SARS-CoV-2 infection fatality rate among elderly retired Danish blood donors - A cross-sectional study

Pedersen, Ole Birger; Nissen, Janna; Dinh, Khoa Manh; Schwinn, Michael; Kaspersen, Kathrine Agergård; Boldsen, Jens Kjærgaard; Didriksen, Maria; Dowsett, Joseph; Sørensen, Erik; Thørner, Lise Wegner; Larsen, Margit Anita Hørup; Grum-Schwensen, Birgitte; Sækmose, Susanne; Paulsen, Isabella Worlewenut; Frisk, Nanna Lond Skov; Brodersen, Thorsten; Vestergaard, Lasse Skafte; Rostgaard, Klaus; Mølbak, Kåre; Skov, Robert Leo; Erikstrup, Christian; Ullum, Henrik; Hjalgrim, Henrik

BACKGROUND: Despite the vast majority of individuals succumbing to infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are elderly, infection fatality rate (IFR) estimates for the age group 70 years older are still scarce. To this end we assessed SARS-CoV-2 seroprevalence among retired blood donors and combined it with national COVID-19 survey data to provide reliable population-based IFR estimates for this age group. METHODS: We identified 60,926 retired blood donors age 70 years or older in the rosters of three region-wide Danish blood banks and invited them to fill in a questionnaire on COVID-19 related symptoms and behaviours. Among 24,861 (40.8%) responders, we invited a random sample of 3,200 individuals for blood testing. Overall, 1,201 (37.5%) individuals were tested for SARS-CoV-2 antibodies (Wantai) and compared to 1,110 active blood donors age 17-69 years. Seroprevalence 95% confidence intervals (CI) were adjusted for assay sensitivity and specificity. RESULTS: Among retired (age 70 years or older) and active (age 17-69 years) blood donors, adjusted seroprevalences were 1.4% (95% CI: 0.3%-2.5%) and 2.5% (95% CI: 1.3%-3.8%), respectively. Using available population data on COVID-19 related fatalities, IFRs for patients age 70 years or older and for 17-69 years were estimated at 5.4% (95% CI: 2.7%–6.4%) and 0.083% (95% CI: 0.054%-0.18%), respectively. Only 52.4% of SARS-CoV-2 seropositive retired blood donors reported having been sick since the start of the pandemic. CONCLUSION: COVID-19 IFR in the age group above 69 years is estimated to be 65 times as high as the IFR for people age 18-69 years.