This study is the second cross-sectional study conducted in the region. In the first cross sectional study, conducted in the winter of 2020, we aimed to estimate the seroprevalence of SARS-CoV-2 IgG antibodies among blood donors in Sarajevo Canton. We also assessed immune durability among seropositive participants after 6 months. In total, of 1015 blood donors aged 18-65 years in Sarajevo Canton between 2 November and 3 December 2020 were recruited and population-weighted seroprevalence in Sarajevo Canton was 19.2% (95% CI: 16.7-21.6%). The aim of this second cross-sectional study is to measure the seroprevalence SARS-CoV-2 antibodies and assess antibody kinetics in the blood donor population after 12 months.

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.

Recruitment will be convenience sampling of blood donors aged between 18 and 65 years. All blood donors visiting the Institute for Transfusion Medicine of the Federation of Bosnia and Herzegovina for routine blood donation will be approached during the study period to participate in the study by the staff (medical doctors) of Institute for Transfusion Medicine of the Federation of Bosnia and Herzegovina at time of donation. Some blood donors will also be called by the staff of the Institute for Transfusion Medicine during the routine actions for donation of missing blood types, from an existing list maintained by staff at the Institute for Transfusion Medicine, and will be also invited to participate in the study by staff upon attendance at the blood clinic. Investigators will endeavor to ensure that the following three age groups can be reported: 18-29, 30-39, 40-65.

If the data is shared by the implementing organization to WHO or any agency or institution providing support for data analysis, data shared will include only the study identification number and not any personally identifiable information.