Development of a Recombinant Omicron BA.1 Subunit Vaccine Candidate in Pichia pastoris

Low-cost and safe vaccines are needed to fill the vaccine inequity gap for future pandemics. Pichia pastoris is an ideal expression system for recombinant protein production due to its cost-effective and easy-to-scale-up process. Here, we developed a next-generation SARS-CoV2 Omicron BA.1-based recombinant vaccine candidate expressed in P. pastoris. The receptor binding domain of Omicron BA.1 spike protein (RBD-Omicron) was produced at 0.35 g/L in supernatant. With a 60% recovery after two-step purification, RBD-Omicron showed 99% purity. After in vitro characterisation of purified RBD-Omicron via chromatography, mass spectrometry, calorimetry and surface plasmon resonance-based methods, it was injected into mice for immunization studies. Three different doses of Alum and CpG adjuvanted RBD-Omicron were investigated and 10 μg RBD-Omicron gave the highest antigenicity. After two doses of vaccination, IgG titers in mice serum reached to more than 10⁶. These serum antibodies also recognized earlier (Delta Plus: B.1.617.2) and later (Eris: EG.5, Pirola: BA.2.86) SARS-CoV2 variants. The long-term immunological response in mice was measured by analyzing serum antibody titers and T-cell response of splenocytes after 60 weeks. Interestingly, IgG titers and Th1 response were significantly high even after a year. Omicron subvariants are dominantly circulating in the world, so Omicron sub-lineage-based vaccines can be used for future pandemics. The RBD-Omicron-based vaccine candidate developed in this study is suitable for technology transfer and transition into the clinic.