Polysaccharide nanoparticles belong to a class of natural polymers composed of carbohydrate monomers connected by glycosidic bonds
[51][1]. With inherent immunomodulatory, biocompatibility, biodegradability, low toxicity, and safety characteristics, polysaccharides have attracted much attention in the preparation of nanovaccines and nanomedicine. Polysaccharide adjuvants mainly include chitosan and its derivatives, in addition to glucan, mannan, inulin, and Chinese medicinal herbs.
Chitosan is a cationic polysaccharide biopolymer that exists in the exoskeleton of crustaceans and is produced by acetylation
[52][2]. Chitosan NPs have a large surface area, are capable of the controlled release of drugs, have excellent antibacterial and other biological properties, are non-toxic to humans, and are environmentally friendly and used as a drug delivery vehicle
[53,54,55][3][4][5]. Chitosan nanovaccines have proven that the vaccines with chitosan as a carrier can stimulate immune responses in animals
[56,57][6][7]. In particular, chitosan is soluble in acidic environments and has adhesive properties. The excellent adhesion of chitosan reduces the nasal clearance of the vaccine
[58,59,60][8][9][10]. Chitosan can prolong the retention time of drugs or vaccines and improve their efficacy. It has significant advantages as an adjuvant for oral or nasal nanovaccines. Priscila Diniz Lopes et al.
[61][11] confirmed that a chitosan-based IBV-cs vaccine, alone or in combination with a heterologous live attenuated vaccine, can cause humoral and cell-mediated immune responses at the primary site of virus replication and can be localized (the trachea) or in the whole body (kidney) and provide effective protection against IBV infection. Santosh Dhakal et al.
[62][12] confirmed that chitosan NPs improve mucosal immunity and influenza vaccine protection in pigs. Mucosal immune response and systemic immunity are generated after nasal vaccination with chitosan-based nanovaccines. Chitosan NPs are theoretically feasible as the delivery system and adjuvant of SARS-CoV-2 nanovaccines. Adel M. Talaat et al.
[63][13] developed a quil-A-loaded chitosan (QAC) nanovaccine for COVID-19. Neutralizing antibodies and IgA were tested in vaccinated mice. The effect of cationic chitosan-based nanovaccines in improving animal humoral immunity is more significant than other chitosan-based nanovaccines
[64][14]. The feasibility of chitosan and its derivatives as SARS-CoV-2 nanovaccine carriers is emphasized in some reviews
[65,66][15][16]. Chitosan can also be associated with other poly nanoparticles, such as association chitosan-polymers. The associated nanoparticles may be an option in nanovaccine development
[67][17].