Sensitivity analysis
Sensitivity analysis was performed to examine the robustness of the results. Complete case analysis was performed (i.e. after inclusion of previously infected HCWs, those with missing vaccination status, or no follow-up data) to cover more possible events and to investigate influences of missing data on the study results (results of the sensitivity analysis are presented in the Supplementary Material ).
RESULTS
Of the 2,332 HCWs who participated in the universal screening programme, 1,967 (84.3%) were female. The median age was 46.9 (IQR: 36.4–55.4) years. Approximately one third (32%) were nurses. One hundred and eighty-four (7.9%) HCWs worked in the COVID-19 area (Table 2 ).
Beginning on March 17, 2020 (the first case of a SARS-CoV-2 infected HCW identified at the CMHI) and up to October 19, 2020 (the start of universal screening of HCWs), we identified 86 HCWs with SARS-CoV-2 infection from the 2,332 HCWs, which corresponds to a baseline prevalence of 3.7%.
In total, 11,797 samples from 2,332 HCWs were tested as part of the universal screening programme between October 20, 2020, to May 6, 2021. After excluding 199 (1.7%) invalid test results, the median number of tests per person was 6 (IQR: 3–7). In addition, 1,298 PCR test results performed outside the screening and up to August 31, 2021, were available for 934 (40.1%) of the 2,332 HCWs (median 1 sample per person, IQR: 1–2).
Overall, 457 (19.6%) of the 2,332 HCWs had positive SARS-CoV-2 PCR test results within the study period, including 452 new infections (19.4%) and 8 (0.2%) reinfections. Almost half (45.1%) of the infections were asymptomatic and 48.6% were identified during the universal screening programme. Among 281 HCWs with laboratory confirmed SARS-CoV-2 infection, for whom data on the probable source of infection were available, the most common source of infection was the community (44.1%), followed by the household (32.7%), while the rate of infection at the workplace was only 14.2%.
Fig. 2 shows the incidence of SARS-CoV-2 infection among HCWs at the CMHI and in the general population in the Mazovian voivodeship and Fig. S1 reports the new weekly cases of SARS-CoV-2 infection among HCWs at the CMHI by testing mode. The weekly incidence of new cases among HCWs at the CMHI fluctuated during the study period, corresponding to the dynamic of SARS-CoV-2 transmission in the community. In particular, most of the cases were identified between epidemiological (epi) week 43 and 49, 2020, which encompassed the peak of the second epidemic wave in Poland. The incidences among the HCWs decreased during the first three months of the vaccination programme and then increased again and peaked at epi week 13, 2021, which corresponded to the peak of the third wave in Poland, caused by variant Alpha (B.1.1.7; Fig. S2 ). This later peak of incidence was relatively lower among HCWs than in the community.