Educational and income inequalities across diseases in Denmark: a register-based cohort study

Background Educational attainment and income are often, but not always, associated with disease incidence. Existing 
research typically examines single diseases, resulting in disparate analyses with little comparability. In this study, we 
aimed to assess educational and income inequalities across diseases in Denmark.
Methods This register-based study included all Danish residents aged at least 30 years between Jan 1, 2013, and 
Dec 31, 2022, who were born after 1920, and with known educational attainment and income. We used a disease-wide 
approach to assess associations between education and income and the incidence of 751 diagnostic codes determined 
upon hospital admission. We estimated age-standardised incidence rates and incidence rate ratios (IRRs) using 
Poisson regression, adjusted for birth cohort and stratified by sex. Participants were followed up until time of 
diagnosis, death, emigration, or until Dec 31, 2022.
Findings 4541 309 individuals aged 30 years and older were registered as living in Denmark between Jan 1, 2013, and 
Dec 31, 2022. 121 083 were excluded due to limited or missing information about educational attainment. 
4 420226 individuals were included in the analysis of educational inequalities (2232200 [50%] were female and 
2 188026 [50%] were male). 23 708 were excluded due to absence of income information, and 4 396518 were included 
in the analysis of income inequalities (2223217 (51%) were female and 2173 301 (49%) were male). Socioeconomic 
differences in incidence rates were observed across all disease groups; incidence rates of most diseases decreased 
with higher educational attainment and income. The magnitude of the socioeconomic inequalities varied substantially.
Among non-communicable diseases, the strongest positive association with regard to education was observed in 
chronic obstructive pulmonary disease for female individuals (low education vs high education, IRR 2·7 [95% CI 
2·4–3·0]) and schizophrenia for male inividuals (low education vs high education, IRR 4·4 [2·2–8·8]), and the 
strongest negative association was in melanoma and other skin cancers for females (low education vs high education, 
IRR 0·7 [0·7–0·8]) and melanoma and skin cancers for males (low education vs high education, 0·7 [0·6–0·8]). With 
regard to income, for females, the strongest positive association was observed in schizophrenia (quartile 1 [Q1] vs
quartile 4 [Q4], IRR 10·1 [6·1–17·2]), whereas the strongest negative association was in melanoma and other skin 
cancers (Q1 vs Q4, IRR 0·5 [0·5–0·6]). For males, the strongest positive assocation was schizophrenia (Q1 vs Q4, IRR 
18·4 [95% CI 8·5–39·9]) and the strongest negative association was also melanoma and other skin cancers (Q1 vs Q4, 
IRR 0·5 [0·5–0·6]). The most prevalent disease category, other digestive diseases, was also strongly positively 
associated with education (low education vs high education, IRR 1·6 [95% CI 1·6–1·6] for females; IRR 1·5 [1·4–1·5] 
for males) and income (Q1 vs Q4, IRR 1·5 [1·5–1·5] for females; IRR 1·3 [1·3–1·4] for males). 
Interpretation Our study provides a detailed representation of the association between two socioeconomic indicators 
and disease incidence. A broad spectrum of diseases, and not only the most prevalent, show socioeconomic disparities. 
This finding highlights the need for not only policies that address specific diseases, but also universal policies 
addressing the root causes of socioeconomic disparities and their health consequences