Fig. 2 Potential elicited immune responses by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine candidate platforms. The six vanguard platforms in COVID-19 vaccine design include viral vector, virus-like particle, nucleic acid (DNA or mRNA), live attenuated, inactivated and subunit protein vaccines. Following vaccination, the viral particles or encoding genes of SARS-CoV2 proteins are harvested by tissue antigen presenting cells (APCs), especially dendritic cells. Afterwards, the engulfed viral antigens are processed and presented to CD4+ T helper (Th) and cytotoxic CD8+ T lymphocytes (CTL) by major histocompatibility complex (MHC) class ΙI and Ι, respectively. Stimulation of Th cells as the conductor of the immune system leads to further induction of CTLs as well as B lymphocyte responses through various soluble and insoluble factors. Accordingly, subsequent the possible SARS-CoV2 infection via angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2), the secreted neutralizing antibodies against SARS-CoV2 surface antigens primarily spike protein and CTLs neutralize the virions and remove the infected cells, respectively. Eventually, by producing memory T cells and long-lived plasma cells, the vaccinated individual becomes immune to re-infection with SARS-CoV2.
Table 3. Immunological characteristics as well as advantages and disadvantages of key vaccine platforms against COVID-19