Development and characterization of a bivalent mRNA vaccine targeting the Delta and Omicron variants of SARS-CoV-2

The emergence of the Omicron variant of SARS-CoV-2 has led to a decrease in vaccine efficacy and contributed to the ongoing epidemic of COVID-19. Consequently, there is an urgent need to develop new vaccines targeting the emerging SARS-CoV-2 variants. To address this, we developed a bivalent mRNA vaccine, ARCoV-Biv, targeting the receptor-binding domains (RBDs) of Delta and Omicron BA.1 variants, which were selected for their strong transmissibility and immune escape potential, necessitating an updated vaccine formulation for broader protection. In vitro characterization showed that ARCoV-Biv exhibited favorable physicochemical properties comparable to the monovalent mRNA vaccines. Then, the immunogenicity of the vaccine candidates under a two-dose prime immunization regimen were further analyzed, demonstrating that ARCoV-Biv induced a broad neutralizing antibody response, with mean neutralization titers (NT50) of 87,556.33 against Delta (B.1.617.2) and 52,634.67 against Omicron BA.1, representing 71.19-fold and 399.72-fold increases, respectively, compared to the levels prior to booster immunization. It also elicited robust cellular immunity against different variants. Furthermore, the booster effect of ARCoV-Biv following the administration of two doses of ARCoV (prototype vaccine) was evaluated. The results showed that ARCoV-Biv significantly enhanced the neutralizing activity of immunized serum against BA.2.12.1 (5,700.67) and BA.4/5 (1,289.5) in addition to the prototype, Delta, and BA.1 variants. These findings suggest that ARCoV-Biv may serve as a foundational platform for future vaccine optimization against newly emerging SARS-CoV-2 strains. Compared to mRNA-1273.214 (Moderna), a bivalent vaccine, ARCoV-Biv demonstrated a higher neutralization breadth against Delta and Omicron BA.1, along with enhanced T-cell responses, indicating potential advantages in cross-variant protection and durability.