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).