inclusion criteria: * 18 years old upon study start * healthcare personnel exposed to patients with covid-19 respiratory disease: physicians, nurses, chemists, pharmacists, janitors, stretcher-bearer, administrative and respiratory therapists. * signed consent for randomization to any study arm.

inclusion criteria: * 18 years old upon study start * healthcare personnel exposed to patients with covid-19 respiratory disease: physicians, nurses, chemists, pharmacists, janitors, stretcher-bearer, administrative and respiratory therapists. * signed consent for randomization to any study arm.

inclusion criteria: - 18 years old upon study start - healthcare personnel exposed to patients with covid-19 respiratory disease: physicians, nurses, chemists, pharmacists, janitors, stretcher-bearer, administrative and respiratory therapists. - signed consent for randomization to any study arm.

inclusion criteria: - 18 years old upon study start - healthcare personnel exposed to patients with covid-19 respiratory disease: physicians, nurses, chemists, pharmacists, janitors, stretcher-bearer, administrative and respiratory therapists. - signed consent for randomization to any study arm.