Enhanced protective efficacy of a novel, thermostable, RBD-S2 vaccine formulation against SARS-CoV-2 and its variants

Abstract

With the rapid emergence of variants of concern (VOC), the efficacy of currently licensed vaccines has reduced drastically. VOC mutations largely occur in the S1 subunit of Spike. The S2 subunit of SARS-CoV-2 is conserved and thus more likely to elicit broadly protective antibody and T-cell responses. However, the contribution of the S2 subunit in improving the overall efficacy of vaccines remains unclear. Therefore, we designed, characterized, and evaluated the immunogenicity and protective potential of a stabilized SARS-CoV-2 Receptor Binding Domain (RBD) fused to a stabilized S2. Designed immunogens were expressed as soluble proteins with significantly higher purified yield than the Spike ectodomain. Squalene-in-water emulsion (SWE) formulated fusions were highly immunogenic. S2 immunization failed to elicit a neutralizing immune response but significantly reduced lung viral titers in mice challenged with the heterologous Beta variant, while RBD-S2 fusions elicited broad neutralization including against the Clade 1a WIV-1 variant, and were highly efficacious. A lyophilized RBD-S2 fusion retained antigenicity and immunogenicity even after incubation at 37 degrees C for a month. In hamsters, SWE-formulated RBD-S2 showed enhanced immunogenicity and efficacy relative to corresponding RBD and Spike formulations.