mRNA vaccines encoding membrane-anchored receptor-binding domains of SARS-CoV-2 mutants induce strong humoral responses and can overcome immune imprinting

To address the limitations of whole-spike COVID vaccines, we explored mRNA vaccines encoding membrane-anchored receptor-binding domain (RBD-TMs), each a fusion of a variant RBD, the transmembrane (TM) and cytoplasmic tail (CT) fragments of the SARS-CoV-2 spike protein. In naive mice, RBD-TM mRNA vaccines against ancestral SARS-CoV-2, Beta, Delta, Delta-plus, Kappa, Omicron BA.1 or BA.5, all induced strong humoral responses against the target RBD. Multiplex surrogate viral neutralization (sVNT) assays indicated broad neutralizing activity against a range of variant RBDs. In the setting of a heterologous boost, against the background of exposure to ancestral whole spike vaccines, sVNT studies suggested that RBD-TM vaccines were able to overcome the detrimental effects of immune imprinting. Omicron BA.1 and BA.5 RBD-TM booster vaccines induced serum antibodies with 12 and 22-fold higher neutralizing activity against the target RBD than their equivalent whole spike variants. Boosting with BA.1 or BA.5 RBD-TM provided good protection against more recent variants including XBB and XBB.1.5. Each RBD-TM mRNA is 28% of the length of its whole-spike equivalent. This advantage will enable tetravalent mRNA vaccines to be developed at well-tolerated doses of formulated mRNA. One Sentence SummarymRNA vaccines encoding membrane-anchored RBDs of SARS-CoV-2 mutants are effective vaccines that can overcome immune imprinting in mice