Neutralization Assay of SARS-CoV-2 Variants

The pseudotyped virus displaying SARS-CoV-2 (Wuhan-1 reference strain and variant strains) S protein express GFP in infected cells. They were prepared as previously described [48 (link)]. Mice sera were 2-fold serially diluted and incubated with pseudotyped virus at 37°C for 1 h. Then the mixture was transferred to pre-plated VERO cell monolayers in 96-well plates. After incubation for 15 h, the transducing unit numbers were calculated on a CQ1 confocal image cytometer (Yokogawa). Neutralization titre was determined by fitting nonlinear regression curves using GraphPad Prism and calculating the reciprocal of the serum dilution required for 50% neutralization of infection. Neutralization titre below the limit of detection was determined as half the limit of detection. In this study, the GFP-encoding pesudotyped virus was only used in the comparison of neutralizing activity against Wuhan-1 reference strain and variant strains.