Figure 8. Inhibition titers against S1-RBD–ACE2 binding by ELISA in the primary 2-dose vaccination and after the booster third dose.

ELISA-based neutralization (inhibition) of S1-RBD–ACE2 binding titers in the primary 2-dose vaccination series of a 196-day phase I trial (60 participants) and in the extension study with a booster third dose. Participants of (A) 10 μg, (B) 30 μg, and (C) 100 μg dose groups (n = 20 per dose group) received 2 assigned vaccine doses, 28 days apart, and a booster third dose of 100 μg at a time over 6 months administered to 50 participants (n = 17 for the 10 μg, n = 15 for the 30 μg, and n = 18 for the 100 μg dose groups). Serum samples were collected at the indicted time points for measuring the inhibition titers against S1-RBD binding to ACE2 by ELISA. The horizontal dotted lines indicate the lower limit of quantification. (D) Good correlation was found between S1-RBD–ACE2 binding inhibition and VNT₅₀. Data are plotted for all prime/boost vaccinated participants (10, 30, and 100 μg dose groups). Data points for participants on day 0 were excluded from correlation analysis. Correlation analyzed by nonparametric Spearman’s correlation method.