evidence: Different Types of COVID-19 mRNA Vaccines mRNA vaccines can be categorized as non-replicating mRNA, self-amplifying mRNA (saRNA) and circular RNA (circRNA) based on their genetic characteristics (Fig. 3).107 Non-replicating mRNA vaccines deliver exclusively genetic information coding for the target antigen, thus containing the 5′-cap, 5′ untranslated region (UTR), 3′ UTR, and 3′-poly(A) tail regions.108 saRNA vaccines can deliver genetic information encoding the target antigen and other genes, e.g., viral RNA polymerase, to enable mRNA to self-replicate.109,110 Based on saRNA technology, safe trans-amplifying RNA (taRNA) vaccines have been optimized and developed.111 In addition, circRNA has recently been developed for COVID-19 mRNA vaccines due to its natural high stability.112 Most COVID-19 mRNA vaccines currently in clinical trials or those already on the market are non-replicating mRNA vaccines. The advantages of non-replicating mRNA vaccines include the use of an RNA molecule of simple structure and shorter length. An optimized or modified mRNA can have greatly enhanced biological activity. Lastly, saRNA vaccines are currently in preclinical and clinical trials. [...] Walsh, E. E. et al. RNA-based COVID-19 vaccine BNT162b2 selected for a pivotal efficacy study. Medrxiv, https://doi.org/10.1101/2020.08.17.20176651 (2020). [...] Mulligan, M. J. et al. Phase I/II study of COVID-19 RNA vaccine BNT162b1 in adults. Nature586, 589–593 (2020).