Figure 1. Vaccine-induced binding and neutralizing antibody responses observed among US healthcare worker participants in the Prospective Assessment of SARS-CoV-2 Seroconversion (PASS) study, January–August 2021. A) MFI levels of vaccine-induced spike IgG binding before and after second vaccination in serum samples diluted 1:400 (n = 227 participants). Horizontal line indicates the positive or negative spike IgG threshold. B) Spike IgG binding antibodies (BAU/mL) quantified from serum samples collected 1 month (mean 36.9 days, range 23–81 days) and 6 months (mean 201.1 days, range 151–237 days) postvaccination (n = 187 participants). Wilcoxon matched-pairs signed rank test performed; y-axis is log2-scale. C) Neutralizing antibody titers against severe acute respiratory syndrome coronavirus 2 wild-type and Delta variant from serum samples collected 1 month (mean 30.8 days, range 28–42 days) and 6 months (mean 200.1 days, range 189–219 days) postvaccination (n = 49 participants). Friedman ANOVA with Dunn’s multiple comparisons performed post-hoc; y-axis is log2-scale. All errors bars represent the geometric mean and 95% CIs. BAU, binding antibody units; IC50, 50% inhibitory concentration; MFI, median fluorescence intensity.