IJE Advance Access originally published online on November 3, 2006
International Journal of Epidemiology 2007 36(1):50-57; doi:10.1093/ije/dyl239
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Socioeconomic differentials in cause-specific mortality among South Korean adolescents
1 Department of Family Medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
2 Department of Preventive Medicine, University of Ulsan College of Medicine, Seoul, Korea.
3 Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, Canada.
* Corresponding author: Department of Family Medicine, Asan Medical Center, University of Ulsan College of Medicine, 388-1 Poongnap-dong Songpa-gu, Seoul, 138-736 Korea. E-mail: hjcho{at}amc.seoul.kr
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Abstract |
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Background There is inconsistent evidence regarding the presence of a socioeconomic differential in adolescent all-cause and cause-specific mortality. This study examines possible socioeconomic mortality differentials in Korean adolescents.
Method A total of 330 321 boys and 311 830 girls aged 10–19, who are health insurance beneficiaries for civil servants and private school teachers of Korean Health Insurance Cooperation, were followed for 9 years (1995–2003). Parental income information was linked to national death certificate data.
Results For boys, all-cause mortality showed a graded inverse relationship with income level in both 10–14 year olds (RR = 1.64, 95% CI: 1.40–1.91) and 15–19 year olds (RR = 1.68, 95% CI: 1.40–1.91). The major contributor was mortality differentials from external causes, with differentials of transport accident death the most important. Mortality from circulatory disease was higher in the lowest income groups in 15–19 year olds (RR = 2.21, 95% CI: 1.09–4.50). A significant socioeconomic gradient of non-external cause mortality was found in 15–19 year olds. For girls, socioeconomic differentials were less evident than boys. The all-cause mortality gradient for girls was smaller than for boys and only significant between the lowest and the highest tertile in both 10–14 year olds and 15–19 year olds (RR = 1.33, 95% CI: 1.02–1.72, RR = 1.38, 95% CI: 1.11–1.72, respectively). There were significant socioeconomic mortality differentials in all external causes and transport accidents and a marginally significant difference in suicide mortality for 10–19 year olds. Mortality from non-external causes showed no social gradient in girls.
Conclusions Socioeconomic differentials in all-cause mortality were observed in adolescents, even in early youth. This pattern might also apply to mortality from non-external causes, especially cardiovascular disease in 15–19 year old males.
Keywords Adolescence, income, mortality, socioeconomic factor, South Korea
Accepted 3 October 2006
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Introduction |
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Health inequality continues to be an important public health issue. Although socioeconomic health inequalities are widely observed in infants,1,2 children,3,4 and adults5,6 across countries, there is mixed evidence on whether there are socioeconomic differentials in mortality7–13 and morbidity14,15 in adolescents.
In general, all-cause and external cause mortality in adolescents shows socioeconomic differentials in favour of higher socioeconomic position (SEP).4,8,11,12 For external cause death there are mixed findings. Some European studies have found evidence that socioeconomic inequalities were present in non-external deaths in adolescents.8,11,12 In a Swedish study, Ostberg showed that all-cause mortality of children of manual workers was higher than that of non-manual workers for both boys and girls aged 1–19 years.10 For boys the class gradient was present in both early (10–14 year olds) and late youth (15–19 year olds). In the US, Wise found a progressive increase in mortality associated with a decrease in family income among 10–14 year olds and, to a lesser degree, in 15–19 year olds.7
West hypothesized that there is relative equality of health (especially mortality other than external causes he called ‘medical mortality’), particularly in early youth (i.e. 10–14 year old). He suggested that this phenomenon of equalization of health in early youth is the result of a combination of the diminishing effect of home background and the increasing effects of school, peer group, and youth culture. He also argued that socioeconomic gradients ‘re-emerge’ in late youth or early adulthood due to health-related social mobility.12
Most studies on socioeconomic health inequalities during adolescence have been conducted in Western countries.4,7–12,16 Some studies used unlinked data, resulting in a possible bias in the results due to numerator-denominator problems.9,12 Several studies in Northern European countries including Sweden and Finland used data linked with personal identification to overcome the problems of bias, but did not analyse social gradients of cause-specific mortality between early (10–14 year olds) and late youth (15–19 year olds), which was the key hypothesis by West.4,8,10,16
In this study, socioeconomic inequalities in mortality among South Korean boys and girls were examined with individually linked mortality follow-up data. This is the first study among Asian populations to examine socioeconomic differentials in cause-specific mortality in adolescents. South Korea is interesting because it experienced rapid economic growth since the 1960s with per capita GNI 254 USD in 1970 to more than 16 251 USD in 2005.17 In Korea, the military government was in power for >20 years from 1961. Following the victory of the civil movement, a democratically elected government has been in control since 1987. Korean adolescents are less independent and more strongly influenced by family than those of Western countries because of the influence of traditional Confucianism, but with the rapid economic development and modernization their relationship with the older generation has changed rapidly and the generation gap has increased.18
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Methods |
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Study subjects
This study was approved by the Institutional Review Board of the Asan Medical Center, Seoul, South Korea. Data from adolescents were obtained from the civil servants and private school teachers' health insurance beneficiary dataset, initiated at the end of 1994 by the National Health Insurance Cooperation, South Korea. The beneficiary data included all government employees, teachers, and their dependents in 1994. Of the total 4 055 177 subjects aged 0 or over, this study included 330 321 boys and 311 830 girls aged 10–19. Study subjects represent 8.1% of total Korean adolescent population in 1994 (7 887 315) and the proportion of males is the same as in the total population (51.3%). In general study subjects are in more stable jobs and have a higher income than the general population. This data contained unique 13-digit personal identification numbers that enabled us to link to mortality data from the Korean National Statistical Office. During the 9 year mortality follow-up between January 1995 and December 2003, a total of 1615 boys and 793 girls died.
Socioeconomic position and outcome variables
The socioeconomic position (SEP) measure for this study was the individual income level, and the level of health insurance contribution was used as a proxy measure of it. Since health insurance contributions in South Korea are imposed proportionally on the monthly salary for government employees and teachers, this SEP measure directly reflects the parental salary level for study subjects. A total of 53 levels in 1994 were initially used for the determination of health insurance contribution. The present study categorized them into tertiles. In 1994 the monthly salary level of the lower third was 
769 USD and that of the higher third was 1160 USD. The outcome variables for this study were all-cause and cause-specific mortality. Using causes of death coded in death certificates per the International Classification of Disease, 10th Revision (ICD-10), four broad causes were identified: (i) all cancers (C00–C97), (ii) diseases of the circulatory system (I00–I99), (iii) all external causes (V01–Y98), and (iv) other causes. For external causes, mortality differentials in transport accidents (V00–V97), accidental drowning (W65–W74), and suicide (X60–X84) were examined.
Analysis
Boys and girls were analysed separately in two age groups, 10–14 and 15–19 years. Cox proportional hazards models were used to estimate relative risks (RR) for mortality associated with income level and their 95% confidence intervals (CI). All statistical analyses were performed with SAS statistical software.
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Results |
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Age-specific number of deaths per person-year by income in both genders
Table 1 shows 54.4 deaths and 28.3 deaths per 100 000 person-year in 10–19 year olds for boys and girls, respectively. Generally the mortality rate was higher in late youth than early youth for both sexes. The inverse relationship between income and death rate was evident only for boys.
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Socioeconomic mortality differentials in boys aged 10–19
Table 2 shows the associations between mortality and income level among boys aged 10–19. A graded inverse relationship was found in all-cause mortality among boys. The lowest income tertile group showed greater mortality risk in both 10–14 year olds (RR = 1.64, 95% CI: 1.40–1.91) and 15–19 year olds (RR = 1.68, 95% CI: 1.40–1.91) compared with the highest income group. The difference from external mortality was the main contributor to this socioeconomic gradient. More than two-thirds of deaths were from external causes in both 10–14 and 15–19 year olds. Excessive risk from external mortality for the lowest group was 17.8 deaths per 100 000 person-year in 10–14 year olds and 24.4 in 15–19 year olds compared with the highest income group. The external cause mortality risk of those in the lowest income tertile was 2.01 times higher compared with that of boys in the highest tertile in 10–14 year olds and 1.69 times higher in 15–19 year olds. Mortality differentials from external causes originated mainly from the mortality gradient of transport accidents. Nearly half of the mortality from external causes was from transport accidents. Compared with the highest income tertile, mortality of the lowest tertile from transport accidents was 2.66 times higher in 10–14 year olds and 2.15 times higher in 15–19 year olds. The socioeconomic gradient in mortalities was not evident for accidental drowning and suicide when it was analysed separately in early and late youth, however, suicidal death showed a significant socioeconomic gradient when we aggregated the subjects in total.
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When we considered causes other than external deaths, there was a significant socioeconomic gradient such that mortality of the lowest income group was 1.38 times (95% CI: 1.12–1.71) the highest income group in 10–19 year olds and 1.51 times (95% CI: 1.13–2.02) in 15–19 year olds. This gradient mainly originated from the results of circulatory disease mortality.
For circulatory disease, socioeconomic gradients in mortality were found among those aged 15–19 (RR = 2.21, 95% CI: 1.09–4.50). Of 56 deaths, cerebrovascular accidents accounted for 28.6% (N = 16), followed by cardiac arrest (23.2%, N = 13) and acute myocardial infarction (17.9%, N = 10). A mortality gradient was also seen in ‘other causes’, especially in 15–19 year olds, in which the lowest income group showed 57% more risk than the highest group. In our data, ‘other causes’ of death among boys aged 15–19 included neurologic diseases (N = 37), infectious and respiratory disease (N = 17), disease of urinary system (N = 10), disorders of myoneuronal junction and muscle (N = 7), congenital anomaly (N = 6), and ill-defined causes (N = 24).
Socioeconomic mortality differentials in girls aged 10–19
Table 3 shows that there was no clear graded socioeconomic differentials of all-cause mortality in girls compared with boys. An increased risk of all-cause mortality was seen in both 10–14 year olds (RR = 1.33 95% CI: 1.02–1.72) and 15–19 year olds (RR = 1.38, 95% CI: 1.11–1.72) in the lowest income level compared with the highest group; however, the size of the gradient was smaller than that of boys.
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Mortality from external causes was 58.0% of all-cause mortality in girls (460 of 793), but the proportion of mortality from transport accidents among external causes was lower compared with boys (34.8% in 10–19 year olds). The proportion of suicidal deaths was higher than transport accident deaths in girls than in boys (39.6%, 182 of 460), but suicide mortality of boys was higher than girls. Separate, age-specific analyses with 10–14 and 15–19 year old girls showed that, with the exception of external causes in the 15–19 year old group, cause-specific mortality gradients were not evident. This is partly due to the small number of deaths. However, there were significant socioeconomic mortality differentials in all external causes and transport accidents, and a marginally significant difference in suicide mortality (RR = 1.40, 95% CI: 0.99–1.99) with all age groups combined. Unlike boys, mortality from non-external cause showed no social gradient among girls.
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Discussion |
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Results of this study showed a clear socioeconomic gradient for all-cause mortality in boys, but a less evident pattern in girls. This gradient originated mainly from mortality from external causes, with mortality of transport accidents being the most important contributor followed by suicide. This study showed that transport accidents were the most common cause of death among adolescents (10–19 year olds) of both sexes in South Korea. In fact, the mortality rate of transport accidents was 15.1/100 000 in 2003, second only to Greece among OECD countries.19 A socioeconomic gradient of transport accidents was also found in Korean adults, where mortality from transport accidents for those with no formal education was 8.2 times higher than those with college or higher education.20
Our results are consistent with other studies that have shown socioeconomic gradients of transport injuries in terms of morbidity and mortality among children and adolescents.21–24 This gradient might be associated with various environmental factors.25 Among them, differential exposure to various hazards would be important. All types of transport injuries including pedestrian, bicycle, motorcycle and car injuries for children and adolescents were more prevalent in groups with lower SEP.22 Impaired driving from alcohol or drug use occurred more frequently in the lower education group, resulting in more severe and fatal injuries.25 In addition, non-use of safety belts, which could be a cause of more serious and fatal injuries,26 was also more prevalent in low SEP adolescents in South Korea.27
Because there is a discrepancy between no consistent gradient in injury morbidity and a clear inverse socioeconomic gradient in injury mortality,24 access to and quality of health care services may also be a contributing factor to the mortality differential. A high proportion of out-of-pocket expenditures in the South Korean health care system (41.9% in 2003)28 may have differentially prevented adolescents in lower SEP from using quality emergency services.
The proportion of suicide death among external causes for girls was higher than boys, but, suicide mortality of boys (8.6/100 000 person-year) was higher than girls (6.5/100 000 person-year), which was consistent with the studies in Western countries.29
Consistent with a study among Finnish youth,16 the socioeconomic gradient of all-cause mortality in early youth (10–14 year olds) was similar to late youth (15–19 year olds) in the present study. This is different however, from studies conducted by West, in which the socioeconomic gradient of all-cause mortality was absent in early youth and ‘re-emerged’ in late youth and early adulthood.9,12 In addition, mortality excluding deaths by external causes in this study showed a different pattern from all-cause mortality. Mortality from non-external causes was significantly higher in the lowest income group of boys in late youth. These findings do not fully support West's hypothesis that mortality tends to be equal in early youth and mortality gradient re-emerges in late youth and early adulthood, but rather partly support the observation that the socioeconomic gradient is smaller for non-external causes of death. Youth culture may have some effects in reducing socioeconomic gradient for non-external cause in early youth, but it is too early to conclude that it is the only cause of equalization of mortality in early youth. Many indicators related to quality of life (e.g. feeling of loneliness, satisfaction of life, and perceived level of stress) and health-related behaviours (e.g. smoking rate, bullying and violence) also show social gradients.30 Further study is needed to investigate the mechanisms underlying them.
For boys in the 15–19 year old group, death from non-external causes showed an inverse relationship with income level, and cardiovascular mortality showed a socioeconomic gradient. Among cardiovascular disease, acute myocardial infarction and cerebrovascular accidents were the main causes of death. Binge drinking can be one of the plausible mechanisms for cardiovascular mortality differentials. Binge drinking is known to be associated with higher risk of stroke,31 coronary events and death from ischemic heart disease,32,33 cardiac arrhythmia,34 and sudden cardiac death,35 even in younger men. Binge drinking is more prevalent in males than females and most prevalent among adolescents and young adults.36 Moreover, the prevalence of binge drinking or alcoholism among adolescents and young adults is higher in lower SEP both in Western countries36–39 and in Korea.27 There is evidence of atherosclerosis even in children and adolescents,40 and smoking, one of the risk factors of atherosclerosis, by Korean boys is relatively high and more prevalent in lower socioeconomic group.30 How much atherosclerosis can contribute to cardiovascular death in adolescents is not certain. However, more studies to investigate the pathways of mortality gradient in adolescents are required.
Compared with girls, boys showed a clear graded pattern of mortality differentials for all-cause and some cause-specific mortalities (i.e. all external causes, circulatory disease and other causes) among 10–19 year olds. In general, the magnitude of mortality differentials measured by RR in boys was greater than those in girls. These findings are compatible with studies in Western countries.4,8,10
One strength of the present study is that we used parental income level as SEP indicator for adolescents. According to West, as parental influence diminished, socioeconomic equalization in adolescent health and a ‘re-emergence’ of the socioeconomic gradient in late youth or early adulthood would be due to lifestyles independent of social class of origin.12 This study did not support this mechanism. Parental SEP was shown to affect socioeconomic gradients until late adolescence in South Korea.
Another strength of this study is the relatively large dataset that was linked individually with national death certificate data. This overcame the problem of numerator/denominator bias. Because there are only a few cohort studies examining the socioeconomic gradient of adolescent mortality outside of Western countries, this study adds to the body of science in examining the mechanism of adolescent mortality differentials. Additionally, cause of death was based on legal death certificates from the Korean National Statistical Office. Because 96.2% of adolescent death was verified by certification of physicians, completeness of cause of death was unlikely to bias the results.41
This study has limitations. First, insurance contribution used as a socioeconomic indicator was only a proxy measure of individual income with other sources of income unknown. Second, additional individual level information from adolescents was not available to fully understand the mechanism of the associations shown in this study. Lastly, the size of the socioeconomic differences in this study may be underestimated because the study population comprised civil servants and private school teachers who were in relatively stable jobs.
In conclusion, socioeconomic differentials are present in all-cause and external cause mortalities among male and female adolescents in Korea. Moreover, socioeconomic inequality is also observed in non-accidental deaths, in particular cardiovascular disease, among 15–19 year old male adolescents. The magnitude of socioeconomic differentials in all-cause and cause-specific mortality among boys was greater than those in girls.
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Acknowledgments |
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This work was supported by Korea Research Foundation Grant (KRF-2004-041-E00103).
Conflict of Interest None declared.
KEY MESSAGES
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