The CoNARTaS cohort is based solely on national registry data and involves no interventions or personal communication with the participants. The data extraction and linkages are approved by the different national registry keeping authorities, data protection agencies and ethical committees as required in each of the four countries. According to the national laws in Denmark and Finland, studies using only registry data do not need ethical approval from the scientific ethical committees. Permission was granted from the regional ethics committees in Norway (REK 2010/1909) and in Gothenburg, Sweden (Dnr 023-09, T431-09, Dnr 214-12, T-422-12, T-516-15, T-233-16, T-300-17, T-1144-17). For all included individuals, the unique personal PIN codes were replaced with serial numbers before data were transferred to the researchers. Thus, no directly person-identifiable data are used or transferred across borders.

For all health-related data on individuals, data security and privacy is a major ethical concern. We use a data platform for storing and analyzing data provided by Statistics Denmark. The project server can be accessed only by group members and requires a two-step login procedure. The group members are not able to extract micro data from the platform and store them at other locations. Thus, the data sharing solution prevents any uncontrolled spread of potentially sensitive data.