Seven-week-old female K18-hACE2 transgenic mice were immunized with 5 or 0.1 μg of mRNA vaccines. A. Scheme of immunizations, blood draw, and virus challenge. B. Binding of sarbecovirus cross-reactive monoclonal antibodies to Wuhan-1 and B.1.1.529 spike and RBD proteins. C-F. Serum IgG responses at three weeks after the second 5 μg (C-D) or 0.1 μg (E-F) dose of mRNA vaccines (control or mRNA-1273) against indicated spike (C, E) or RBD (D, F) proteins (n = 12, two experiments, boxes illustrate geometric mean values, dotted lines show the limit of detection (LOD)). G. Serum neutralizing antibody responses three weeks after second vaccine dose as judged by focus reduction neutralization test (FRNT) with WA1/2020 D614G (left panel) and B.1.1.529 (right panel) in mice immunized with 5 or 0.1 μg of control (n = 4) or mRNA-1273 (n = 12) vaccines (two experiments, boxes illustrate geometric mean titer (GMT) values, dotted lines show the LOD). H. Paired analysis of serum neutralizing titers against WA1/2020 D614G and B.1.1.529 from individual mice (data from G) from samples obtained three weeks after the second 5 μg (left panel) or 0.1 μg (right panel) dose of mRNA-1273 (n = 12, two experiments, dotted lines show the LOD). GMT values are indicated at the top of the graphs. Statistical analyses. C-F: Mann-Whitney test; G: One-way ANOVA with Dunn’s post-test; H: Wilcoxon signed-rank test (** P < 0.01; *** P < 0.001; **** P < 0.0001).