Feasibility of cohort studies in Estonia

Objectives-To describe the methodology and feasibility of follow up for vit al status in retrospective cohort studies in Estonia. Methods-A cohort of 7412 workers who had been employed at two factories in Tallinn between 1946 and 1988 was followed up for vital status from the dat e of first employment until death, emigration, or the end of the study, 31 December 1995, whichever occurred first. The cohort was electronically link ed with the National Population Registry of Estonia that was created in 199 2 and includes personal identification numbers of Estonian citizens and res idents, and the Mortality Database that contains information from death cer tificates issued in 1983-95. A manual search was carried out on several non -computerised population data sources and archives. Results-By 31 December 1995, the vital status of 6780 (91.5%) subjects coul d be traced (4495 (60.6%) subjects were alive, 1993 (26.9%) had died, and 2 92 (3.9%) had emigrated). Analysis by calendar period of leaving work showe d that the proportion of subjects traced was lowest in the group of workers who had left work between 1946 and 1955 (58.4%), especially those whose ag e at leaving work was <30 (53.2%) or >60 years (42.3%).Among subjects who l eft work in 1956-65, 1966-75, and 1976-88, the follow up rate was 84.7%, 94 .6%, and 98.2%, respectively. Conclusions-The findings, which are especially important for occupational e pidemiology, confirm the feasibility of conducting retrospective cohort stu dies in Estonia. Most of the issues discussed in the paper apply to other f ormer Soviet countries.