Rapid evaluation of heterologous chimeric RBD-dimer mRNA vaccine for currently-epidemic Omicron sub-variants as booster shot after inactivated vaccine

•Scientific questions: The continuous mutation of SARS-CoV-2 leads to severe escape that gradually renders the vaccines based on prototype SARS-CoV-2 ineffective, which urges the development of a new generation of broad-spectrum vaccines for boosting vaccination.•Evidence before this study: Previously, we developed the COVID-19 protein subunit vaccine ZF2001® based on the RBD-dimer of prototype SARS-CoV-2, which has been approved for use in China, Uzbekistan, Indonesia and Columbia. To broaden the cross-protection efficacy, we upgraded the antigen into a hetero-chimeric prototype (PT)-Beta or Delta-BA.1 RBD-dimer, and proved its efficiency with protein subunit and mRNA vaccine platforms.•New findings: In this study, the designs of hetero-chimeric RBD-dimer mRNA vaccines were further explored, and their broad-spectrum activities as booster jabs following two doses of inactivated vaccine in mice were evaluated. The results demonstrated that the chimeric vaccines greatly boosted neutralizing antibody levels and specific T-cell responses against the variants, and PT-Beta was superior to Delta-BA.1 RBD as a booster in mice.•Significance of the study: These results provide basis for choosing effective antigens for booster jabs. With continuous mutations of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the severe immune escape of Omicron sub-variants urges the development of next-generation broad-spectrum vaccines, especially as booster jabs after high-level vaccination coverage of inactivated vaccines in China and many other countries. Previously, we developed a coronavirus disease 2019 (COVID-19) protein subunit vaccine ZF2001® based on the tandem homo-prototype receptor-binding domain (RBD)-dimer of the SARS-CoV-2 spike protein. We upgraded the antigen into a hetero-chimeric prototype (PT)-Beta or Delta-BA.1 RBD-dimer to broaden the cross-protection efficacy and prove its efficiency with protein subunit and mRNA vaccine platforms. Herein, we further explored the hetero-chimeric RBD-dimer mRNA vaccines and evaluated their broad-spectrum activities as booster jabs following two doses of inactivated vaccine (IV) in mice. Our data demonstrated that the chimeric vaccines significantly boosted neutralizing antibody levels and specific T-cell responses against the variants, and PT-Beta was superior to Delta-BA.1 RBD as a booster in mice, shedding light on the antigen design for the next-generation COVID-19 vaccines.