Stability of lyophilized and spray dried vaccine formulations

Liquid formulations of vaccines are subject to instabilities that result from degradation processes that proceed via a variety of physical and chemical pathways. In dried formulations, such as those prepared by lyophilization or spray drying, many of these degradation pathways may be avoided or inhibited. Thus, the stability of vaccine formulations can be enhanced significantly in the absence of bulk water. Potential advantages of dry vaccine formulations include extended shelf lives and less stringent cold-chain storage requirements, both of which offer possibilities of reduced vaccine wastage and facilitated distribution to resource-poor areas. Lyophilization and spray drying represent the most common methods of stabilizing vaccines through drying. This article reviews several lyophilized and spray dried vaccines that address a diverse set of pathogens, as well as some of the assays used to quantify their stability. Recent dry vaccine trends include needle-free delivery of dry powder via non-parenteral routes of administration and the incorporation of advanced vaccine adjuvants into formulations, which further contribute to the goal of increasing vaccine distribution to resource-poor areas. Challenges associated with development of these newer technologies are also discussed.