Vaccine Formulation: From Adjuvants to RNA Delivery Systems

Vaccine formulation integrates antigens, adjuvants, and delivery systems to enhance immunogenicity and shape the magnitude, breadth, and durability of immune responses. Aluminum salts entered vaccine research in the 1920s, and later adjuvants such as MF59, AS01, AS04, and CpG shifted formulation research from empirical testing toward mechanism-based design through innate immunity and systems vaccinology. mRNA vaccines expanded formulation science because they depend on sequence engineering, nucleoside modification, and lipid nanoparticle delivery to support intracellular antigen expression and immune activation. LNP-encapsulated, nucleoside-modified mRNA vaccines entered clinical use against COVID-19, marking a major milestone for RNA therapeutics and RNA delivery systems^{[1][2][3][4][5][6]}.
Mechanistically, adjuvants improve vaccine responses by activating innate immunity, promoting antigen uptake and presentation, inducing cytokines and chemokines, recruiting immune cells, and strengthening T-cell and B-cell responses. Some adjuvants act as immunostimulants, while others function as delivery systems; TLR agonists activate pattern-recognition receptor signaling, and emulsion or particulate platforms modify antigen exposure, trafficking, and lymph-node delivery. mRNA-LNP formulations protect mRNA, promote cellular uptake, and enable cytoplasmic delivery so host cells translate encoded antigens. Both the mRNA molecule and LNP components contribute to innate immune sensing, immunogenicity, and reactogenicity, making formulation variables central to experimental design^[2][4][5][6].
Disease applications include infectious disease prevention, COVID-19 vaccines, influenza, Zika, RSV, HIV, cancer vaccines, and immunotherapy research. In phase 3 trials, the two-dose BNT162b2 regimen conferred 95% protection against COVID-19 in people aged 16 years or older, and mRNA-1273 showed 94.1% efficacy in preventing COVID-19 illness. Key gaps remain: adjuvant mechanisms are still only partly understood, LNP tissue targeting and physiological barriers require optimization, and RNA stability and delivery efficiency continue to constrain platform expansion. Future experiments should compare adjuvant combinations, ionizable lipid chemistry, antigen coding sequence, administration route, and innate immune activation thresholds to guide safer and more precise vaccine formulation development^{[1][2][4][5][6][7][8]}.