COVID-19 Vaccines Currently under Preclinical and Clinical Studies, and Associated Antiviral Immune Response

Abstract

With a death toll of over one million worldwide, the COVID-19 pandemic caused by SARS-CoV-2 has become the most devastating humanitarian catastrophe in recent decades. The fear of acquiring infection and spreading to vulnerable people has severely impacted society's socio-economic status. To put an end to this growing number of infections and deaths as well as to switch from restricted to everyday living, an effective vaccine is desperately needed. As a result, enormous efforts have been made globally to develop numerous vaccine candidates in a matter of months. Currently, over 30 vaccine candidates are under assessment in clinical trials, with several undergoing preclinical studies. Here, we reviewed the major vaccine candidates based on the specific vaccine platform utilized to develop them. We also discussed the immune responses generated by these candidates in humans and preclinical models to determine vaccine safety, immunogenicity, and efficacy. Finally, immune responses induced in recovered COVID-19 patients and their possible vaccine development implications were also briefly reviewed.

Keywords: COVID-19; SARS-CoV-2; T-cell responses; convalescent patients; neutralizing antibodies; vaccine development; vaccine platforms; vaccine response.

Figure 1

Structural features of SARS-CoV-2. Spike (S) glycoprotein, the membrane (M) protein, and envelope (E) protein are embedded in the viral envelope. The RNA genome is complexed with the nucleocapsid (N) protein. Virus spike trimer is enlarged to depict its key subunits (S1 and S2) and N-terminal domains (NTD) and C-terminal domains (CTD) in the S1 subunit encompassing receptor-binding domain (RBD). S protein targets the host cell receptor, ACE2, through RBD in S1 subunit (Created with Biorender.com).

Figure 2

Graphical representation of the vaccine candidates with respect to their clinical stages of development. The number of candidates on the x axis are compared for each of the major vaccine platforms shown on the y axis. The clinical stage for the vaccine candidates in each platform is depicted by color-coded legends on the right of the graph. The graph is constructed based on the data obtained from the World Health Organization; draft landscape of COVID-19 candidate vaccines and Coronavirus Vaccine tracker. VLPs-virus-like particles, RNA-ribonucleic acid, DNA-deoxyribonucleic acid.

Figure 3

Different vaccine platforms for the COVID-19 vaccine. Left; viral and non-viral vaccine delivery platforms are shown. Right; antigen induced activation of antigen-presenting cells (APCs); a nexus of innate and adaptive immune arms activating T-cell and B-cell immunity by fundamental immunological pathways are represented to show how vaccine-elicited immune responses lead to the clearance of infection. (Created with Biorender.com).