Vaccine Delivery Systems
INTRODUCTION Vaccination continues to be the most successful procedure for the prevention of infectious diseases. It is important that the vaccine does not cause disease or negative side effects in the host while stimulating an immune response capable of protection against the pathogen. Advances in biotechnology have stimulated the development of more highly sophisticated subunit vaccines that induce a greater specific immunity to infectious agents. However, subunit vaccines need potent adjuvants to be effective, and a means to deliver vaccines in a way to induce the most appropriate immune response (mucosal, cell-mediated, opsonizing or neutralizing antibody responses) necessary to induce protective immunity. Consequently, delivery techniques and formulations that overcome these difficulties have been a topic of investigation for much of the past century. Today, most human and veterinary vaccines such as diphtheria, pertussis, tetanus toxoid, measles, and chickenpox (herpes zoster) are administered by the parenteral route, that is, by intramuscular (IM) or subcutaneous (SC) needle injections. Parenteral administration suffers from a number of disadvantages such as needle-stick injuries, subsequent risk of transmission of diseases, difficulties in mass immunization, and suboptimal induction of mucosal immune responses. Needle injections in livestock are also associated with tissue damage, and it is estimated that IM delivery of killed vaccines results in an $8 loss per injection site in cattle alone (1). In recent years, there has been an increased focus in developing less invasive vaccine delivery systems. This chapter will address advances in one-dose or self-boostering vaccines and delivery systems for both mucosal and cutaneous immunization (Table 1) that have the potential for improving the efficacy of vaccines for both human and veterinary applications.
MUCOSAL VACCINATION Although a majority of infectious agents invade their hosts at mucosal sites, most vaccines are administered parenterally to induce circulating antibodies that usually do not cross mucosal sites. Induction of immunity at mucosal sites prevents host invasion and the serious pathological manifestations associated with infections, for example, cytokine release as well as endotoxin or other toxins’ life-threatening effects on the host (19). Despite the obvious advantages of protective mucosal immunity to many pathogens, the practical difficulties of delivery of mucosal vaccines have minimized clinical success of such vaccines.