Vaccine Adjuvants: History, Role, Mechanisms of Action, and Side Effects

The present chapter aims at offering a comprehensive overview of adjuvants in vaccines, addressing their historical context, contemporary developments, and their role and mechanisms of action in enhancing immune responses. It also highlights the relevance of innate immunity and systems vaccinology in advancing our understanding of adjuvants. The term “adjuvant” has Latin origins, deriving from “adjuvare,” which means “to help,” “to aid,” or “to assist.” This etymological origin aptly reflects the role of adjuvants in aiding (eliciting and enhancing) the immune response to vaccine antigens. Adjuvants are classified based on various criteria, including traditional versus novel adjuvants, delivery systems, and pathogen-derived adjuvants. This classification provides a comprehensive framework for understanding the diversity of adjuvant types. Adjuvants play a crucial role in amplifying immune responses, with alum historically serving as the primary adjuvant. Recent advancements in understanding their mechanisms, including pattern recognition receptors (PRRs) and systems vaccinology, are reshaping vaccine development paradigms. Commonly reported side effects of adjuvanted vaccines include mild local reactions and temporary systemic effects, such as pain, redness, fever, and fatigue. Serious side effects are rare, and vaccine safety monitoring is robust, emphasizing the overall benefits of vaccination. In summary, this chapter provides a broad overview of adjuvants in vaccines, encompassing their history, classification, mechanisms of action, and potential side effects. It underscores the significance of adjuvants in modern vaccine development and the importance of vaccination in preventing infectious diseases.