OBJECTIVES: The 'healthy migrant effect' (HME) hypothesis postulates that health selection has a positive effect on migrants' health outcomes, especially in the first years after migration. We examined the potential role of the HME by assessing the association between residence duration and disease occurrence.

METHODS: We performed a historical prospective cohort study. We included migrants who obtained residence permits in Denmark between 1 January 1993 and 31 December 2010 (n = 114 331). Occurrence of severe conditions was identified through linkage to the Danish National Patient Register. Hazard Ratios (HRs) were modelled for disease incidence by residence duration since arrival (0-5 years; 0-10 years; 0-18 years) adjusting for age and sex.

RESULTS: Compared with Danish-born individuals, refugees and family reunited immigrants had lower HRs of stroke and breast cancer within 5 years after arrival; however, HRs increased at longer follow-up. For example, HRs of stroke among refugees increased from 0.77 (95% CI: 0.66; 0.91) to 0.96 (95% CI: 0.88; 1.05). For ischaemic heart disease (IHD) and diabetes, refugees and family reunited migrants had higher HRs within 5 years after arrival, and most HRs had increased by end of follow-up. For example, HRs of IHD among family reunited migrants increased from 1.29 (95% CI: 1.17; 1.42) to 1.43 (95% CI: 1.39; 1.52). In contrast, HRs for TB and HIV/AIDS showed a consistent decrease over time.

CONCLUSION: Our analyses of the effect of duration of residence on disease occurrence among migrants imply that, when explaining migrants' advantageous health outcomes, the ruling theory of the HME should be used with caution, and other explanatory models should be included.