estimates
We used a survival analysis framework with calendar time as the underlying time scale, assigning time zero to the first day of each observation period. We excluded person-time at-risk from the date of receipt of a vaccine dose until 13 days after. Follow-up started at the beginning of the observation period and ended at the earliest occurrence of any of the following event: 1) date for the outcome of interest, 2) death of any cause, 3) date of discontinuation in the administrative database (e.g., emigration), or 4) administrative censoring (eight weeks after time zero).
We estimated adjusted hazard ratios (aHR) of each outcome and 95% confidence intervals (CI) with Cox proportional hazards models. The adjustment variables included sex, age group (in 5-year age bands), previous SARS-CoV-2 infection, comorbities (with the exception of Luxembourg) and other variables relevant at each study site (Supplementary material, Appendix 2). We computed VE as: VE = (1-aHR) x 100%, using the unvaccinated as the reference group in the primary analysis. To estimate the rVE of booster doses, we considered those with complete primary vaccination ≥169 days ago as well as those with first booster ≥90 days ago as a reference. All estimates were stratified by age group (65-79 or ≥80- years old).
For data protection reasons, sites reported aHR estimates only when at least 5 events per vaccination status category were observed. All sites fulfilled ethical and data protection requirements according to their national legislation (Supplementary material, Appendix 3).