(Figure 3I-L).
The convalescent plasma samples were analyzed with VSV-ΔG bearing truncated SARS-CoV-2 Spike protein (VSV-ΔG-Sdel21). Spike pseudotyped viruses acquire the host spectrum of coronavirus and a rapid read out of the results is likely by the transfer of the GFP gene as a measure of infection. Administration of neutralizing antibodies against Spike pseudotyped VSV-ΔG virus reduce the number of GFP-positive cells. Neutralization activity of plasma was calculated based on GFP signal. Neutralization activities of plasma samples were depicted in
Figure 4 . With this assay, samples 2, 3, 4, 6, 7, 8, 9, 10, and 11 showed high neutralization activity with 50% inhibition above 1:270 dilutions.
To confirm data obtained by pseudovirus neutralization assay, we performed Plaque Reduction Assay (PRA) against SARS-CoV-2 Wuhan variant by using the same convalescent plasma samples. PRA is the gold standard phenotypic method to determine antiviral activity of plasma samples. This assay relies on quantifying the titer of neutralizing antibody through the number of plaques forming units (pfu) generated in a monolayer of virus-infected cells. Isolated SARS-CoV-2 was used to infect Vero E6 cells in the presence of decreasing concentrations of convalescent plasma and the 50% plaque reduction titer was determined. Several samples (# 7, 8, 9, and 11) showed high neutralization activity over a PRA50 of 1:320 (Table 2 ).