The COVID-19 pandemic, caused by SARS-CoV-2, highlighted the need for effective and rapidly deployable vaccines. mRNA vaccines emerged as a powerful tool, offering advantages like rapid development and the ability to target emerging variants. However, challenges remain, particularly regarding cold-chain requirements and accessibility in low- and middle-income countries (LMICs). The Omicron BA.1 variant presented a significant challenge, exhibiting substantial immune evasion. To address this, GEMCOVAC-OM, a monovalent, Omicron-specific (BA.1), thermostable, self-amplifying mRNA (samRNA) vaccine, was developed. Unlike conventional mRNA vaccines, samRNA vaccines contain a replicase that amplifies mRNA within the cell, allowing for lower doses. GEMCOVAC-OM is also lyophilized, stable at 2-8°C, and delivered intradermally via a needle-free injection system. This phase 2/3 study aimed to evaluate the safety and immunogenicity of GEMCOVAC-OM as a heterologous booster in adults previously vaccinated with either BBV152 or ChAdOx1 nCoV-19. The study's importance lies in assessing the potential of this novel vaccine platform to overcome the limitations of existing mRNA vaccines and provide effective protection against emerging SARS-CoV-2 variants, especially in resource-constrained settings.

The rapid evolution of SARS-CoV-2, particularly the emergence of immune-evasive variants like Omicron, necessitated the development of updated vaccines. Studies demonstrated that the Omicron variant significantly evaded neutralization by antibodies induced by vaccines targeting earlier strains. This prompted the development of Omicron-adapted vaccines, predominantly mRNA-based due to their advantages in rapid development and modification. Existing mRNA vaccines, however, often require stringent cold-chain storage, limiting their accessibility in LMICs. Self-amplifying mRNA (samRNA) technology emerged as a promising solution, offering the potential for reduced dose requirements and enhanced thermostability. Several studies have explored the potential of samRNA vaccines for various infectious diseases, showcasing their ability to induce robust immune responses. This research builds on this foundation by evaluating the efficacy and safety of a samRNA-based Omicron-specific vaccine designed for broader accessibility and applicability.

This study employed a prospective, multicenter, open-label, randomized, seamless phase 2/3 design. Phase 2 compared GEMCOVAC-OM to GEMCOVAC-19 (a prototype vaccine targeting the D614G variant) in 140 participants (70 per group). Phase 3 incorporated a safety cohort (3140 participants; 3000 GEMCOVAC-OM, 140 ChAdOx1 nCoV-19) and an immunogenicity cohort (420 participants; 280 GEMCOVAC-OM, 140 ChAdOx1 nCoV-19) to assess GEMCOVAC-OM against ChAdOx1 nCoV-19. Participants were healthy adults aged 18 years and older who had received two doses of either BBV152 or ChAdOx1 nCoV-19 at least 4 months prior. GEMCOVAC-OM (10 µg) was administered intradermally using a needle-free injection system, while ChAdOx1 nCoV-19 was administered intramuscularly. Immunogenicity was assessed using various assays including plaque reduction neutralization test (PRNT50), ELISA for anti-spike IgG antibodies, and intracellular cytokine staining (ICS) for cellular immune responses. Safety was assessed by monitoring adverse events (AEs) throughout the study. Statistical analyses included ANCOVA (using baseline titers as covariates), paired t-tests, Wilcoxon signed-rank tests, and the Miettinen-Nurminen method. The primary endpoint in phase 3 was the demonstration of noninferiority, and subsequently superiority, of GEMCOVAC-OM compared to ChAdOx1 nCoV-19 in terms of neutralizing antibody titers and seroconversion rate at day 29.

GEMCOVAC-OM was well-tolerated in both phase 2 and 3, with no serious vaccine-related adverse events. In phase 2, GEMCOVAC-OM demonstrated significantly higher anti-spike IgG antibody titers and seroconversion rates compared to GEMCOVAC-19 at day 29 (p<0.0001). In phase 3, at day 29, GEMCOVAC-OM showed significantly higher neutralizing antibody titers against the BA.1 Omicron variant compared to ChAdOx1 nCoV-19 (p<0.0001). The least squares geometric mean ratio (LSGMR) of neutralizing antibodies was 1.58 (95% CI 1.36-1.84), exceeding the predefined non-inferiority margin. Seroconversion rates were also significantly higher with GEMCOVAC-OM (39.5% vs. 19.5%). At day 90, GEMCOVAC-OM maintained significantly higher neutralizing antibody titers than ChAdOx1 nCoV-19 (LSGMR 2.09, p<0.0001). GEMCOVAC-OM also elicited significantly higher anti-spike IgG antibody titers at days 29 and 90 compared to both GEMCOVAC-19 (phase 2) and ChAdOx1 nCoV-19 (phase 3). Cellular immune responses, assessed by ICS, showed significantly higher levels of IFNγ CD4+ T cells, TNFα CD4+ T cells, and IL-2+ CD4+ T cells with GEMCOVAC-OM compared to the comparators. A significant increase in Omicron-specific B cells was also observed with GEMCOVAC-OM at day 29. Sex-disaggregated analysis showed no significant differences in humoral responses between men and women receiving ChAdOx1 nCoV-19. However, women receiving GEMCOVAC-OM exhibited numerically higher neutralizing antibody titers at day 90.

This study demonstrates that GEMCOVAC-OM, a novel samRNA Omicron-specific vaccine, is safe and highly immunogenic when administered as a booster dose. The superior performance of GEMCOVAC-OM compared to ChAdOx1 nCoV-19, a vaccine targeting the ancestral Wuhan strain, highlights its effectiveness against the immune-evasive Omicron variant. The higher neutralizing antibody titers and seroconversion rates observed with GEMCOVAC-OM are consistent with findings from other Omicron-adapted mRNA vaccines. The robust cellular immune responses, particularly the increased expression of Th1 cytokines (IFNγ and TNFα), suggest durable immunological memory. The thermostability and needle-free delivery of GEMCOVAC-OM address crucial accessibility challenges, particularly in LMICs. The findings support the potential of this vaccine to enhance global COVID-19 vaccination efforts.

GEMCOVAC-OM demonstrates a safe and effective Omicron-specific booster response. Its samRNA platform, thermostability, and needle-free delivery offer advantages for wider accessibility. Future research should focus on long-term efficacy and broader real-world evaluations, particularly in diverse populations and settings. The success of this vaccine platform suggests its potential application in combating future viral outbreaks.

The study population was predominantly male, potentially limiting the generalizability of results to female populations. The comparator in phase 3 was ChAdOx1 nCoV-19, not another Omicron-specific mRNA vaccine; this makes direct comparison of vaccine platforms difficult. The open-label design could introduce bias in safety assessment. Accurate data on past COVID-19 infections were unavailable, hindering the analysis of prior infection's impact on vaccine responses. Finally, ethical considerations precluded a placebo arm, preventing a direct efficacy study; an immunobridging approach was employed.