[The healthy migrant effect: role of selection and late entry bias].

Background: First-generation immigrants frequently have a lower mortality than the host population, in spite of a low socio-economic status. This is usually explained by (self-) selection into migration. If this were the case, the immigrants' mortality risk would increase with time under observation. A persistently low mortality could be due to a late entry bias: if migrants are enrolled in a study years after immigration, sick or socio-economically unsuccessful individuals may already have returned to their countries of origin. Mortality risk would then be inversely associated with length of stay in the host country before enrollment.
Methods: We assessed the mortality risk of immigrants from Mediterranean countries to Germany in the German Socio-economic Panel, in relation to time under observation (1-15 years) and length of stay in Germany before enrollment (0-34 years), using the Cox regression.
Results: In 1984-98, 2624 immigrants aged 16-83 years accrued 21,858 person years; 59 died. The hazard ratio, adjusted for age, sex and marital status, for each additional year under observation was 0.93 (95 % CI: 0.87-0.99); and for each additional 10 years in Germany before enrollment 0.49 (95 % CI: 0.27-0.89) in the age group >/= 50 years.
Conclusions: We found no evidence for a mortality increase with time under observation, suggesting that the healthy migrant effect is not primarily due to (self-)selection. The initial mortality advantage could be due to international differences in mortality patterns. A late entry bias does contribute to the persisting mortality advantage of older immigrants.