Cross sectional analysis of mortality by country of birth in england and wales, 1970-92BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7082.705 (Published 08 March 1997) Cite this as: BMJ 1997;314:705
- a Epidemiology Unit, Department of Epidemiology and Population Sciences, London School of Hygiene & Tropical Medicine, London, WC1E 7HT
- Correspondence to: Dr Wild
- Accepted 18 December 1996
Objective: To compare mortalities for selected groups of immigrants with the national average.
Design: Analysis of mortality for adults aged 20-69 in 1970-2 and 1989-92 using population data from 1971 and 1991 censuses. Mortality of Scottish and Irish immigrants aged 25-74 was also compared with mortality in Scotland and Ireland for 1991.
Setting: England and Wales.
Main outcome measures: Standardised mortality ratios for deaths from all causes, ischaemic heart disease, cerebrovascular disease, lung cancer, and breast cancer.
Results: In 1989-92 mortality from all causes was higher than the national average for Scottish immigrants, by 32% for men and 36% for women; for Irish immigrants it was higher by 39% for men and 20% for women; and for Caribbean born men it was lower by 23%. Ischaemic heart disease and lung cancer accounted for 30-40% of the excess mortality in Scottish and Irish immigrants. For south Asians, excess mortality from circulatory disease was balanced by lower mortality from cancer. Standardised mortality ratios for cerebrovascular disease in 1989-92 were highest for west African immigrants (271 for men and 181 for women).
Conclusions: Widening differences in mortality ratios for migrants compared with the general population were not simply due to socioeconomic inequalities. The low mortality from all causes for Caribbean immigrants could largely be attributed to low mortality from ischaemic heart disease, which is unexplained. The excess mortality from cerebrovascular and hypertensive diseases in migrants from both west Africa and the Caribbean suggests that genetic factors underlie the susceptibility to hypertension in people of black African descent.
Differences in mortality of immigrants in England and Wales widened between 1970 and 1992 and could not be explained by socioeconomic differences
Mortality from all causes was higher than average for Scottish and Irish immigrants, and some of the excess could be accounted for by smoking habits, reflected by increased mortality ratios for lung cancer
Mortality ratios for stroke and hypertension were highest in west African immigrants, indicating that better detection and treatment of hypertension is required in this group
Mortality from all causes was low in Caribbean immigrants, largely due to low mortality from ischaemic heart disease
Mortality from ischaemic heart disease remained high in south Asian immigrants and did not decline as fast as in the general population
Country of birth has been recorded on death certificates in England and Wales since 1970. Two previous studies of immigrant mortality indicated differences by country of birth that varied with cause of death.1 2 Our analysis of mortality by country of birth in 1989-92 provides an updated version of previous analyses and permits examination of mortality trends by country of birth.
Subjects and methods
Data on populations
We used published population data from the 1971 and 1991 censuses of England and Wales for analyses of mortality of immigrant groups and from 1991 censuses of Scotland, Northern Ireland, and the Republic of Ireland for analyses of deaths in these countries.3 4 5 6 7 Our study included immigrants from Scotland, Ireland, Africa, the Caribbean Commonwealth, and the Indian subcontinent (south Asia).
We analysed separately data for immigrants from east African countries and for those from west and south African Commonwealth countries (abbreviated to east Africa and west Africa respectively). Previous studies combined data on all immigrants from east Africa, 68% of whom are of south Asian origin, and from west Africa, 73% of whom are of black African origin.3 White immigrants were the second largest ethnic group from both of these groups of countries (16% and 22% of immigrants from east Africa and west Africa respectively).3 The countries included in “east Africa” were Kenya, Malawi, Tanzania, Uganda, and Zambia, while those included in “west Africa” were Gambia, Ghana, Sierra Leone, Nigeria, Botswana, Lesotho, Swaziland, and Zimbabwe. We combined the Republic of Ireland and Northern Ireland and grouped together the countries in south Asia (Bangladesh, India, Sri Lanka, and Pakistan) to avoid numerator-denominator bias (that is, when country of birth is recorded differently on census and death certificate). This has been identified as a particular problem with data on death certificates for people born in these groups of countries.1 In our tables we presented the countries in the order in which they appeared in the census.
Data on deaths
The Office of Population Censuses and Surveys provided data on deaths by country of birth, sex, and age for people aged 20-74 years who died in England and Wales between 1970 and 1992. We obtained data on deaths by sex and age for England and Wales for 1971, and for Scotland and the Irish republic and Northern Ireland for 1991 from the World Health Organisation's statistics annuals, which provide data in 10 year age bands for people aged 25-74.4 8 4 We recorded the underlying cause of death, coded according to the International Classification of Diseases, eighth and ninth revisions (ICD-8 and ICD-9).10 11
We examined deaths of people aged 20-69 by country of birth for 1970-2 and 1989-92. Consistent with previous studies of immigrant mortality, the reference death rates we used for calculating standardised mortality ratios were generated using numbers of deaths and population figures by sex and five year age group for the whole population of England and Wales during 1989-92. We calculated standardised mortality ratios and 95% confidence intervals by sex and country of birth for people aged 20-69 using conventional methods.12
Population data for Scottish and Irish immigrants in England and Wales were not available for the age range 25-74 for 1971, and we therefore could not compare mortality trends from 1971 to 1991 for immigrants with their countries of origin. We calculated standardised mortality ratios for Scotland and Ireland and for Scottish and Irish immigrants aged 25-74 in 1991 using death rates in England and Wales in 1991 as the standard.
Between 1971 and 1991 the proportions of Caribbean and south Asian immigrant populations in older age groups increased, because the largest influx of immigrants from these countries occurred before 1965. Of the 20-69 year old population counted in the census, the proportion aged 50-69 increased among Caribbean immigrants from 12% in 1971 to 49% in 1991 and increased among south Asian immigrants from 18% to 31%. The size of the Caribbean immigrant population aged 20-69 stayed roughly constant (237 085 in 1971 and 241 265 in 1991), but the south Asian population in this age range almost doubled (351 120 in 1971 and 634 187 in 1991). As a result of these factors, the numbers of deaths of immigrants born in the Caribbean and south Asia increased between 1971 and 1991.
With continuing migration to and from Scotland and Ireland, the age distributions of Scottish and Irish immigrants remained roughly constant.
The proportions of deaths in England and Wales accounted for by immigrants from the countries studied were 5.5% in 1970-2 and 9% in 1989-92. The age distributions of the populations by country of birth differed from that of the whole population of England and Wales as shown in table 1).
Mortality from all causes
Mortality ratios for deaths from all causes were higher than the national average for all the immigrant groups except for Caribbean immigrants (table 2)). Mortality from all causes fell in all groups between 1971 and 1991, with the national average declining 31% for men and 24% for women. Declines were least for men and women born in Scotland (16.5% and 1.5% respectively) and greatest for men and women born in the Caribbean (41% and 49%). The relatively small decline for Scottish immigrants, a group with high mortality ratios from all causes, and the large decline for Caribbean immigrants, whose mortality from all causes was already low, indicates widening differences in mortality by country of birth over the 20 year period.
This pattern was generally repeated for specific causes of death, except for the substantial fall in cerebrovascular mortality ratios for Caribbean immigrants. Ischaemic heart disease, cerebrovascular disease, and lung cancer accounted for 40% of the excess deaths in Scottish and Irish immigrants. For Caribbean immigrants the lower mortality from ischaemic heart disease outweighed the higher mortality from cerebrovascular disease; lower mortality from ischaemic heart disease and lung cancer accounted for 88% of the difference in mortality from all causes between Caribbean men and the national average.
Ischaemic heart disease was the leading cause of death for men and women for all population groups with the exception of women immigrants from west Africa, who were relatively young. Standardised mortality ratios for ischaemic heart disease were highest for south Asian men and women and east African men and lowest for Caribbean and west African immigrants in 1989-92 (see table 3)). Table 4) shows standardised mortality ratios for ischaemic heart disease by different age groups for 1989-92: differences between men by country of birth were even greater for the youngest age group (20-44 years).
From 1971 to 1991 the national average standardised mortality ratios for ischaemic heart disease fell by 29% for men and by 17% for women. Caribbean immigrants showed a steeper decline (38% for men and 40% for women), and south Asian immigrants showed a shallower decline (20% for men and 7% for women).
Mortality ratios for cerebrovascular disease were higher than the national average for all immigrant groups in 1989-92, the highest ratios being for west African immigrants (table 5)). The national average decline in age standardised mortality from cerebrovascular disease over the study period was 50% for men and 52% for women. The greatest decline occurred in Caribbean immigrants (57% for men and 66% for women), but their standardised mortality ratios for cerebrovascular disease remained elevated. Scottish, Irish, and south Asian immigrants showed smaller declines in mortality from cerebrovascular disease than the national average.
Analyses of mortality from hypertensive diseases (ICD codes 401-405) were based on small numbers of deaths. They showed a similar pattern of differences between immigrant groups as did those for cerebrovascular disease, though the actual differences were more pronounced. Standardised mortality ratios were 813 for west African men (95% confidence interval 503 to 1242, based on 21 deaths), 373 for Caribbean men (296 to 462, 82 deaths), 944 for west African women (432 to 1792, 9 deaths), and 668 for Caribbean women (524 to 840, 73 deaths).
Mortality ratios for lung cancer were high for men and women from Scotland and Ireland and low in the other immigrant groups (table 6)). In men the mortality ratios declined by between 18% (for Scottish men) and 41% (for south Asian men). In women, however, the mortality ratios increased: the national average by 47%, by 64% for Scottish women, and by 25% for Irish women. Relative risks of mortality from lung cancer by country of birth were roughly equal in 1971 and 1991.
There were relatively small numbers of deaths from breast cancer in immigrant groups, but women born in Ireland, the Caribbean, and south Asia showed low mortality ratios (table 7)). From 1971 to 1991 the national average mortality ratio for breast cancer did not change, and low rates in Caribbean and south Asian women persisted.
Comparison with mortality in Scotland and Ireland
Mortality ratios for all causes and for ischaemic heart disease were higher in Scotland and Ireland than in England and Wales (table 8)). Scottish men and Irish men and women who were immigrants showed higher standardised mortality ratios for all causes than did the comparable groups resident in Scotland and Ireland. Irish immigrants showed greater excess mortality from all causes over the population resident in the country of origin than did Scottish immigrants. Mortality from ischaemic heart disease was higher for residents in Scotland than for Scottish immigrants but was higher for Irish immigrants than for residents in Ireland.
Validity of results
The estimation of trends in mortality involves the comparison of two standardised mortality ratios, which is valid only if the proportional hazard model applies. When relative risks differ between age groups, as for mortality from ischaemic heart disease, it is more appropriate to report age-specific mortality ratios as in table 4).13 The low standardised mortality ratios for all causes in Caribbean immigrants were mainly attributable to low mortality from ischaemic heart disease. It is unlikely that the effect of return migration of sick people could be entirely responsible for the low mortality because about half of all fatal coronary events occur in the absence of previous symptoms.14
Overall, no census data were obtained for 2.2% of the population in 1991. Completeness of enumeration varied with population characteristics, and underenumeration was greatest in Afro-Caribbean men aged 20-29.15 16 17 The effect of such underenumeration would be to increase apparent mortality in affected populations, which could not explain the low death rates found for Caribbean immigrants.
Reasons for different mortality patterns
Mortality patterns for 1989-92 were similar to those reported around the 1971 and 1981 censuses.1 2 One possible explanation for high mortality ratios in men and women born in Scotland and Ireland is selection of less fit individuals at migration. Ill health is less likely to inhibit migration to England and Wales for people born in Scotland and Ireland than it is for people from more distant countries for which there are restrictions on immigration. The persistent excess mortality in second generation Irish people in England and Wales,18 19 however, is not consistent with a selection explanation.
Previous studies have shown that social class did not account for the increased mortality in Irish immigrants, which was higher than the average for England and Wales within each social class.18 20 Mortality from lung cancer can be used as a proxy measure of smoking in populations, and the high standardised mortality ratios for lung cancer in Scottish and Irish immigrants suggest that smoking habits may account for some of the excess of ischaemic heart disease.21
Adverse environmental conditions (deprivation, poor working conditions, lack of social support, unhealthy lifestyle, etc) in the country of origin may have long term effects on immigrants and may persist or develop in the host country.22 The low mortality from all causes that we observed in Caribbean immigrants cannot be explained by a favourable social class distribution because a higher proportion of people of Afro-Caribbean ethnic origin are manual workers (70%) than in the whole population (52%).3
High standardised mortality ratios for ischaemic heart disease and cerebrovascular disease in south Asian men and women are consistent with the high rates of ischaemic heart disease in this group worldwide and are associated with increased prevalences of central obesity and insulin resistance.23 It is not clear why Afro-Caribbean people, in whom prevalences of diabetes and hypertension are high, have relatively low rates of ischaemic heart disease.24 25 The relative advantage of Caribbean men with regard to mortality from all causes and from ischaemic heart disease was preserved from 1971 to 1991, in contrast with the expectation that mortalities of immigrant populations would approximate to those of the host population over time.
High rates of cerebrovascular disease in Caribbeans and west Africans are consistent with survey data showing that they have higher mean blood pressures than Europeans in London24 and high prevalences of hypertension in their countries of origin.26 As migrants from the Caribbean and from west Africa have not shared a common environment for the past 300 years, a genetic explanation for the susceptibility to hypertension in people of west African descent is likely. The high mortality from diseases related to hypertension in migrants from west Africa does not support the hypothesis that black people in the United States are more prone to hypertension as a result of selective survival of slaves able to retain salt.27 The extremely high mortality from cerebrovascular disease in Caribbean immigrants noted around the 1971 census became less pronounced in 1991, suggesting that detection and control of hypertension have improved in this group. In a recent survey more than two thirds of Afro-Caribbean men and women with hypertension were found to be receiving treatment.24
The findings of our study suggest a need for general health promotion programmes targeted at Irish and Scottish immigrants and specific health promotion programmes focused on preventing cerebrovascular disease in west Africans.
We thank Tim Devis and Nicola Hayden of the Office of National Statistics for supplying the death data and Seeromanie Harding for her comments.