Figure 5.

Cross-reactive antibody responses against the Delta variant after 2 doses of mRNA vaccine. A, Binding IgG antibody responses to spike RBD from the original Wuhan strain and the Delta variant, measured by ELISA, 1 month following the second vaccine dose (peak; green) and 3 months after this dose (durability; red) in HCW (blue) and older adults (orange) who were COVID-19 naive at study entry, and convalescent participants (N Sero+; grey). Median values are shown below each group. P values (computed using the Wilcoxon paired test for within-group comparisons, and the Mann-Whitney U test for between-group comparisons) are shown above each comparison performed. The number of pairs compared in the Wilcoxon paired test is shown below the relevant time points. B, ACE2 competition results in the same individuals, measured by ELISA. Values represent AU/mL calibrated against an external standard, reported in log₁₀ units. Abbreviations: ACE2, angiotensin-converting enzyme 2; AU, arbitrary unit; BAU, binding antibody units; COVID-19, coronavirus disease 2019; Delt, Delta; ELISA, enzyme-linked immunosorbent assay; HCW, health care worker; ns, not significant; RBD, receptor-binding domain; ULOQ, upper limit of quantification; Wuh, Wuhan.