Antibody responses from participants. Shown are anti-RBD antibody titers expressed as the mean values of the participants (left) or by individual over time (right). Levels <0.48 units per mL are considered seronegative in healthy volunteers.¹³ Fourteen participants (34%) were seronegative at baseline. Twenty-seven participants (66%) were seropositive at baseline. All initially seronegative participants converted to seropositive after vaccination, although postvaccination IgG levels were lower than in the participants who were initially seropositive (Wilcoxon signed rank sum test, P = .0062).