Two formulations of coronavirus disease‐19 recombinant subunit vaccine candidate made up of S1 fragment protein P1, S2 fragment protein P2, and nucleocapsid protein elicit strong immunogenicity in mice

INTRODUCTION: Coronavirus disease (COVID‐19) is ongoing as a global epidemic and there is still a need to develop much safer and more effective new vaccines that can also be easily adapted to important variants of the pathogen. In the present study in this direction, we developed a new COVID‐19 vaccine, composed of two critical antigenic fragments of the S1 and S2 region of severe acute respiratory syndrome coronavirus 2 as well as the whole nucleocapsid protein (N), which was formulated with either alum or alum plus monophosphoryl lipid A (MPLA) adjuvant combinations. METHODS: From within the spike protein S1 region, a fragmented protein P1 (MW:33 kDa) which includes the receptor‐binding domain (RBD), another fragment protein P2 (MW:17.6) which contains important antigenic epitopes within the spike protein S2 region, and N protein (MW:46 kDa) were obtained after recombinant expression of the corresponding gene regions in Escherichia coli BL21. For use in immunization studies, three proteins were adsorbed with aluminum hydroxide gel and with the combination of aluminum hydroxide gel plus MPLA. RESULTS: Each of the three protein antigens produced strong reactions in enzyme‐linked immunosorbent assays and Western blot analysis studies performed with convalescent COVID‐19 patient sera. In mice, these combined protein vaccine candidates elicited high titer anti‐P1, anti‐P2, and anti‐N IgG and IgG2a responses. These also induced highly neutralizing antibodies and elicited significant cell‐mediated immunity as demonstrated by enhanced antigen‐specific levels of interferon‐γ (INF‐γ) in the splenocytes of immunized mice. CONCLUSION: The results of this study showed that formulations of the three proteins with Alum or Alum + MPLA are effective in terms of humoral and cellular responses. However, since the Alum + MPLA formulation appears to be superior in Th1 response, this vaccine candidate may be recommended mainly for the elderly and immunocompromised individuals. We also believe that the alum‐only formulation will provide great benefits for adults, young adolescents, and children.