Fig. 3. Antigen expression in different types of mRNA vaccines.

A The vaccine immunogen is encoded by a non-replicating RNA flanked by 5′ and 3′ UTRs (S protein). B Self-amplifying RNA (saRNA) encodes four nonstructural proteins (nsp 1–4) and a subgenomic promoter derived from the alphavirus genome. saRNA encodes a replicase and amplifies vaccine-encoding transcripts. C Trans-amplifying RNA (taRNA) uses two transcripts to enable self-amplification of replicase and the immunogen. D Circular RNA (circRNA) is circularized by the autocatalytic Group I ribozyme.²²³ The exon 2 is ligated upstream to exon 1, and a coding region is inserted between the exon-exon junction. During splicing, the 3′-OH of a guanosine nucleotide engages in a transesterification reaction at the 5′ splice site. The 5′ intron is excised, and the 3′-OH at the end of the intermediate engages in a second transesterification reaction at the 3′ splice site, resulting in the circularization of the immunogen mRNA. Upon entering the cell, the internal ribosome entry site (IRES) of circRNA initiates protein translation. The figures are created with BioRender.com