Broad Humoral Immunity Generated in Mice by a Formulation Composed by Two Antigens From Delta Variant of SARS-CoV-2

Abstract Due to the rapid development of new variants of SARS-CoV-2 virus, as well as the real threat of new coronavirus zoonosis events, the development of a preventive vaccine with a broader scope of functionality is highly desirable. Previously, we reported the functionality of a nasal formulation based on the preparation of the nucleocapsid protein with the ODN-39M and combined with RBD, both antigens from Delta variant of SARSCoV-2. This combination induces a cross-reactive immunity in mucosal and systemic compartments until sarbecovirus level. In the present study, we explored the magnitude of the immunity generated in Balb/C mice by the same formulation, but adding alum as adjuvant, so as to enhance the humoral immunity against the two antigens. Animals were immunized with three doses of the bivalent formulation, administered by subcutaneous route. The humoral immunity was tested by ELISA and by a Surrogate of Viral Neutralization test. The cell-mediated immunity was also explored. High levels of antibodies against both antigens (N and RBD) were obtained upon immunization. Additionally, the anti-RBD Abs with neutralizing capacity reacted against the three SARS-CoV-2 variants of RBD assayed, including Omicron. At the same time, the Abs also recognized the nucleocapsid proteins from: SARS-CoV-1 and SARS-CoV-2 Delta and Omicron. Taken together, these results make the bivalent formulation tested, an attractive component of a pancorona vaccine able to broaden the scope of humoral immunity against both antigens. This will be particularly important in the reinforcement of immunity from previously vaccinated and/or infected populations.