A transferable SARS-CoV-2 IRES module enables dual translation initiation for enhanced antigen expression in COVID-19 mRNA vaccines

mRNA vaccines are a versatile platform for infectious disease prevention and therapeutic applications, yet their performance is limited by exclusive reliance on cap-dependent translation, which is markedly suppressed under hypoxia and cellular stress. Here, we report a hybrid 5' untranslated region (5'UTR) that enables dual translation initiation via both cap-dependent and internal ribosome entry site (IRES) mechanisms. This element integrates a minimal stem-loop 4.5-5 module (SL4.5-5) from the SARS-CoV-2 genomic 5'UTR, in which a conserved 5'-UUUCGU-3' motif within the SL5 loops is essential for function. Incorporating the SL4.5-5 module downstream of conventional 5'UTRs confers cap-independent translation capacity and enhances overall translation efficiency under translation-restrictive conditions such as hypoxia. When applied to the 5'UTRs of clinically validated COVID-19 vaccines, this module improves antigen expression in both modified and unmodified mRNAs. Notably, unmodified Omicron BA.5 and XBB.1.5 mRNA vaccines containing this element elicited potent humoral and cellular immune responses at sub-microgram doses, comparable to those induced by the approved N1-methylpseudouridine-incorporated mRNA vaccine, raxtozinameran. These findings identify SL4.5-5 as a modular IRES element that enables dual translation initiation, promoting efficient protein synthesis under cap-dependent translation-restrictive conditions and expanding the functional landscape of mRNA vaccines and therapeutics beyond cap-dependent limitations.