Department of Tropical Hygiene and Public Health, Heidelberg University. Im Neuenheimer Feld 324, 69120 Heidelberg, Germany. e-mail: oliver.razum{at} urz.uni-heidelberg.de
SirRingbäck Weitoft et al.1 present intriguing evidence indicating that the lower mortality among immigrants compared to the host population in register studies could largely be explained by inaccurate denominator figures. Their findings suggest that immigrants to Sweden, particularly those from Turkey and Southern Europe, frequently remain registered on the national population registry even after returning to their home countries. Thus, an inflated denominator basis would lead to an underestimate of their mortality rate.
In Germany, too, the age-adjusted mortality among the 2 million Turkish residents is markedly lower than that of Germans, according to register studies.2 To assess whether this could be due to tbe bias reported by Ringbäck Weitoft et al. we analysed the mortality experience of immigrants in the German Socio-Economic Panel (SOEP). In the SOEP, a representative sample of households is followed up annually. Between 1984 and 1997, 23 769 individuals (25% of them immigrants) had spent a median of 8 years on the panel; 1310 deaths (6.3% among immigrants) were reported. We compared the mortality of immigrants from Turkey and Southern Europe, most of them former guest workers and their family members, to that of Germans in a Poisson regression model.3 Forward-stepwise estimation led to the selection of the variables sex, time period (19911997 versus 19841990), and age in 10-year bands as a continuous and a quadratic term. No significant interactions of relevance were found.
Table 1 shows crude mortality rates and adjusted relative risk estimates of immigrants from Turkey and Southern Europe versus German panel members. Immigrants have a significantly lower mortality risk than Germans even after age adjustment. So far, there is no evidence of a secular increase in their mortality.
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With migration on the rise world-wide, epidemiological studies will increasingly include immigrants in the study population. The healthy migrant effect thus becomes a potential effect modifier that needs to be properly understood and quantified. Future research should identify factors determining the size and possible attenuation with time of the mortality advantage among immigrants. Besides, a numerator bias, e.g. due to a selective re-migration of critically ill individuals to their home countries, should be ruled out.
Acknowledgments
The data of the German Socio-Economic Panel (SOEP) used in this publication were provided by the Deutsches Institut für Wirtschaftsforschung (DIW), Berlin.
References
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