Fig. 2: Induction of sustained humoral immune response with broad-spectrum neutralizing activities by RBD-HR/trimer vaccine.

a The schematic representation of the mouse immunization and sample collection protocol. b The assay of endpoint titers of anti-RBD IgG induced by RBD-HR/timer vaccine was performed by ELISA (n = 6 mice per group). c Neutralizing antibody titers against pseudoviruses in sera from n = 6 mice vaccinated with 10 μg adjuvant-formulated RBD-HR/trimer vaccine were determined by pseudovirus neutralization assay. d Neutralizing antibody titers in sera from n = 5 rats immunized with 60 μg of RBD-HR/trimer vaccine against authentic SARS-CoV-2 virus were determined. e Neutralization antibody titers of sera from mice immunized with three doses of mRNA, three doses of RBD-HR/trimer, or two doses of mRNA followed one dose of RBD-HR/trimer (n = 6 mice per group). f Representative graphs of flow cytometry represent blockade of RBD-Omicron binding to cell-surface human ACE2 receptor by the immunized sera. Negative control: cells stained with PE-conjugated anti-human IgG Fc antibodies only; Positive control: in the absence of sera; PBS/Sera: in the presence of sera from mice treated with PBS; RBD-HR/Sera: in the presence of sera from mice immunized with RBD-HR/trimer vaccine. g The flow cytometry analysis of the inhibition of binding between RBD-Prototype, RBD-Delta or RBD-Omicron with cell-surface receptor ACE2 in the presence of immunized sera (n = 5 biological replicates). Representative images (h) and quantitative analysis (i) of syncytia in the cell-cell fusion mediated by SARS-CoV-2 S protein, in the presence or absence of immunized sera (n = 5 mice per group). Serum samples in (c), and (f) to (i) were collected on day 56 after the first immunization, and mice sera in (e) were collected on day 84. Scale bars represent 100 μm in (h). Data are presented as geometric mean values ± SD in b–e and g, and presented as mean with SEM in i. P values in b were determined by One-way ANOVA followed by Dunnett’s multiple comparisons test, and in e and i were conducted by One-way ANOVA analysis followed by Tukey’s multiple comparisons test. Source data are provided as a Source Data file.