Increasing IgG antibodies to SARS-CoV-2 in asymptomatic blood donors through the second COVID-19 wave in Karachi associated with exposure and immunity in the population

Introduction One million cases of COVID-19 have been reported in Pakistan until August 1, 2021. However, SARS-CoV-2 PCR testing capacity is limited, and the true level of SARS-CoV-2 infections is unknown. Most individuals have asymptomatic or mild COVID-19 and remain undiagnosed. Volunteer healthy blood donors can be a control population for assessment of SARS-CoV-2 exposure. We determined COVID-19 seroprevalence during the second pandemic wave in Karachi. Materials and Methods We enrolled 558 healthy blood donors at the Aga Khan University Hospital blood bank between December 2020 and February 2021. Serum IgG reactivity were measured to spike and receptor binding domain (RBD) proteins. Clinical history was taken from individuals with a positive IgG result. Results Of the 558 study subjects, 553 (99.1%) were males and 5 (0.9%) were females with a mean (± SD) age of 29.0 ± 7.4 years (range 17–53 years). Positive IgG responses to spike were detected in 298/558 blood donors (53.4%). Of the spike IgG positive cases, 93/298 individuals (31.2%) had IgG antibodies to RBD. Clinical information was available for 63.7% of spike IgG positive cases. Of these, 37% had flu-like symptoms, 25% had travelled, individuals had contact with COVID-19 suspected (18%) or confirmed (15%) cases. Within the year prior to blood donation, 29% of individuals had laboratory confirmed COVID-19. Conclusions The seroprevalence of 53.4% of IgG antibodies to spike and 31.2% of IgG to RBD of SARS-CoV-2 in healthy blood donors indicates high rates of exposure and gives insights into protective immunity in the population.