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 .