Migration is a phenomenon reshaping society structure in numerous countries 1. It is of particular Public Health importance because the health status of migrants frequently differs from that of the general population of the country these people come to, the "host country", as well as from that of the population of the country they come from, the "country of origin" 23.

Since the early 1990s, almost 2 million ethnic German "resettlers" (Aussiedler) have migrated from the FSU to Germany 4. Being descendants of people who left what is now Germany to Russia centuries ago, Aussiedler suffered strong discrimination during and after the two World Wars, but partially managed to keep their own ethnic identity. In the aftermath of the collapse of the Soviet Union, they began to migrate to Germany in large numbers. German legislation favours these migrants through social benefits and by giving them German nationality upon arrival in the country. This distinguishes them from other migrants, such as labour migrants, who receive much less support from the state 5.

So far, there are no data on overall and cause-specific mortality in this population group. The present study aims at filling this gap by providing data on overall mortality and mortality from CVD, the most frequent cause of death in the industrialized world 6, for Aussiedler from the FSU, and by comparing it to that of Germany's general population. In a more general context, a hypothesis on the health status of migrants is developed and tested in this population.

There are numerous studies from different countries showing that the health status and mortality of migrants differ from that of the host country's general population. Often, migrants have a lower mortality which gradually converges towards mortality levels prevailing in the host country with increasing duration of stay and over generations. This pattern is particularly common for CVD and other chronic conditions 237. A closer look, however, reveals that such an initial mortality advantage almost exclusively occurs in migrants from populations where mortality from these diseases is lower than in the host country. One example can be found in the United States, where this is the case for migrants from Latin America and Asian-Pacific islands, both regions with lower chronic disease mortality 7. Conversely, migrants from a country where chronic disease mortality is higher than that in the host country have an initially higher mortality. This, for example is the case for migrants from Finland to Sweden 8. Similar data are reported from Canada. There, migrants from Scandinavian countries, where CVD mortality is high, had an increased risk of dying from CVD compared to those from regions with a lower CVD mortality like Latin America and Asia 9. In general, overall and CVD mortality in migrants seems to converge from an initial level similar to that in the country of origin towards the level in the host country 3. It can be claimed that this pattern is determined by factors from three broad categories, namely factors in the country of origin, factors in the host country, and migration-specific factors.

For Aussiedler from the FSU, these factors potentially determine overall and CVD mortality as follows. First, in the countries of origin, the FSU, overall mortality is substantially higher than in Western Europe, which is to a large extent a consequence of extraordinarily high CVD mortality. In fact, FSU countries have the highest CVD mortality worldwide with mortality rates being about three times that of Western European countries. For example, the age-standardized CVD mortality rate in the FSU countries in 2000 was 772.05 per 100,000, whereas in the European Union it was 249.89 per 100,000. Respective figures for all-cause mortality were 1371.74 and 665.91 6. Thus, the largest part of the East-West mortality gap is attributable to deaths from CVD 10. The reasons for this phenomenon are not fully understood, but most probably the driving force is a combination of various risk factors like smoking, nutritional deficiencies and binge drinking 1011. Mortality in the FSU seems to differ between certain population subgroups with different ethnic and religious backgrounds, and at least some of these observed differences are due to concomitant disparities in socioeconomic characteristics 1213. For Aussiedler in the FSU, there are no specific mortality data but there is some information on SES. The few existing studies in the countries of origin showed that economically Aussiedler fared somewhat better than the general population. There were no differences with regard to employment or education, but the importance of family and religion seemed to be higher among Aussiedler than the general population 1415. In summary, there is little evidence for differences in SES which could explain a substantially different risk factor profile. Because the gap in mortality averages between the two countries is very big, overall and CVD mortality among Aussiedler is probably higher than in Germany's general population before migration to Germany. Due to a presumed "time lag", i.e. a long latency between exposure and onset of disease, for some risk factors 1617, CVD mortality should remain high, or even increase, compared to that of the general population of Germany, after the Aussiedler's arrival in Germany.

Second, studies have shown that Aussiedler from the FSU in Germany, in spite of their privileged status compared to other migrants, have a lower SES than the country's general population. This has manifold reasons among which there might be the mere fact of being a migrant, i.e. a person who has to adapt to a new environment and society 181920. Low SES is independently associated with high overall and CVD mortality in many populations around the world 162122. Therefore, this observed socio-economic disadvantage is another potential reason for a higher overall and CVD mortality of Aussiedler from the FSU as compared to Germany's general population.

Third, factors related to migration itself might have an effect on the subsequent mortality of migrants. In many instances, migrants seem to be a selected sample of their population of origin. Such selection can be based on health status, but there are arguments for either direction of this effect. Possibly, particularly healthy people decide to migrate, which is referred to as the "healthy migrant effect", or more sick people do so because they may be hoping for better treatment in the country of destination 3. It is important to notice that such selection only occurs compared to the population of origin, and not compared to the host country's population 323. In the case of Aussiedler from the FSU, it is hard to quantify how much health selection took place during migration. Since the large majority of Aussiedler in the FSU has now migrated to Germany 24, the overall effect of selection is probably rather small. Another potential migration-specific factor with beneficial influence on health status is the formation of a strong ethnic identity within families and social networks in the host country 25, and indeed there is some evidence for the formation of such a specific identity among Aussiedler from the FSU in Germany 2627.

In summary, we expected that Aussiedler from the FSU in Germany have a higher overall and CVD mortality than Germany's general population. Migration-specific factors might have an alleviating effect on their mortality.

To test this hypothesis, and to provide data for resource allocation and health planning in Germany as well as in other countries which experience in-migration at large scale, we conducted a retrospective cohort study among a representative sample of Aussiedler from the FSU. We investigated possible mortality differentials between subgroups of this population and assessed the effect of covariables available in this study.

We conducted a retrospective cohort study with the aim of recruiting a study population as representative as possible of the totality of Aussiedler from the FSU who came to Germany. Because the focus of our study was on adult mortality, we confined the study population to persons aged at least 15 years upon arrival in Germany. The sampling and follow-up procedures have been described in detail elsewhere 28. In short, we obtained a dataset containing individual data of all Aussiedler from the FSU (n = 281,356) who had come to North-Rhine Westphalia, Germany's most populous federal state, between 1990 and 2001. The final cohort for analysis consisted of all those for whom an automated record linkage with the population registry of their first residence was possible (n = 34,394). Assignment of Aussiedler to the different federal states and municipalities within Germany is done according to a quota considering the population size of the respective state and municipality. For example, North-Rhine Westphalia, where about 21% of the German population live, receives 20.8% of all Aussiedler coming to Germany 24. Arriving Aussiedler can express a preference for a state but respective assignment is by no means guaranteed. This can be considered as a quasi-random procedure and therefore we assume that our selection technique delivers a random sample of all Aussiedler who arrived in Germany during the study period 528.

For the cohort, we assessed vital status, i.e. if a person is either alive or deceased, at the end-of-follow-up, December 31, 2002. All observations were censored at that date. We calculated person time with the date of arrival in Germany as start and either the predefined end of follow-up, the date of death or the date of last known vital status, as end of the observation period. Person time was allocated to strata of approximately equal width formed by different values of the covariables available in our dataset (sex, age, study period, year of arrival in Germany, duration of stay in Germany, family size at arrival and population size of first residence). The definition of strata is given in table 1. Stratification according to family size upon arrival in Germany was done in the categories "arrived alone" – "arrived within a family of two to four members" – "arrived within a larger family". The rationale for this stratification is based on the finding that in Germany, singles, i.e. people without partner or spouse as well as divorced or widowed persons, have a significantly higher mortality than those living with a partner 29, and on the fact that for Aussiedler large family networks appear to play a crucial role in everyday life 25. As cut-off point for the stratification of population size of the city of arrival, we used a population size of 100,000.

For deceased participants, we ascertained cause of death coded according to ICD in an anonymous way at the state statistical office of North-Rhine Westphalia 30. In the rare case of non-availability of cause of death (4.2% of requested cases), we retrieved death certificates at local health authorities. The listed causes were subsequently coded according to ICD by professional coders at the Saarland cancer registry. We obtained all causes of death according to ICD9 for deaths which occurred before December 31, 1997, and according to ICD10 for deaths thereafter. We defined CVD deaths as deaths with an ICD9 code between 390 and 459 or an ICD10 code between I00 and I99 3132.

We calculated SMRs for all causes and CVD with observed and expected deaths for each calendar year, sex, and five-year age group. The number of expected deaths was based on publicly available mortality data and mid-year population figures for the same five-year age groups of the entire resident population of Germany 33. Regional mortality data for the state of North-Rhine Westphalia are available, but figures are highly similar to those for the whole of Germany 3334. Since our aim was to draw conclusions on a country level, we decided to use data of Germany's entire resident population to calculate expected deaths.

We performed SMR calculation in the whole cohort as well as in the different substrata. Since the percentage of cases with missing information on the cause of death was negligible, we did not apply a correction technique 35 for CVD SMRs. We calculated 95 % confidence intervals for SMRs with approximate methods when the number of observed cases was 50 or larger and with exact methods if it was smaller 36. In addition, to compare secular trends of CVD mortality in the two populations, we used direct standardisation. As standard population, we employed a truncated form (only age groups above 15 years) of the European Standard Population 37. We calculated corresponding 95 % confidence intervals from the standard errors of the age-specific rates under the assumption that their covariances are zero 36.

To assess the simultaneous influence of the several measured covariables, we used Poisson regression to obtain univariate and multivariate-adjusted estimates for the effect of covariables on SMRs 36. Here, the SMR is modelled in dependence of the values of the covariables by assuming that the number of events in a given stratum is Poisson-distributed and by using the natural logarithm of expected cases for each stratum as an offset term. The strata are formed by cross-classification of all covariables as given in table 1. From the coefficients we calculated rate ratios. Corresponding 95 % confidence intervals were obtained with maximum likelihood methods. We used SAS version 8.02 38 for all statistical analyses.

To our knowledge, this study is the first of its kind which assesses mortality among Aussiedler from the FSU in Germany. In contrast with the postulated hypothesis on the health status of migrants, the findings show that this migrant group has a significantly lower overall and CVD mortality than Germany's general population. CVD is the most important cause of death in this population, and SMR patterns for all causes of death and CVD are highly similar. This suggests that the measured covariables are associated with CVD and overall mortality in a similar way. Stratification and multivariate analyses reveal that there are considerable differences in SMRs between different age groups, calendar years, and between large and small families.

The health of migrants is a complex phenomenon 39, and thus there are no stringent simple explanations for the surprising results. The findings are in contrast to results from studies in the United States, Canada and Sweden which suggest that post-migration CVD and overall mortality in migrants is determined by mortality in the countries of origin 789. Our hypothesis which predicted a higher overall and CVD mortality might have been based on incorrect assumptions. Although overall and CVD mortality are excessively high in the FSU, a finding which is unlikely to be due to non-reliable data 4041, our assumption that this is also the case for the population of Aussiedler in the FSU may have been wrong. Specific data shedding light on this matter do not exist, and the few available studies on SES among Aussiedler in the FSU 14 might have not sufficiently been able to measure socio-economic differences that could explain mortality differentials between Aussiedler and the population average in the FSU. The special importance of "traditional" family and religious values might determine certain lifestyle-dependent risk factor profiles leading to a lower mortality. A potentially alike pattern can be found among Russian Jews for whom similar values seem to play an important role in everyday life. In Moscow, it was found that they have a mortality advantage compared to the city's general population 12. This might be one of the reasons for the finding that, contrary to what was expected 42, Jewish migrants from Russia to Israel have a similar to slightly lower CVD and overall mortality when compared to the general population of Israel, whose mortality is much lower than the FSU average 43. Nevertheless, to fully explain the low mortality found among Aussiedler in our study, based on the mortality data for the entire population of the FSU given in the introduction, their overall mortality in the FSU would need to be only 40 % and their CVD mortality only a third of the average in these countries. Intuitively, this seems improbable. However, it is known from many populations around the globe that there are huge disparities in mortality between subgroups. In most countries, women have a mortality rate which is about half that of men. Within the city of Chicago, life expectancy differs with a magnitude of more than 20 years across neighbourhoods 44. It is possible that such substantial differences exist also between Aussiedler in the FSU and the general population, but data to support this hypothesis are not available. Another possibility to verify if there are "ethnicity-specific" rather than "country-of-origin-specific" characteristics determining the mortality of Aussiedler would be to compare the mortality of Aussiedler from the FSU to that of Aussiedler from Poland and Romania in Germany, with whom they share genetic, cultural and environmental characteristics. So far, there are no mortality data for the latter.

The second assumption of our hypothesis, that socio-economic disadvantages of Aussiedler lead to higher overall and CVD mortality compared to the general population, might also be erroneous. Most studies measured only "conventional" socio-economic markers such as income, housing and occupational characteristics and did not assess more subjective attributes (e.g. life satisfaction and other psychosocial factors) 181920. "Subjective SES" is in some settings more strongly related to health outcomes than objective markers 16. Thus, higher perceived SES among Aussiedler from the FSU might contribute to lower mortality than in Germany's general population. There are hints that life satisfaction among Aussiedler is indeed higher than in their German peers 4546, something which could be favoured by the formation of an own specific "cultural identity"25 during the transition from life in the FSU to life in Germany, providing strong social cohesion with beneficial effects on health 2627. Only further in-depth studies have the potential to test for this speculative assumption.

Strong health selection during migration from the FSU to Germany, in the sense of the frequently postulated "healthy migrant effect" 3, is a rather improbable reason of the observed low mortality. It would need to be very strong in order to explain the observed low SMRs, something which is unlikely because the vast majority of the once existing population of Aussiedler in the FSU has migrated to Germany 24. Selective return migration of sick or socio-economically disadvantaged people with a higher risk of dying, another alleged explanation for observed low overall and CVD mortality in some migrant populations 394748, can be excluded in our study population, of which less than one % moved back to the FSU.

Differences in SMRs between younger and older Aussiedler might be due to a particularly poor risk factor profile in young adults, similar to what is observed in the FSU 4950, or due to age-dependent selection. In older age, where diseases, and especially CVD, often present in a limiting manner, people with prevalent disease might decide against migrating. In younger age, where sudden cardiac deaths without previous limiting disease are more common 6, there might be no such selection.

Observed secular differences in SMRs are much stronger for CVD than for all causes of death. They result mostly from a CVD mortality decline in the German population which Aussiedler from the FSU do not experience. The reasons for this decline, which is common to many Western countries are not fully understood, but advances in treatment and prevention seem to be driving forces 51. Our results imply that Aussiedler from the FSU do not benefit from these advances to the same extent as Germany's general population.

The significantly lower SMRs in Aussiedler arriving in large families and the much smaller difference in SMRs between singles and families with two to four members are also unexpected. Health effects of family and social ties are as complex a matter as migrants' health 52. The data available in this study are insufficient to provide convincing explanations. The fact that large family networks seem to play an important role for Aussiedler 15 might have helped the ones actually migrating within the texture of a large family to deal better with post-migration stress. Such family resources might be particularly helpful given that by migrating Aussiedler undergo a transition from a more "collectivistic" towards a more "individualistic" society. Without specific data, however, this hypothesis is bound to remain speculative. Consequently, to learn more about the observed association, studies focusing on the quantity and quality of inter-personal relationships of migrants, and of Aussiedler in particular, are urgently needed.

Due to its size, our study is able to provide reliable data on this population's overall and CVD mortality. Although the sampling scheme was partly guided by pragmatic concerns, a comparison shows that there are no appreciable differences with regard to the measured covariables between cohort members all Aussiedler from the FSU. One exception is that Aussiedler who had been assigned to larger cities were overrepresented in our cohort. Since we did not find any mortality differences between large and small cities, this should not have biased our results. We thus believe that our cohort constitutes a representative sample of the population under study 28.

We could not further follow up 8.5 % of cohort members after they had moved to another municipality. This would cause an appreciably bias only if the mortality in this subgroup was very different to the main part of the cohort which we think is very unlikely. The German population registry system is known to be not fully accurate 53, but potential mistakes in registration affect data from the cohort and the general population similarly. The same holds true for information on cause of death which is crucial for the calculation of CVD SMRs. Misclassification seems to be a problem 5455, but there is no indication for systematic differences between ethnic groups. Cause of death information in our study was highly complete, which minimizes bias due to unascertained cause of death and allowed us to forego SMR correction. Aussiedler receive German citizenship upon entering the country, so they are included in the comparison data for the general population of Germany. This biases SMRs towards the null 56. However, since Aussiedler from the FSU constitute less than two % of Germany's population, any resulting bias is negligible. We were confined to exclusively using data from population registries, so it was not possible to account for specific risk factors on an individual level, something which would have been highly desirable in the context of this population. Finally, we could only analyze mortality, and no objective and subjective morbidity measures.

In contrast to what was expected, overall and CVD mortality among Aussiedler from the FSU is lower than in Germany's general population. There is no single stringent explanation, which underpins the fact that the reasons for health differences between migrants and the population of host countries, a widespread phenomenon, are still far from being sufficiently understood. We observed a secular rise in CVD SMRs, possibly because Aussiedler from the FSU do not benefit from improved CVD prevention and treatment. This, together with the fact that CVD SMRs are already high among younger Aussiedler, might lead to a considerable burden of CVD in this population in the future, which suggests that Public Health should pay more attention to this group. There is a definite need to assess possible reasons of shortcomings in CVD prevention for Aussiedler. The results of such assessment should help to improve future preventive measures, especially for younger Aussiedler. To be able to draw a clearer picture of their health status, further studies of quantitative as well as qualitative nature, including the assessment of individual risk factors, family structure and social networks, are required to learn more about health risks, but also health resources, in this and other migrant populations.

CVD cardiovascular disease

FSU former Soviet Union

ICD International Classification of Diseases

SES socio-economic status

SMR standardized mortality ratio

The author(s) declare that they have no competing interests.

HB and OR conceived of the study and the research questions. All authors participated in designing the study. UR and CK collected the data. UR and CK carried out the statistical analyses with the support of HB and OR. All authors interpreted the results, drafted and revised the manuscript and read and approved its final version.