Table 1.

Results of preclinical COVID-19 mucosal vaccines candidates ¹.

Vaccine		Adjuvant	Schedule	Animal Model	Efficacy	Ref.
Live	Live oral	None	Post-pyloric administration of SARS-CoV-2 by esophagogastroduodenoscopy	Rhesus macaques	Limited virus replication in the gastrointestinal tract and minimal to no induction of mucosal antibody titers in rectal swabs, nasal swabs, or bronchoalveolar lavage.	[82]
Subunit	Recombinant RBD protein	None	Intranasal	Mice	High titers of serum IgG and nAb as well as a significant mucosal immunity	[83]
	Recombinant RBD protein using self-assembling Helicobacter pylori–bullfrog ferritin nanoparticles, purified from mammalian cells and assembled into 24-mer nanoparticles	None	Intranasal	Ferrets	No fever, body weight loss, or clinical symptoms; rapid clearance of infectious virus in nasal washes and lungs as well as of viral RNA in respiratory organs.	[84]
	RBD + 2 domains of the viral nucleocapsid protein (N)	Heat-labile enterotoxin B (LTB)	Three-dose vaccination schedule	Mice	Enhanced post-dose-3 nAb, IgG, and IgA production to S- and N-protein-stimulated IFN-γ and IL-2 secretion by T cells	[85]
			Heterologous subcutaneous prime with S1 protein and oral booster	Rats	A single oral booster following two subcutaneous priming doses elicited serum IgG and mucosal IgA levels
	S1 nanoparticles	IL-15 and TLR agonists	IM-primed/intranasal (IN)-boosted mucosal vaccine	Rhesus macaques	Weaker T-cell and antibody responses, but higher dimeric IgA and IFNa. No detectable subgenomic RNA in upper or lower respiratory tracts	[86]
	S1 protein from the beta variant in PLGA	CP15	Intranasal after WA strain priming 1 year before	Rhesus macaques	Serum- and bronchoalveolar lavage (BAL)-IgG, secretory nasal- and BAL-IgA, and nAb against the original strain and/or beta variant	[87]
Virus-like particles (VLP)	Outer membrane vesicles of Salmonella typhimurium conjugated with the mammalian cell culture-derived RBD (RBD-OMVs)	None	Intranasal	Golden Syrian hamster (Mesocricetus auratus)	High titers of blood IgG to RBD as well as detectable mucosal responses; no weight loss, lower virus titers in bronchoalveolar lavage fluid, and less severe lung pathology.	[88]
	VLPs displaying RBD (CuMVTT-RBD)	Tetanus-toxin; TLR7/8 ligands.	Intranasal	Mice	Strong RBD- and spike-specific systemic IgG and IgA responses of high avidity; Strong mucosal antibody and plasma cell production in lung tissue	[89]
	Thermostable VLP (e-VLPs) harnessed with variable surface proteins (VSPs) from Giardia lamblia, affording them resistance to degradation and expressing pre-fusion stabilized form of S and membrane protein (M) expression	None	I.m. prime-oral boost	Mice and hamsters	Complete protection from a viral challenge; dramatically boosted the IgA mucosal response of intramuscularly injected vaccines.	[90]
Adenoviral vectors	Adenovirus type 5 AdCOVID™	None	Single-dose intranasal	Mice	Elicits systemic and mucosal immunity	[91]
	Human adenovirus type 5	None	Single dose intranasal	mice and ferrets	Complete protection in the upper and lower respiratory tracts.	[92]
	Chimpanzee adenovirus encoding prefusion-stabilized Spike	None	Single dose intranasal	hACE2 transgenic mice	High levels of nAbs, systemic, and mucosal IgA and T cell responses, and almost entirely prevents infection in both the upper and lower respiratory tracts; durable high nAb and Fc effector antibody responses in serum and S-specific IgG and IgA secreting long-lived plasma cells in the bone marrow. At 9 months after vaccination, serum antibodies neutralized SARS-CoV-2 strains with B.1.351, B.1.1.28, and B.1.617.1 spike proteins and conferred almost complete protection in the URT and LRT	[93,94]
	Adenovirus 5- and 19a-vectored vaccines	None	Intranasal vaccinations with adenovirus 5- and 19a-vectored vaccines following a systemic DNA or mRNA priming	Mice	Strong systemic and mucosal immunity; high levels of IgA and tissue-resident memory T cells in the respiratory tract. Mucosal neutralization of VOC was also enhanced. Importantly, priming with mRNA provoked a more comprehensive T cell response consisting of circulating and tissue-resident memory T cells after the boost, while a DNA priming induced mostly mucosal T cells.	[95]
vaccinia vectors	Mucosal homologous plasmid and a heterologous immunization strategy using a plasmid vaccine and a Modified Vaccinia Ankara (MVA) expressing Spike (S) and nucleocapsid (N) antigens.	None	Mucosal	Mice	nAb in serum and bronchoalveolar lavage; induction of Th1 and Th17 responses and polyfunctional T-cells expressing multiple type-1 cytokines (e.g., IFN-γ, TNFα, and IL-2) in the lungs and spleen	[96]
	Pre-fusion-stabilized Washington strain Spike, expressed from a highly attenuated, replication-competent vaccinia virus construct, NYVAC-KC.	None	Intranasal	Mice	Fully protected against disease and death from the mouse-adapted strain of SARS-CoV-2, SARS2-N501YMA30, contains a spike that is also heavily mutated, with mutations at four of the five positions in the Omicron spike associated with neutralizing antibody escape (K417, E484, Q493, and N501).	[97]
Lentiviral vectors	Spike	None	Systemic prime-intranasal boost	hACE2 transgenic mice and golden hamsters	>3 log10 decrease in the lung viral loads and reduces local inflammation	[98]
Rhabdoviral vectors	VSV-SARS2(+G) virions generated by G protein trans-complementation	None	Oral	Cynomolgus macaques	Compared to parental VSV-SARS2, G-supplemented viruses were orally active in virus-naive and vaccine-primed cynomolgus macaques, powerfully boosting SARS-CoV-2 nAb titers	[99]
Live attenuated influenza virus vectors	LAIV-CA4-RBD LAIV-HK68-RBD	None	Systemic prime-intranasal boost	K18-hACE2 mice	Higher systemic and mucosal immune responses, including bronchoalveolar lavage IgA/IgG and lung polyfunctional memory CD8 T cells, including against VOC	[100]

¹ Wording in many of the table cells was taken verbatim from the cited reference to maintain the exact meaning as in the original report.