Characterization of an Oil-in-Water Adjuvant and its Interaction with Antigen Protein in Candidate COVID-19 Vaccine

Adjuvants are an essential component in the development of highly effective vaccines. Oil-in-Water (O/W) adjuvants are a class of emulsion suitable for human use to enhance and/or prolong optimal immune response of vaccines. This study characterizes the physical and chemical properties, evaluates the stability of an in-house made O/W squalene-based adjuvant to deepen the understanding of how they may relate to the function of candidate vaccine; Experimental methods and results were used and reported, covering centrifugal stability constant, Turbiscan (a light signal-based) stability, emulsion droplet merging speed, surface tension and In situ liquid-transmission electron microscopy, some of which are reported for the first time for studying this type of adjuvant. Furthermore, the interaction between the emulsion adjuvant and the model protein antigens was explored using the Turbiscan stability analysis, isothermal titration calorimetry, size exclusion high performance liquid chromatography, In situ liquid-phase transmission electron microscopy, and methods characterizing high-order structure of the model antigen protein (Fourier transform infrared spectroscopy and fluorescent spectrometry), confirming lack of interaction and alternation of antigen structure; Finally, results of adjuvant stability assessment show that the in-house made O/W adjuvant is remarkably stable, due to the optimized component ratio and production process, and highly suitable for used in COVID-19 vaccine.