Recruitment and Sample Collection Eligible participants were selected from former participants in the Bus Santé study, including household members. All household members 5 years and up were eligible to participate. 2,834 individuals in Geneva, Switzerland were enrolled in the study between April 6-May 8, 2020 to assess seroprevalence for SARS-CoV-2 anti-S1 IgG antibodies by ELISA. The authors took 2 3mL peripheral venous blood from individuals over 14 years, and 2 1.5mL from individuals under 14.

Laboratory Analysis The anti-SARS-CoV-2 IgG antibodies were tested with ELISA (Euroimmun; Lübeck, Germany # EI 2606-9601 G) to assess the S1 domain of SARS-CoV-2 spike protein. The sera was diluted at 1:101 and processed (93% sensitivity and 100% specificity). The authors used recombinant immunofluorescence assay for all potentially indeterminate individuals to conduct a confirmatory test.

Statistical Analysis The authors assessed seroprevalence in the general population with Bayesian logistic regression model, accounting for age and sex. Week 2 was chosen as the reference group since it was the first week statistically different from week 1.