Swedish Covid-19 Investigation for Future Insights - a Population Epidemiology Approach Using Register Linkage (SCIFI- PEARL)

Background: In response to the Covid-19 pandemic, we designed and initiated a nationwide linked multi-register, regularly updated, observational study for timely response to urgent scientific questions. Aim: To describe the SCIFI-PEARL (Swedish Covid-19 Investigation for Future Insights - a Population Epidemiology Approach using Register Linkage) linked database encompassing essentially all known diagnosed Swedish Covid-19 patients plus a large general population comparison cohort and outline its utility in the current and future phases of the pandemic. Methods: Individuals with Covid-19 from the entire country are identified on a regularly updated basis, from different sources: all individuals from SmiNet, the national database of notifiable diseases, with positive SARS-CoV-2 polymerase chain reaction (PCR) test results; patients identified in the healthcare system by condition (ICD-10) or procedure codes in the National Patient Register or Cause-of-Death Register; patients identified through several disease-specific national quality registers (NQRs); and in two regions additionally patients identified in primary care. A comparison population was obtained by stratified random sampling from Swedish national population registers. Data from all these registers plus the National Prescribed Drug Register, the Cancer Register, national sociodemographic registers, some additional NQRs, the National Vaccination Register, and further data sources, are then linked to all study subjects (Covid-19 cases and population cohort). New cases in the study population and all data for all subjects are updated every few months, as required. Conclusion and Utility: The SCIFI-PEARL study cohort captures Swedish residents with Covid-19 on an ongoing basis, includes a representative general population comparison cohort, and links to a broad range of national and regional healthcare data for a comprehensive longitudinal view of the Covid-19 pandemic. By combining high-quality national registers with short time delay and continuous repeated linkage and updating, the project brings timely and internationally relevant data for epidemiological research on SARS-CoV-2. Our efforts provide an example and important learnings for similar efforts internationally in the future.