A Novel Circular Delta‐XBB.1.5 RBD Dimeric Protein Subunit Vaccine Mediated by Split Intein Elicits an Immune Response and Protection Against Multiple SARS‐CoV‐2 Variants in Mice

ABSTRACT SARS‐CoV‐2 continues to mutate, leading to breakthrough infections. The development of new vaccine strategies to combat various strains is crucial. Protein cyclization can enhance thermal stability and may improve immunogenicity. Here, we designed a cyclic tandem dimeric receptor‐binding domain protein (cirRBD2) via the split intein Cth‐Ter. Cyclization does not affect the antigen epitopes of RBD but results in better thermal stability than that of its linear counterpart (linRBD2). Compared with the mice immunized with linRBD2, those immunized with two doses of 5 μg of cirRBD2 produced significantly greater levels of broad‐spectrum neutralizing antibodies, and generated a considerable cellular immune response. In the VEEV‐VRP‐hACE2‐transduced mouse model, two doses of 5 μg of cirRBD2 provided protection against infection with BA.5, XBB.1.9, and partial protection against EG.5 which has more mutations. This study developed a novel circular RBD dimer subunit vaccine for SARS‐CoV‐2 that exhibits broad‐spectrum neutralizing activity against various variants. A similar strategy can be applied to develop vaccines for other pathogens, especially for thermally stable vaccines.