Breadth. (a) Graphs showing IC₅₀ neutralization activity of antibodies detected at 1.5 mo (n = 16) or 6 mo (n = 18) against indicated mutant SARS-CoV-2. (b) Graphs showing IC₅₀ neutralization activity of antibodies at 1.5 mo (n = 35) or 6 mo (n = 36) against WT (Wuhan-Hu-1 WT), Delta-RBD (L452R/T478K), and Omicron BA.1, compared with mRNA vaccinees at prime and 1.3 and 5 mo after Vax2 (Cho et al., 2021; Muecksch et al., 2022). The E484K, K417N/E484K/N501Y, and L452R/T478K substitutions, as well as the deletions/substitutions corresponding to viral variants, were incorporated into a spike protein that also includes the R683G substitution, which disrupts the furin cleavage site and increases particle infectivity. Neutralizing activity against mutant pseudoviruses was compared with a WT SARS-CoV-2 spike sequence (NC_045512), carrying R683G where appropriate. All experiments were performed at least in duplicate and repeated twice. Red bars and lines indicate geometric mean values. Statistical significance in panel a was determined by two-tailed Mann–Whitney U test and in panel b by two-tailed Kruskal–Wallis test with subsequent Dunn’s multiple comparison.