Data source and curation
Raw data was prepared by the Croatian Institute for Public Health (CIPH) from nationwide databases on: (i) COVID-19 laboratory test results (polymerase chain reaction [PCR]-based or rapid antigen tests [RAT]) and COVID-19 patients diagnosed on clinical/epidemiological criteria (without laboratory tests); (ii) COVID-19 vaccinations; (iii) all hospitalizations; (iv) deceased individuals; (v) Central Health Information System (CEZIH) - primary healthcare database maintained by the Ministry of Health. All subjects diagnosed with COVID-19 between February 25 2020 and October 15 2021 were identified, and data were linked to the hospitalizations database, database of deceased persons and to their primary healthcare data (January 1 2019 - October 31 2021 for all) (Figure 1A). We received anonymized merged database (Figure 1B) and we: a) excluded patients who were diagnosed by a PCR test first performed when seeking hospital services, so that only patients who could be reasonably considered as “outpatients” (diagnosed at public mass testing points, or clinical/epidemiological diagnosis by family physicians) were retained; b) excluded subjects <16 years of age; and c) excluded subjects for whom data on sex, date of birth, COVID-19 testing date/result/date of diagnosis, or vaccination status/dates were missing or were erroneously entered. We identified subjects with more than one COVID-19 episode: we considered that positive PCR/RAT tests or ICD-10 code U07.1/U07.2 entries or hospitalizations related to COVID-19 that were ≥30 days apart indicated two separate COVID-19 episodes. Only the first documented COVID-19 episode for each subject was included (Figure 1B). We set the cut-off date for COVID-19 diagnosis at August 15 2021, to allow for a follow-up period long-enough for outcomes to occur (until October 31). Finally, we identified patient subsets of interest (see Table 1 for details), their outcomes and their matching covariates (detailed in Table 2) (Figure 1B).