Virus Neutralization Assay Results
To determine whether serum samples in which SARS-CoV-2 antigen-specific IgG was detected were able to neutralize SARS-CoV-2, we performed a live-virus neutralization assay. We selected samples from four categories, which were determined based on the corresponding ELISA results: I. Positive for SARS-CoV-2- and IBV-specific IgG (comprised of 10 vaccine implementers and one poultry worker); II. Positive for SARS-CoV-2-specific IgG (comprised of four COVID-19 patients and one vaccine implementer); III. Negative for SARS-CoV-2- and IBV-specific IgG (comprised of one vaccine implementer, two poultry workers, and one pre-pandemic control); IV. Negative for SARS-CoV-2-specific IgG and positive for IBV-specific IgG (comprised of two vaccine implementers). The IgG ratios of samples from each group are presented in Figure S7A. Only samples from group II (antibody-positive group against SARS-CoV-2) were able to neutralize in vitro SARS-CoV-2-mediated killing of Vero E6 cells. The geometric mean of neutralization titers were 1:16, 1: 35.78, 1: 80, 1: 16, and 1: 80 for VI17, C12, C13, C14, and C16, respectively (Table 1). Representative examples of positive, negative, and poultry farm personnel serum samples are shown in Figure S7B.