Figure 4.

Serum endpoint immunoglobulin G (IgG) ELISA titers against autologous full-length spike (S) protein. (A). Total IgG S antibodies were measured in mice sera 2 weeks after the third immunization. Serum starting concentration was 1:50. (B) Total IgG S antibodies were measured in mice sera 2 weeks after the fourth immunization. The highest dilution that gave an optical density (OD)₄₅₀ twofold higher than that of the prebleed sera (week 0) was designated as the antibody endpoint titer. Antibody titers were expressed as mean endpoint titers ± standard error of the mean (SEM) for each vaccine group with an individual scatter dot plot (n = 6). Data were compared by one-way ANOVA followed by Tukey’s multiple comparison test. ns: no significant difference. The asterisks refer to the level of significance: **** p < 0.0001; ns: no significant difference.