Indirect immunofluorescence assay (IFA)

Vero cells were infected with SARS-CoV-2 at a MOI of 0.1 in 96-well plates for 24 h. The cells were fixed with 4% formaldehyde, permeabilized with 0.5% Triton X-100, and blocked with 5% BSA. The 1:100 diluted human plasma samples were used as the primary antibody. Alexa Fluor 488-labeled anti-human IgG (Life Technologies, Eugene, OR, USA) was used as the secondary antibody. The nuclei were stained with DAPI (Sigma-Aldrich). The images were obtained by scanning with an Operetta High Content Screening system (PerkinElmer, Waltham, MA, USA) and the number of SARS-CoV-2 infected Vero cells and fluorescent intensity were quantified by using the same software.