COVID-19 Vaccines in Clinical Trials and their Mode of Action for Immunity against the Virus.

For nearly two decades, coronaviruses have caused many health and economic problems, while no effective commercial vaccine has yet been developed. It is worth mentioning that despite some mutations and recombination in SARS-CoV-2, its genotype is very close to the original strain from Wuhan, China. Therefore, the development of an effective vaccine would be promising. It might be hypothesized that BCG vaccination is performed in high-risk populations before the commercialization of an effective SARS-CoV-2 vaccine. However, the development of an effective vaccine without considering the adverse immune reactions derived from antibody-dependent or cell-based immune enhancement may threaten vaccinated people's lives and long-term side effects must be considered. To this end, targeting of the receptor-binding domain (RBD) in spike and not whole spike, glycolization of FC receptors, PD-1 blockers, CPPs, etc., are promising. Therefore, the subunit vaccines or RNA vaccines that encode the RBP segment of the spike are of interest. To enhance the vaccine efficacy, its co-delivery with an adjuvant has been recommended. Nanoparticles modulate immune response with higher efficiency than the soluble form of antigens and can be functionalized with the positively charged moieties and ligands of targeted cells, such as dendritic cells, to increase cellular uptake of the antigens and their presentation on the surface of immune cells. This research aimed to discuss the COVID-19 vaccines entering the clinical trial and their mode of action effective immunity against the virus and discusses their advantages compared to each other.