inclusion criteria after taking a written informed consent: - a healthcare worker at high risk for covid19 exposure (defined below): - persons primarily working in emergency departments (physicians, nurses, ancillary staff, triage personnel) ; - persons primarily working in intensive care units (physicians, nurses, ancillary staff, respiratory therapists) ; - persons performing aerosol generating procedures (i.e. anesthesiologists, nurse anesthetists, gastroenterologists performing endoscopy, pulmonologists performing bronchoscopy); - first responders (i.e. emts, paramedics) ; - persons working in the departments of general medicine, pulmonology, infectious disease and isolation wards.

inclusion criteria after taking a written informed consent: - a healthcare worker at high risk for covid19 exposure (defined below): - persons primarily working in emergency departments (physicians, nurses, ancillary staff, triage personnel) ; - persons primarily working in intensive care units (physicians, nurses, ancillary staff, respiratory therapists) ; - persons performing aerosol generating procedures (i.e. anesthesiologists, nurse anesthetists, gastroenterologists performing endoscopy, pulmonologists performing bronchoscopy); - first responders (i.e. emts, paramedics) ; - persons working in the departments of general medicine, pulmonology, infectious disease and isolation wards.