Viral Vaccine Adjuvant Strategies for Shaping Durable Immunity Across the Human Lifespan

Vaccination remains one of the most effective strategies for preventing infectious diseases. Yet, the success of modern vaccines increasingly depends on the rational design of adjuvants that enhance and shape immune responses. In this review, we examine current and emerging adjuvant strategies for viral vaccines across the human lifespan. Traditional adjuvants, particularly aluminum salts, have long served as the foundation of vaccine formulations. Still, their limitations have driven the exploration of novel platforms, including emulsions, nucleic acid-based adjuvants, and advanced particulate delivery platforms with intrinsic immunostimulatory properties. These newer approaches act through diverse mechanisms, such as activating innate immune pathways via pattern recognition receptors (PRRs) and stimulating antigen-presenting cells (APCs), thereby improving both humoral and cellular immunity. Recent advances in molecular biology, nanotechnology, and systems vaccinology have deepened mechanistic understanding and enabled more precise modulation of immune responses. However, significant challenges remain, including incomplete knowledge of adjuvant mechanisms, limited diversity among licensed adjuvants, safety concerns, and inconsistent efficacy across age groups. In particular, immune immaturity in infants and immunosenescence in older adults highlight the need for age-specific adjuvant strategies. The review identifies critical gaps in comparative studies, long-term safety data, and the development of adjuvants capable of inducing broad and durable immunity. Further, this article integrates licensed and emerging viral vaccine adjuvants through a lifespan framework. Addressing these limitations through interdisciplinary research and precision-based approaches will be essential for advancing next-generation vaccines and improving global preparedness for emerging infectious diseases.