Inclusion Criteria:  All individuals identified for recruitment into the investigation, resident of Canton Sarajevo between 18-65 years, irrespective of prior COVID-19 infection.  Exclusion Criteria:  Refusal to give informed consent or to attend follow-up appointments; currently experiencing COVID-19 symptoms or in the last 14 days; and close contact with a confirmed case of COVID-19 within 14 days prior to donation.