Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of Coronavirus Disease 2019 (COVID-19), has continued to circulate globally since its emergence in 2019, leading to recurrent waves of infection and the emergence of multiple variants of concern (VOCs). Successive viral evolution has introduced extensive mutations, particularly in the spike (S) glycoprotein, which remains the primary target of most mRNA vaccine strategies. Early vaccines encoded the ancestral S protein and provided significant protection during the initial pandemic phases. However, as viral evolution progressed, the effectiveness of ancestral formulations declined. Current vaccine strategies rotate full-length S protein constructs adapted to the variant dominant at the time, providing improved but still limited breadth and durability of immune protection. This underscores the need for vaccine approaches that extend beyond the full-length S protein.

T-cell responses play a critical role in controlling viral replication and preventing severe disease by eliminating infected cells and supporting long-term immune memory. Structural proteins such as nucleocapsid (N) and membrane (M) are highly conserved across Sarbecoviruses, abundantly expressed, and are capable of eliciting strong immune responses in prior studies. Incorporating these non-S protein antigens into vaccine designs provides an opportunity to evaluate their role in shaping broader immune responses.

This dissertation focuses on the development of mRNA vaccine platforms encoding both S and non-S protein SARS-CoV-2 antigens. One construct employed a chimeric S protein design, in which the backbone of one variant was combined with the receptor-binding domain (RBD) of another, to explore how such configurations influence immune recognition while maintaining overall structural integrity. Additional constructs included N and M proteins to investigate their potential roles in cellular immunity. These immunogens were encoded in nucleoside-modified mRNA and formulated with lipid nanoparticles (LNPs) to enable efficient encapsulation and delivery.

Evaluation in a human angiotensin-converting enzyme 2 (ACE2) transgenic mouse model compared these mRNA-LNP vaccines against LNP-only controls, assessing humoral and cellular responses to explore potential for cross-protection. Findings from these studies provide insights into the potential of using both S-based and non-S protein–based antigens within mRNA platforms to broaden immune coverage and inform future vaccine development against current and emerging SARS-CoV-2 variants.