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Advances in the delivery of COVID-19 vaccines

Several systems are being developed and investigated for the delivery of COVID-19 vaccines. A crucial challenge is the ability to maintain vaccine efficacy through the use of an effective delivery system. Some of these vaccine delivery systems such as lipid nanoparticles (LNPs) have been approved for the use by regulatory authorities in numerous countries. LNPs are currently used for the delivery of Moderna and Pfizer/BioNtech vaccines. LNPs consist of four constituents: Cholesterol for LNP stabilization, cationic lipids for the protection of messenger RNA (mRNA) molecules from nuclease degradation, and helper phospholipids that aid the formation and intracellular release of mRNA and PEGylated lipids that reduce nonspecific interactions. Researchers have also used virus-like particles (VLPs) for COVID-19 vaccine delivery. VLPs consist of several hollow viral proteins without the viral genome. VLPs are structurally identical to the native virus and can activate the human adaptive immune response. The nanosized VLPs self-assemblies have investigated as potential platforms for the delivery of COVID-19 vaccines. Liposomal vesicles are amphiphilic since the polar headgroups of phospholipids are oriented toward water molecules and the hydrophobic chains are in the internal area of the vesicles. The rationale behind the utilization of liposomes as vaccine delivery systems is their versatility and flexibility. Messenger RNA coding for SARS-CoV-2 spike protein can be entrapped into liposomes that are designed to remain stable in the bloodstream until their uptake by phagocytic cells. Other vaccine delivery approaches such as the use of microneedles and electroporation provide transdermal vaccine transport enable COVID-19 vaccines to cross the skin but not the cells of deep-lying tissues.