## 1 Introduction  
The remarkable success of lipid nanoparticle (LNP)-based mRNA vaccines during the COVID-19 pandemic has accelerated research into safe and effective mRNA delivery methods [1, 2]. While LNPs have greatly advanced vaccine technology by stabilizing and facilitating the cellular uptake of mRNA, concerns regarding their long-term effects and potential immunogenicity have emerged [3, 4]. Reported adverse effects associated with LNP-based mRNA vaccines include rare but clinically significant conditions such as severe allergic responses, myocarditis, vaccine-associated immune thrombotic thrombocytopenia, and various autoimmune diseases [5, 6, 7]. In addition to safety concerns, the ultra-cold storage requirements for LNP vaccines pose transportation and storage barriers, thereby affecting their global distribution and accessibility [8].  
Given the limitation of LNP-based delivery, electroporation (EP) has emerged as a promising alternative for administering naked nucleic acid vaccines. EP enables direct intracellular delivery by transiently increasing cell membrane permeability through controlled electrical pulse [9, 10]. Intramuscular EP (IM-EP) has already demonstrated potential in nucleic acid vaccine delivery, and substantial progress has been made through extensive research on DNA vaccines [11]. It has been shown to enhance both cellular and humoral immune responses more effectively than traditional IM injection alone [12; 13]. In addition, IM-EP-mediated immunization has demonstrated enhanced immunogenicity in nonhuman primate models and further shown potential for infectious disease applications, including human papillomavirus [14; 15]. Collectively, these findings illustrate the broad utility of IM-EP for DNA vaccines; however, mRNA vaccine delivery via IM-EP remains comparatively limited and requires further investigation to realize its full potential.  
EP has been widely explored as a physical delivery strategy for small interfering RNA (siRNA) and naked mRNA, but the majority of these studies have focused on in vitro or ex vivo settings. In vitro studies demonstrated that EP efficiently delivers siRNA by transiently permeabilizing the cell membrane, enabling effective gene silencing [16; 17]. Ex vivo experiments, including studies utilizing dendritic cells loaded with tumor antigens and activated through mRNA EP, have shown that EP can facilitate the delivery of naked mRNA into immune cells, enhancing antigen expression and immunogenicity [18]. While there have been in vivo attempts to use IM-EP for the delivery of self-amplifying RNA, such as in HIV vaccine models, these studies were limited to immunogenicity assessments without evaluating protective efficacy against viral challenge [19]. Nonetheless, considering the advantages of naked mRNA, such as enhanced cellular uptake without the need for complex carriers, enabling efficient antigen expression with a simplified and potentially safer, further development of IM-EP-based delivery strategies remains necessary. Unlike DNA vaccines, for which delivery protocols have been extensively optimized, IM-EP-mediated delivery of naked mRNA is still under explored, particularly in the context of infectious disease models.  
In this study, we evaluated IM-EP as an alternative platform for mRNA vaccine delivery using a SARS-CoV-2 mouse model. Mice were immunized with naked mRNA encoding the SARS-CoV-2 spike protein via IM-EP to assess the induction of humoral and cellular immune responses and their protective efficacy against viral challenge. Our findings demonstrate that IM-EP effectively elicits strong humoral and cellular immunity, leading to complete protection against lethal SARS-CoV-2 infection.