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