Study design and setting
Using a common protocol , Belgium, Denmark, Luxembourg, Navarre (Spain),
Norway and Portugal constructed population cohorts based on data
collected routinely in EHR. Countries were recruited based on their
assessed capability to join the study and a formal outreach performed by
the ECDC. We used individual deterministic linkage to cross-match
administrative population and statistical office databases with
registers for COVID-19 vaccination, SARS-CoV-2 testing,
hospitalisations, deaths and clinical data. Belgium was unable to
include unvaccinated individuals, but followed the same protocol to
compare individuals with different vaccination status to provide
relative VE (rVE) estimates. A description of the EHR used by each study
site to monitor COVID-19 VE is provided in the supplementary material
(Supplementary material, Appendix 1).
We estimated COVID-19 VE with monthly frequency. For each month, the
observation period covered eight weeks to allow sufficient events to
provide precise estimates and to be sensitive to changes in VE over
time. Overall, the observation period was between October 2021 and
November 2022. Between October 2021 and March 2022, we piloted a common
protocol for an outcome of COVID-19 hospitalisation in four study sites:
Denmark, Navarre (Spain), Norway, and Portugal . From March 2022
onwards, we added the outcome of COVID-19-related death, from April 2022
onwards, we added rVE estimates and, from June 2022 onwards, we added
two study sites (Belgium and Luxembourg).