To estimate the income loss from having two or more diseases, over and above the independent and separate effects of having a single disease.
We used linked health income data from 2006–2007 to 2015–2016 for 25–64 years, for the entire New Zealand population. Fixed effects OLS regression was used to estimate within-individual income loss for diseases separately, and to estimate if having two or more diseases together resulted in reduced (subadditive) or additional (superadditive) income impacts (relative to adding together the income impacts for each disease when experienced singly).
Of the 169 comorbidity pairs for both sexes, 28 (17%) had a statistically significant superadditive (n=14) or subadditive (n=14) effect of having two diseases. The combined total income gain from deleting all diseases and comorbidities was US$2.269 billion (95% CI US$$2.125 to US$2.389 billion), or a 3.61% (95% CI 3.38% to 3.80%) increase in income. Of this, 8.8% or US$200 million (95% CI US$193 to US$207 million) was attributable to a tendency for comorbidity interactions to increase income loss more than expected for common disease pairings.
This national longitudinal study found that disease is associated with income loss, but most of this impact is due to the distinct and independent impact of separate diseases. Nevertheless, there was a tendency for two or more diseases to disproportionately increase income loss more than the summed impacts of each of these diseases if experienced singly.