Fig. 4: Study design and composition of the study population.

a Time scale of study enrollment. b Percentages of blood samples from study participants collected during the indicated time frames of the study. c Numbers (%) indicate the proportion of study participants from respective administrative districts. The color graduation indicates the relative representation of the resident district population within the study population from 0.7 to 1.9 in steps of 0.2. Administrative boundaries from © GeoBasis-DE / BKG (2020), Data licence Germany—attribution—version 2.0. d Shown is the quotient of the relative proportions of study participants per district (black), SARS-CoV-2 PCR-positive cases per district (light blue), COVID-19 death cases per district (red), respectively, and the relative proportion of residents in that district, or the quotient of the relative proportion of study participants per district (blue) and the relative proportion of SARS-CoV-2 PCR-positive cases per district by October 15, 2020. e Sex and age characteristics of study participants in relation to the resident population (Supplementary Table 1). f Proportion of individuals aged 70 years or older living in nursing or retirement homes in the study sample and the overall population25.