Subunit antigen delivery: emulsion and liposomal adjuvants for next-generation vaccines

Introduction
Developing new vaccines to combat emerging infectious diseases has gained more significance after the COVID-19 pandemic. Vaccination is the most cost-effective method for preventing infectious diseases, and subunit antigens are a safer alternative to traditional live, attenuated, and inactivated vaccines.

Areas covered
Challenges in delivering subunit antigens and the status of different vaccine adjuvants. Recent research developments involving emulsion and liposomal adjuvants and their compositions and properties affecting their adjuvancy.

Expert opinion
Lipid-based adjuvants, e.g. emulsions and liposomes, represent a paradigm shift in vaccine technology by enabling robust humoral and cellular immune responses with lower antigen doses, a property that is particularly critical during pandemics or in resource-limited settings. These adjuvants can optimize vaccine administration strategies by potentially reducing the frequency of booster doses, thereby improving patient compliance and lowering healthcare costs. While emulsions excel in dose-sparing and broadening immune responses, liposomes offer customization and precision in antigen delivery. However, the broader clinical application of these technologies is not without challenges. Stability issues, e.g. the susceptibility of emulsion-based adjuvants to freezing and their reliance on cold-chain logistics, pose significant barriers to their use in remote/underserved regions. Future developments will likely focus on improving manufacturing scalability and cost-effectiveness.