Due to the situation this study, originally planted until September 2020, was extended with 7 months until April 2021. This extension was approved by the ethical committee University hospital Ghent, Belgium.

The novel SARS-CoV-2 (Severe Acute Respiratory Syndrome-associated Coronavirus type 2) is rapidly spreading over the world causing a condition called Coronavirus disease 2019 (COVID-19). Hospital health care workers (HCW) have received detailed instructions to protect themselves against this infection, but it is likely that at least some of the HCW will get infected either at their workplace or elsewhere. It would be useful to document prevalence and seroprevalence of SARS-CoV-2 and their monthly evolution among Belgian active hospital HCW during a period of 12 months starting beginning of April 2020 and the number of new cases (incidence) of COVID-19 and SARS-CoV-2 seroconversions among Belgian hospital HCW during a period of 12 months which are the primary objectives of this study.

Additionally the study will validate serological tests (subject to change/addition depending on the evolution of scientific research) against the plaque reduction neutralization test (PRNT) (gold standard); validate the saliva sample (sampling with Oracol or equivalent) against the standard naso/oro pharyngeal swabbing (NOPS) to perform RT-qPCR for SARS-CoV-2 diagnostic purposes, as well as against the standard serology (serum); validate the nasal swab against the standard (NOPS) to perform RT-qPCR for SARS-CoV-2 for diagnostic purposes; investigate potential risk factors for the infection; quantify the proportion of asymptomatic cases among new cases that develop during a period of 12 months.

To reach these primary and secondary objectives the researchers will conduct a prospective cohort study in which a random selection of HCW currently working in Belgian hospitals will be tested monthly (with for the first month an additional testing point at 14 days). The testing will include four kind of laboratory tests, being; a molecular (PCR) test on a nasopharyngeal, a saliva and nasal sample (for nasal and saliva sample only testing until 50 PCR positive and 50 PCR negative samples are collected) and a serological test for which a blood sample is needed. To asses risk factors, at each testing point a questionnaire providing basic socio-demographic and health characteristics of the HCW including presence of symptoms since the previous testing point and the HCW's involvement in caring for COVID-19 patients will be completed.

Laboratory data and epidemiological data (questionnaire) will be collected simultaneously. In each of the participating hospitals, one contact person will be designated to coordinate the study locally (ideally a staff member of the local infection and prevention control team) and communicate with the researchers. The sample size calculation indicated the inclusion of 17 randomly selected Belgian hospitals and in each hospital 50 randomly selected HCW. HCW can only be enrolled in the study after give a written informed consent.

Laboratory testing will be performed at the Virology Diseases and Immune Response services of Sciensano, Ukkel, and the Virology Laboratory of the Institute of Tropical Medicine, Antwerp. The laboratory test results will be communicated by the laboratory to each participant. Each participant and sample will have a unique code assigned. Data collected through the questionnaire will be send psuedonymised (using the unique code) to the epidemiologist at Sciensano. The laboratory will prepare a list of test results by unique code to be linked for further analysis with the questionnaire data. None of the researchers who will analyse the data will be involved in data collection, nor in the care of COVID-19 patients.

This study is a Belgian multicentric study executed by Sciensano, Brussels, in collaboration with the Institute of Tropical Medicine, Antwerp.

Inclusion Criteria:

• Any permanent medical and paramedical staff working in the hospital who provided a signed informed consent to participate in the study.
• Participants must have a social security insurance (mandatory in Belgium).
• This study only include adults.

Exclusion Criteria:

• Staff hired on a temporary (interim) basis will be excluded as follow-up over time will be compromised.
• Administrative staff or technical staff will also be excluded.
• HCW who were not active during the inclusion period will automatically be excluded.

Study population include health care workers currently working in Belgian hospitals.

According to current national guidelines, HCW in contact with persons at risk for COVID-19 experiencing mild respiratory symptoms without fever are allowed to work if clinical condition permits, while wearing a mask and observing usual hygiene measures. Healthcare workers experiencing respiratory symptoms and fever are systematically tested and if positive, isolated at home, if negative, eventually working wearing a mask if the clinical state allows it. Those who recovered from a documented infection are also allowed to work. HCW who are not in contact with persons at risk for COVID-19 are not tested and systematically isolated in case of any unusual respiratory symptom. This implies that at baseline, the study population will only include asymptomatic or paucisymptomatic HCW applying proper infection prevention measures.