Abstract
The global reliance on the cold chain system for vaccine storage and distribution presents a major logistical and financial challenge, particularly in low- and middle-income countries (LMICs). Nearly all currently available vaccines need to be stored and transported at temperatures between 2–8 °C from the point of production and until administration to maintain potency and efficacy. This strict temperature requirement imposes significant barriers to equitable vaccine access and increases costs associated with refrigeration, transport, and wastage due to temperature excursions. The development of thermostable vaccine formulations, therefore, represents a promising strategy to overcome these limitations. This thesis investigates the use of carbohydrates, amino acids, their combinations, and deep eutectic solvents (DES) as stabilizing excipients to improve the thermostability of flavivirus vaccines, specifically vaccines against Dengue, Yellow Fever, and Japanese Encephalitis. Accelerated stability studies were conducted by desiccating vaccine samples containing different excipient formulations for 24 hours at ambient temperature (~21℃), followed by quantification of plaque-forming units (PFU) after inoculation in Vero host cells to evaluate viral viability and stability. Among the carbohydrates tested, trehalose and sucrose provided the greatest stabilization, while histidine demonstrated the most stabilizing effect among the amino acids tested. Combinations of trehalose with histidine and sucrose with histidine produced synergistic effects, preserving viral infectivity more effectively than the individual components. Deep eutectic solvents formulated with choline chloride (ChCl) or choline acetate (ChOAc) also provided stability, particularly when combined with sucrose and histidine, though their stabilizing effects were less pronounced when compared to the sugar and amino acid formulations. These results suggest that carbohydrates and amino acids, particularly in combination, can serve as effective and biocompatible stabilizers to improve vaccine thermostability. By reducing susceptibility to degradation, such formulations could maintain vaccine potency under ambient conditions. These findings can have significant implications for global immunization efforts, as it may reduce vaccine waste, lower distribution costs, and lessen the burden of cold chain infrastructure, ultimately improving vaccine accessibility and coverage in LMICs.
