The gene encoding the recombinant ferritin engineered from Helicobacter pylori non-heme ferritin and 2nd to 9th residues of bullfrog (Rana catesbeiana) ferritin lower subunit was a gift from Gary Nabel (44 (link)). Gene encoding Spike of SARS-CoV-2 (GenBank NC_0101080) codon-optimized for human codon usage (GenBank MC_0101081) was purchased from Genscript (pUC57–2019-nCoV-S). RBD was used to generate a fragment encoding RBD-SSGGASVLA linker-recombinant ferritin. For expression plasmid, a commercially available pFUSE vector (Invivogen) was engineered to replace human ferritin light chain gene promoter with SV40 promoter. Genes encoding the recombinant ferritin and the RBD-linker-ferritin fragment were cloned into the plasmid vector.