Fig. 5. Yeast-derived RBD-dimer elicited durable protective antibodies in mice.

a Schedule of mouse immunization and challenge experiments. Two groups of six BALB/c mice were immunized with alum-formulated RBD-dimer protein (5 μg/dose) or PBS at weeks 0, 2, and 4. Serum samples were taken at weeks 4, 6, and 21. Four days after the last serum collection, the mice were challenged with live SARS-CoV-2. b RBD-specific antibody titers in the antisera were measured by ELISA. Anti-PBS sera did not show any binding activity at the lowest serum dilution (1:100; dashed line) and was assigned a titer of 50. c Neutralizing antibody titers of the antisera against SARS-CoV-2 pseudovirus. Anti-PBS sera did not show any neutralization at the lowest serum dilution (1:200; dashed line) and was assigned a NT₅₀ value of 100. d Active immunization with RBD-dimer provided protection against live SARS-CoV-2 infection. Left panel: study outline. After transduction with Ad5-hACE2, the vaccinated mice were challenged with live SARS-CoV-2 and then analyzed for viral loads in the lungs by qRT-PCR. Right panel: qPCR results are shown as viral RNA levels relative to GAPDH, using 2^−ΔCt method. Each symbol represents one mouse. For panels b–d, data are presented as means ± SEM. P values were analyzed with unpaired t-test and indicated as follows: *P < 0.05; **P < 0.01; ****P < 0.0001.