration: The Leading Role of the Environment Confirmed

Abstract

BackgroundThe prevalence of asthma and allergic diseases is rising worldwide. Evidence on potential causal pathways of asthma and allergies is growing, but findings have been contradictory, particularly on the interplay between allergic diseases and understudied social determinants of health like migration status. This review aimed at providing evidence for the association between migration status and asthma and allergies, and to explore the mechanisms between migration status and the development of asthma and allergies.Methods and FindingsSystematic review on asthma and allergies and immigration status in accordance with the guidelines set by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The pooled odds ratio (OR) of the prevalence of asthma in immigrants compared to the host population was 0.60 (95% CI 0.45–0.84), and the pooled OR for allergies was 1.01 (95% CI 0.62–1.69). The pooled OR for the prevalence of asthma in first generation versus second generation immigrants was 0.37 (95% CI 0.25–0.58). Comparisons between populations in their countries of origin and those that emigrated vary depending on their level of development; more developed countries show higher rates of asthma and allergies.ConclusionsOur findings suggest a strong influence of the environment on the development of asthma and allergic diseases throughout the life course. The prevalence of asthma is generally higher in second generation than first generation immigrants. With length of residence in the host country the prevalence of asthma and allergic diseases increases steadily. These findings are consistent across study populations, host countries, and children as well as adults. Differences have been found to be significant when tested in a linear model, as well as when comparing between early and later age of migration, and between shorter and longer time of residence.

Prevalence of non-food allergies among non-immigrants, long-time immigrants and recent immigrants in Canada

“…9 Previous studies have found that the risk of developing allergic diseases increases in those immigrating from low-income countries to Western countries, 9 and the prevalence increases with longer residence in the host country. 10 These results support the "healthy immigrant effect" found in various fields of health research, where recent immigrants tend to be healthier than the native population, but the differences vanish over time with exposures in the host country or the adoption of adverse health behaviours. The mechanism behind differences in allergic conditions by immigration status is still unclear, but the "biodiversity hypothesis" provides a possible explanation.…”

“…25 Second-generation immigrants have been found consistently to have higher rates of allergies than first-generation immigrants. 10 Third, some studies had very small sample sizes, resulting in imprecise estimates from which it was difficult to draw conclusions. 23,25 In general, results from this study are consistent with previous studies with similar study design, that is, studies using self-reported allergies as the outcome, looking at first-generation immigrants only and having a relatively large sample size.…”

“…Only a few studies have looked at allergies among adolescent or adult immigrants, and results from these studies have been imprecise and inconclusive. 10 Thus, it remains unclear whether the effect of immigration status on allergy development applies to the general population. To address this knowledge gap, data gathered from the Canadian Community Health Survey (CCHS) were used to conduct a population-based study on the relationship between immigration status (non-immigrant, recent immigrant and long-time immigrant) and the prevalence of non-food allergies, which should be dominated by respiratory allergies given their considerably higher prevalence compared with atopic dermatitis.…”

A systematic review of socioeconomic position in relation to asthma and allergic diseases

“…However, this does not explain why the prevalence of asthma is generally lower in low-income countries, where especially residents in urban areas in tropical climates may be more exposed to indoor and outdoor pollution than in higher income countries. Research on migrants has shown that the prevalence of asthma in people migrating to high-income countries rises over time, with increased length of exposure to determinants of health in the host country [62]. More research is needed to determine which environmental factors in westernised societies drive this change in susceptibility to asthma.…”

Ethnic and socio-economic differences in the prevalence of wheeze, severe wheeze, asthma, eczema and medication usage at 4 years of age: Findings from the Born in Bradford birth cohort

“…Within Western countries however, further differences in prevalence between native born populations and non-Western migrants, with migrants generallyon average haveing lower prevalence rates thanrelative to native born populations in Western countries [2]. HEvidence does suggest however, that the prevalence of wheeze and asthma symptoms may be higher in migrants compared to their peers in their countries of origin, and prevalence and severity the risk of development and symptoms increase in increaseseverity with each additional year of residence in high-prevalence countires [2][3][4].…”