The primary 2-dose series (days 0 and 28) of the 196-day phase I trial and the extended booster third dose of 100 μg administered on mean day 286 (days 255–316). (A) In the participants of the 100 μg group, the VNT50 observed 14 days after booster reached 3,992 against live SARS-CoV-2 WT and 2,358 against live Delta. Similar high anti-WT and anti-Delta VNT50 levels were observed for the lower 30 and 10 μg dose groups. (B) In the participants of the 100 μg group, the pVNT50 observed 14 days after booster against pseudo-SARS-CoV-2 WT and against pseudo-SARS-CoV-2 variants, including Omicron. (C) Antibody persistence after 2 doses (phase I trial): The anti-WT neutralizing VNT50 decayed slowly, with a t1/2 of 187 days, based on the first-order exponential model fitting (SigmaPlot) over days 42–196 (r2 = 0.9877; the decay rate constant Kel = –0.0037; t1/2 = 0.693/Kel).