Vaccine delivery systems: roles, challenges and recent advances

Abstract

The overwhelming majority of vaccine antigens are biological macromolecules, such as proteins and polysaccharides, typically with a molecular weight greater than 10,000. As such, they need to be delivered to the body in the correct conformation in order to elicit the desired immune response and to effectively target the immune cells. Currently, most vaccines are administered parenterally via the intradermal, subcutaneous or intramuscular route, the choice largely dependent on whether the antigen is in the adsorbed or nonadsorbed state. However, these routes have major drawbacks, including pain associated with the use of needles, the potential for needle contamination, practicalities of needle disposal and the need for a primary healthcare worker. There is now a particular focus on the development of mucosal vaccines, designed for direct application to mucosal surfaces such as those present in the mouth, nose, vagina and rectum. Often, simple antigen solutions are immunologically ineffective when delivered by these mucosal routes, owing to difficulties associated with mucosal retention and uptake. A diverse range of formulation strategies, including microspheres, liposomes, nanoparticles and virus-like particles, are now being actively investigated. In addition to the design and selection of the antigen candidate, the choice and preparation of the antigen delivery system is crucial to achieve the end goal of vaccination. In this chapter, an overview of the role and considerations for the design of vaccine delivery systems is presented, with particular focus on the challenges and recent advances in the field of colloidal and nano-sized delivery systems.