Study design, setting, and participants
This was a cohort, retrospective study with secondary data, conducted
with the Strengthening the Reporting of Observational Studies in
Epidemiology (STROBE) guidelines for reporting observational
studies11, performed among HCWs at the CMHI in Warsaw
(see Supplementary Material ).
The CMHI is the largest tertiary paediatric hospital and research
institute in Poland, with nearly 600 beds and approximately 2,430 HCWs,
including employees and trainees. As previously
described12, beginning on March 9, 2020, a set of
infection prevention and control measures were implemented at the CMHI
to contain the spread of SARS-CoV-2 within the hospital.
The SARS-CoV-2 RT-PCR testing for symptomatic personnel, coupled with
contact tracing, was implemented starting from March 17, 2020. In
addition, since October 20, 2020 (the beginning of the second wave of
the COVID-19 epidemic in Poland), a universal SARS-CoV-2 screening
programme was deployed at the CMHI for all HCWs using a RT-PCR assay of
nasopharyngeal swabs. Up to May 6, 2021 (corresponding to the end of the
third pandemic wave), there were seven rounds of screening which made it
possible to identify and isolate asymptomatic and presymptomatic HCWs.
The HCW vaccination programme with the Pfizer-BioNTech BNT162b2 vaccine
began on January 4, 2021, and up to February 28, 2021, approximately
80% of HCWs received at least one dose of the vaccine. All HCWs
(clinical and non-clinical) who had at least one PCR test performed as
part of the universal screening were included in the study.
Demographic, occupational, epidemiological (if applicable), and
laboratory data were collected for each participant up to August 31,
2021. All data were analysed anonymously in April 2023. Details on the
data sources are provided in the Supplementary Material .
Fig. 1 illustrates the study timeline. The study population and
definition of the follow-up period for the incidence of SARS-CoV-2
infection in HCWs before vaccination (study phase 1) and after
vaccination (study phase 2) are presented in Table 1 .