13. Nucleic acid-based vaccines
Novel genetic engineering techniques have facilitated the use of nucleic acids (DNA and RNA) as vaccine candidates. DNA-based vaccines are made by inserting the encoding gene of a foreign antigen into the plasmid DNA, while RNA-based vaccines are made up of mRNA expressing a microbial antigen in a lipid nanoparticle coating. Finally, the expressed proteins are delivered to the CD8+ T lymphocytes with the help of MHC class Ι and induce robust T cell responses [27]. Although plasmid DNA have been used as valuable expression platforms for decades, the RNA is one of the emerging vehicles in vaccine development (Fig. 2) [119]. Presently, 54 candidate (30 RNA-based and 24 DNA-based) vaccines of this generation have been developed against SARS-CoV2, of which only 8 DNA-based and 7 RNA-based vaccines have been licensed for clinical trials [52].