Fig. 1.

Potent and broad neutralization against SARS-CoV-2 variants elicited by sequential immunization with a wild-type inactivated vaccine and a mutant RBD subunit vaccine in mice. a Schematic diagram of the RBD dimer vaccine named I-P-R-F [6]. The structural elements show IFNα, Pan-DR epitope (PADRE), RBD, and immunoglobulin Fc fragment. The RBD regions of three kinds of I-P-R-F, including WT and Beta and Delta variants, are shown on the right, and mutations are labeled as sticks. b Schematic representation of the immunization schedule and blood sample collection of mice in 8 groups (Groups A-H, n = 10 in each group). c Schematic representation of mutations located in viral spike proteins of the Beta, Delta, Alpha, Iota, Kappa, and A.23.1 variants. d Neutralization titers against SARS-CoV-2 WT and variants of vaccine-elicited mouse serum. The data are shown for different pseudoviruses. e Neutralization titers against SARS-CoV-2 WT and variants of vaccine-elicited mouse serum. The data are shown for different groups and all comparisons are made to the WT readings. Geometric mean titers, in ID₅₀ values, were calculated and are written in each column. The error bars are standard deviations, and the symbols represent individual mice. The dotted line indicates the limit of detection (1:40 dilution) for the assay. Non-neutralizing serum was assigned a value of one-half of the limit of detection for visualization (1:20 dilution). Statistical analysis was performed with paired or unpaired t tests using GraphPad Prism 8.0 software. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001