Physicians, nurses and hospital attendants currently working at the University Clinic of Respiratory and Allergic Diseases Golnik were invited to participate in the pilot study. Nasopharyngeal swabs were obtained three times per week for two consecutive weeks and tested with a point-of-care rapid antigen test and reverse transcription polymerase chain reaction (RT-PCR). In addition, serum samples were obtained at the beginning of the study and 2 weeks after last swab was obtained in order to evaluate the serological status of participating health care workers.

Health care workers (HCW) currently working at the University Clinic of Respiratory and Allergic Diseases Golnik were invited to enroll in the pilot study. HCW directly involved in COVID-19 patient care and those who had a previous laboratory confirmed SARS-CoV-2 infection were not eligible for inclusion in the study. For each participating HCW, nasopharyngeal swabs were obtained. Swabs were collected on Mondays, Wednesdays and Fridays for two consecutive weeks with a maximum of six samplings per person scheduled.

Swabs were inserted into universal transport medium (UTM) and tested with SARS-CoV-2 Rapid Antigen test (Roche Diagnostics GmbH, Mannheim, Germany), in accordance with the manufacturer's instructions. Briefly, samples were thoroughly vortexed and 350 μL of UTM were mixed with the extraction buffer included in the testing kit. Three drops of the mixture were applied to the specimen well of the lateral flow test device and results were visually read after a 15 min incubation at room temperature.

Nasopharyngeal swab samples were subsequently tested for the presence of SARS-CoV-2 RNA without intermitting freeze-thaw cycle. Viral RNA was isolated from the specimens using the QIAamp Viral RNA Mini Kit (QIAGEN GmbH, Hilden, Germany). PCR amplification was set up on CFX96 Touch Real-Time PCR Detection System (Bio-Rad Laboratories - Dubai Branch, Dubai, United Arab Emirates) and a commercially available PCR kit Allplex™2019-nCoV (Seegene, Seoul, South Korea) was used. Allplex™2019-nCoV assay enables simultaneous detection and identification of three target genes, specific for SARS-CoV-2 (e.g. the E, RdRp, and N gene). Samples were considered positive for SARS-CoV-2 only if all three targeted genes were amplified.

In addition, serum samples were collected at the beginning of the study and 14 days after the last nasopharyngeal swab was obtained. The presence of antibodies against SARS-CoV-2 were evaluated using IDK anti-SARS-CoV-2 IgM and IgG ELISA Kit (Immundiagnostik AG, Bensheim, Germany), following manufacturer's instructions.

Inclusion Criteria:

• health care worker

Exclusion Criteria:

• previous known COVID 19 infection

Physicians, nurses and hospital attendants currently working at the University Clinic of Respiratory and Allergic Diseases Golnik were invited to enroll in the pilot study.

IPD are available on request from PI.