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).