Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE that controls the blood pressure and electrolyte balance by stimulating the angiotensin II (Ang II) type I receptors in the rennin-angiotensin aldosterone system (RAAS). There are also increased fibrosis, inflammation, thrombosis, and pulmonary damage [18]. ACE2 converts Ang II, produced from Ang I to Ang 1–7. This Ang 1–7 activates the Mas receptor which is known to protect against lung injury [19, 20]. By interacting with ACE2, the S protein’s receptor binding motif reaches host cells. Endocytosis occurs in this complex and forms an endosome after SARS-CoV-2 activates the transmembrane serine protease 2 (TMPRSS2) or S priming. After that, the endosome is acidified, allowing the virus’s encapsulated single-stranded RNA to be released into the cytoplasm for reproduction and translation. There is a higher binding affinity of SARS-CoV-2 to these receptors. This could be the explanation for easier transmissibility. SARS-CoV-2 increases the nuclear factor kappa B (NF-kB) after infection. The pro-inflammatory cytokines interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) are released as a result [18]. A number of factors have been linked to changes in ACE2 expression as well as the severity and progression of COVID-19. The expression of ACE2 in the lungs and SARS-CoV-2 viral load increase with age. This is the potential explanation for the increased risk of contracting severe COVID-19 in older adults (aged > 60 years). Male predominance is also observed among COVID-19 patients which could be due to gender disparities in ACE expression on chromosomes, sex hormone-driven immune system control, or RAAS [19, 20].

Also known as heat shock protein A5 or binding immunoglobulin protein is a chaperone protein, released from the endoplasmic reticulum. Under normal conditions, glucose-regulated protein 78 (GRP78) is located in the lumen of the endoplasmic reticulum and results in inhibition of protein synthesis, increased protein folding and cell death after binding to inactivating enzymes. GRP78 is a potential getaway for entry of virus in COVID-19. It controls the release of unfolded protein response under cellular stress conditions, which leads to the entry of SARS-CoV-2. Once GRP78 is translocated to the plasma membrane, it further facilitates the entry of viruses via motifs on S protein and molecular docking [21, 22]. SARS-CoV-2 binds to GRP78 via a molecular docking mechanism that occurs when the GRP78 substrate-binding domain area engages with SARS-CoV-2 [23].

COVID-19 patients have increased gene expression for GRP78 and there is increased release of GRP78 from damaged airway cells, which results in severe lung injury and trauma during the infection. This may be responsible for augmented inflammation in COVID-19 patients. Thus, GRP78 mediates binding to the S protein of SARS-CoV-2 and thus may represent a potential target for therapeutic treatment for COVID-19 [18]. According to studies, the celecoxib derivative AR12 (OSU-03012) can be used as a treatment alternative to reduce protein renaturation and S protein synthesis by inhibiting GRP78 ATPase activity with the celecoxib derivative AR12 (OSU-03012) [24, 25].

It’s a protease produced by the host cells that primes the S protein, allowing binding to ACE2. The important cellular surface receptor mediating the SARS-CoV-2 entry is ACE2. Once the S protein of SARS-CoV-2 binds to ACE2, it undergoes conformational changes followed by proteolytic degradation by host cell proteases, notably TMPRSS2 [26]. Thus, it increases infectivity and transmission of the virus. In addition to cathepsin B/L mediated endocytosis, TMPRSS2 cleaves the S protein and also activates the entry of SARS-CoV into host cells [27]. As the virus binds to ACE2, researchers discovered that fusing of plasma and viral membrane during the entrance mechanism causes TMPRSS2 to trigger extracellular priming of S protein [28]. The expression of TMPRSS2 is required for viral dissemination and pathogenicity. This gene’s genetic variation may influence the host’s predisposition to infection and virus clearance. The male gender is more susceptible to infection because the expression of this receptor is much more in androgen sensitive organs including the prostate and testis. This could be the plausible reason for the difference in infection rates between men and women [28].

Ezrin (EZR) is a protein encoded by the EZR gene and belongs to the family EZR-radixin moesin. The role of EZR has been described in the transmission of human immunodeficiency virus (HIV) but in SARS, it causes decreased entry of the virus by interacting with the SARS-CoV S protein [29], causing reduced viral entry. This can be considered as a potential therapeutic alternative to prevent SARS-CoV-2 infection. As EZR inhibits ACE2 and the Toll-like receptors (TLRs), the crucial receptors participating in the pathogenesis of COVID-19, an EZR agonist or molecule can be considered as a strategic therapeutic approach to affect SARS-CoV-2 viral entry [18]. It suppresses the inflammation seen in viral pneumonia [29], a significant pathophysiological consequence seen in COVID-19. As a result, more research is needed to fully comprehend its role in the SARS-CoV-2 virus infection.

TLRs belong to innate immune receptors that activate the innate immunity and regulate cytokine expression [30]. TLRs have 10 members in the human family. TLRs are present in the cell membrane, and endosomes are their other location for receptors like TLR3, TLR7, TLR8, and TLR9. TLRs are expressed by a variety of immune cells, including dendritic cells (DCs), macrophages, natural killer (NK) cells, and cells of adaptive immunity, such as T cells and B cells [31, 32]. TLR3 detects double-strand RNA (dsRNA), TLR4 detects lipopolysaccharide (LPS), TLR7/8 detects single-strand RNA (ssRNA), and TLR9 detects unmethylated cytosine-guanine (CpG) DNA [31]. TLR signals are transduced by two main pathways: myeloid differentiation primary response 88 (MyD88) and Toll/interleukin-1 receptor/resistance protein (TIR) domain-containing adaptor-inducing interferon (IFN) [TRIF, also known as Toll-IL-1R-containing adaptor molecule 1 (TICAM)]. TNF receptor associated factor (TRAF) and interleukin 1 receptor associated kinase (IRAK) are signaling proteins that activate NF-kB and interferon regulatory factor (IRF), resulting in the production of type I IFN and proinflammatory cytokines like IL-1, IL-6, TNF, and IL-12 [30].

The SARS-CoV-2 virus enters the body via the naso-oral pathway and infects cells in the lungs that express the ACE2 receptor, such as pulmonary epithelial cells and type II alveolar cells. After the virus penetrates the target cell, the host immune system identifies the complete virus or its surface epitopes, prompting an innate or adaptive immune response [42].

Following COVID-19 infection, there is a robust innate immune response, as evidenced by increased levels of C-reactive protein (CRP) and serum amyloid A (SAA) [43]. Innate immune cells such as macrophages, monocytes, and DCs become activated after recognizing damage-associated molecular patterns (DAMPs) are pathogen-associated molecular patterns (PAMPs) through their pattern recognition receptors (PRRs) [44]. PRRs on immune cells, primarily TLRs 3, 7, and 8, Nod-like receptors (NLRs), and retinoic acid-inducible gene (RIG)-like receptors (RLRs), are the first to recognize the virus, resulting in increased activation of various signaling pathways and transcription factors, as well as IFN production. The expression of cytokines, chemokines, and adhesion molecules is triggered when PRRs bind to PAMPs.

The generation of antiviral immunity in the lungs is controlled by nucleic acid detecting receptors and subsequent intracellular signaling pathways. Endosomal receptors (TLR3, TLR7, TLR8) and cytosolic RNA sensors [melanoma differentiation-associated gene 5 (MDA5) and RIG-I] detect viral RNA, triggering a signaling cascade that activates several transcription factors (such as neurofeedback (NFB) IRF3, IRF7, and others). IFN receptor signaling activates interferon-stimulated genes (ISGs) in both virally infected and adjacent cells via JAK/STAT pathways, increasing the innate immune response [45].

The nuclear factor-B (NF-B) and IRF3/7 signaling pathways are activated, resulting in increased production of pro-inflammatory cytokines such as TNF, IL-1, IL-6, IL-17, and IFN. These cytokines are well-known for their importance in virus clearance. There are increased levels of interferon gamma-inducible protein 10 (IP-10) fibroblast growth factors 2 (FGF2), granulocyte-colony stimulating factor (GCSF), granulocyte-macrophage colony-stimulating factor (GMCSF), monocytes chemoattractant protein-1 (MCP-1), MIP-1, TNF-α, platelet-derived growth factor subunit B (PDGFB) [46].

Certain cytokines, such as IL-6, IL-7, IL-1, IL-10, IP-10, and TNF determine the severity of the infection and contribute to a severe inflammatory response known as cytokine storm, which leads to severe conditions such as acute respiratory distress syndrome (ARDS), disseminated intravascular coagulation, or multiple organ failure. C-X-C motif chemokine ligand 8 (CXCL8), CXCL1, CXCL2, CXCL10, C-C motif chemokine ligand 2 (CCL2), and CCL7 are involved for neutrophil recruitment, and CXCL6, CXCL11, CXCL2, CCL3, CCL4, CCL7, CCL8, and CCL20 for monocyte and other immune cell recruitment [47, 48].

Along with the abovementioned cytokine storm, the severity of COVID-19 illness also depends on dysregulated or excessive immune response and macrophage activation syndrome.

The adaptive immune system, also known as the acquired immune system responds to infections in an antigen-specific manner. Cellular immunity, which is mediated by T cells, and humoral immunity are the two principal arms of this system. SARS-CoV-2 infection activates macrophages and causes them to secrete inflammatory cytokines, which causes T and B cells to differentiate. Multiple T cell subsets [such as T-helper 1 (Th1) and Th17] are activated, releasing cytokines that help to amp up the immunological response. Stimulation of Th1/Th17 cells with viral epitopes in the helper T cell subset may lead to aggravated inflammatory responses resulting in “cytokine storms” that lead to consequences like pulmonary edema and pneumonia [48].

Lympopenia, increased neutrophil-to-lymphocyte ratio (NLR) cytokine release syndrome (CRS), lymphocyte exhaustion and dysfunction, antibody-dependent enhancement (ADE), and abnormalities of monocytes and granulocytes are the major events implicated in the dysregulation of immune response and the immunopathogenesis of COVID-19 in these patients. In severe cases, T cell subsets such as CD4⁺ , CD8⁺ , and memory T cells are reduced, reflecting an insufficient antiviral response following SARS-CoV-2 infection [43].

Cytotoxic T cells recruited to the site of infection try to kill virus-infected cells in the lungs and are hyper-activated and exhausted whilst CD4⁺ T cells have decreased inflammatory response during SARS-CoV-2 infection. SARS-CoV-2 is unable to multiply within T cells, resulting in fruitless infection, cell death, protracted virus clearance, and a weakened adaptive immune response. B cells/plasma cells identify viral proteins and create SARS-CoV-2-specific antibodies, which could give systemic immunity [53]. Other immune cells such as NK cells, memory and regulatory T cells (Tregs), and B cells are lost in severe immunological disorders [53].

The humoral response to SARS-CoV-2 was discovered to be similar to that of other coronavirus infections, involving the generation of immunoglobulin G (IgG) and IgM antibodies. At the outset of SARS-CoV-2 infection, B cells elicit an early response against the N but antibodies against the S protein were found 4–8 days after the onset of symptoms. The absence of glycosylation sites on N results in the production of N-specific neutralizing antibodies at an early stage of acute infection is highly immunogenic. Studies have shown that SARS-CoV-specific IgA, IgG, and IgM antibodies were detected after the onset of symptoms. IgG levels persist for a longer time; however, IgM levels began to fall after 3 months [53].

Antibodies against the S1 glycoprotein, particularly its receptor attaching domain, neutralize SARS-CoV-2 by blocking the virus from binding to ACE2 and thereby severely limiting virus cell entrance. The existence of neutralizing antibodies against SARS-CoV-2 is the strongest current indicator of protection against reinfection or breakthrough infection in vaccinated people [54].

Age itself is associated with an increase in the systemic and tissue levels of different pro-inflammatory mediators and reactive species of oxygen, in realizing a condition defined as “inflame-aging” [55].

COVID-19 causes more severe symptoms in older persons, however, it also affects younger people. It has been observed that young people and children shed equivalent levels of SARS-CoV-2 virus to older adults during infection [56]. This could mean that younger people can tolerate higher viral loads better, or that the immune response to infection in older people has a bigger factor in severity and symptoms. This disparity in symptoms between age groups could be due to younger people’s decreased expression of the SARS-CoV-2 cell entry receptor, ACE2. ACE2 is a protein on human cell surfaces through which SARS-CoV-2 gains entry into the cells. The predominant cell type infected by SARS-CoV-2 is respiratory epithelial cells, and the intrinsic antiviral responses of these cells alter with age. The senescence of respiratory epithelial cells as people get older may help to sustain viral infection and local tissue inflammation [54, 55].

Children develop symptoms that may be milder due to more active innate immune responses, healthier respiratory tracts due to less exposure to air pollution or cigarette smoke, and fewer comorbidities than adults [57]. IFN, which inhibits viral reproduction, and macrophages, which engulf and digest viruses, are released as part of the innate immune response. In addition, children express fewer ACE2 receptors in their airway cells than adults.

DCs (both myeloid and plasmacytoid subsets) in the periphery exhibit a considerable reduction with age. Because DCs are the most essential antigen-presenting cells (APCs) and serve as a link between innate and adaptive immunity, their loss over time contributes significantly to the increased susceptibility to infections as people get older [56].

SARS-CoV-2 adapts to the host and evades the immune system, causing changes in its genome. Amino acid substitutions that cause major changes in the structure and stability of epitopes in the REM sleep behavior disorder (RBD) may be more significant, as these mutations may result in lower antibody-target protein binding affinity and energy. As a result, antibodies that disrupt key epitopes in the RBD are critical in degrading the RBD’s ACE2 receptor binding [59].

As SARS-CoV-2 replicates in humans under selective pressure from natural and vaccine-induced immunity, variants of concern (VOCs) with greater transmissibility or virulence continue to arise [60].

The newly discovered variant B.1.617 is divided into three sublineages (B.1.617.1, B.1.617.2, and B.1.617.3), each with its own mutational profile. Only sublineage B.1.617.2 or Delta is currently recognized as VOC on a global scale. S mutations T19R, G142D, 157–158, L452R, T478K, D614G, P681R, and D950N characterize it. The S protein of the B.1.351 virus largely mimics the structure of the G614 trimer, with nearly comparable biochemical stability. The RBD has not changed significantly as a result of N501Y, K417N, or E484K, however, the lack of salt bridges between Lys417 and ACE2 Asp30 and Glu484 and ACE2 Lys31 mitigates N501Y’s greater receptor affinity [58, 59, 61].

The structural proteins [membrane (M) protein and N] or Nsps (Nsp1, Nsp3b and Nsp6) or PLpro are used by SARS-CoV-2 and other coronaviruses to suppress the signaling cascade (papain-like protease). The JAK/STAT pathway is inhibited by these viral accessory proteins, resulting in gene suppression by IFN-stimulated response element (ISRE) promoters. SARS-nucleocapsid CoV’s protein (N) interferes with IRF3’s function. RNA binding activity at the first recognition stage of viral RNA may contribute to immune evasion, even though it does not form a complex with RIG-I or MDA5. On the one hand, the production of pro-inflammatory cytokines and type I IFNs creates an antiviral immune microenvironment, controlling viral synthesis and infection; however, the SARS-CoV-2 shuts down these signaling pathways to counteract the immune response via various strategies, such as leukopenia [62]. The immunopathogenesis of COVID-19 has been summarized in Table 1.

Studies have shown that due to the reduced synthesis of type I and III IFNs with sufficient ISG expression, as well as increased chemokine secretion, SARS-CoV-2 infection causes an overall decrease in antiviral gene transcription. COVID-19 severity could be caused by a decrease in the innate antiviral response, as well as hyper-inflammation. SARS-CoV-2 infection decreases the immune response against the virus by increasing the fatigue of effector T cells, in addition to reducing T cells (Table 2) [46].

After RLR activation, TANK-binding kinase 1 (TBK1) is recruited to the RLR/mitochondrial antiviral signaling protein (MAVS) signalosome to activate the transcription factors (TFs), IRF3, and NF-B, which activate IFN and IFN gene expression. By altering this signaling complex, a few SARS-CoV-2 proteins have been identified that antagonize dsRNA recognition. In HEK293T cells, Nsp6 and Nsp13 have been discovered to decrease IFN promoter reporter activity in response to RIG-I caspase recruitment domain family member (CARD). This could be due to a decrease in IRF3 (S396) phosphorylation in Nsp6 or Nsp13 expressing cells after pI:C transfection. Few studies have found Nsp13-TBK1 interactions, but none have found Nsp6-TBK161 interactions or Nsp6 suppression of IFN induction. These inconsistencies could be explained by differences in Nsp6 expression levels between experiments, implying that protein expression levels during infection are crucial for immunological antagonism [62].

Activated NF-kB works as a transcriptional activator for a variety of pro-inflammatory cytokines with an NF-kB-response element after translocation into the nucleus. After being phosphorylated by ubiquitin kinases, the IRF3 homodimerizes and travels inside the nucleus, activating type I IFN transcription. Type I IFNs activate the JAK/STAT signaling pathway through the IFN a/b receptor (IFNAR), which is followed by the cytoplasmic protein JAK1 and tyrosine kinase 2 (TYK2) kinases phosphorylating STAT1 and STAT2. STAT1 and STAT2 heterodimers translocate into the nucleus and recruit ISRE transcription to their promoter.

Attenuated vaccines are based on a living microbe weakened not to cause disease. Microbes that have been attenuated retain their ability to multiply in vivo, resulting in restricted illness and they effectively stimulate the immune system and induce an intense and persistent immune memory that is efficacious in preventing infection. The YF-S0 vaccine, which uses a live-attenuated yellow fever 17D (YF17D) vaccine as a vector to express a non-cleavable pre-fusion form of the SARS-CoV-2 S antigen is an example of an attenuated vaccination for SARS-CoV-2 [68].

Modern therapeutics, based in Boston and the National Institute of Allergy and Infectious Diseases, collaborated to develop mRNA-1273. This is one of the first vaccine candidates which entered the clinical trials in just 63 days after the genome of SARS-CoV-2 was sequenced. A mRNA molecule in this vaccine design is capable of synthesizing a lipid nanoparticle (LNP) encapsulated pre-fusion version of the SARS-CoV-2 S protein. This form of the LNP encapsulated S protein makes its uptake easier by the host cells. The second candidate is mRNA-BNT162b2. Pfizer created the BNT162b2 mRNA platform in conjunction with BioNTech in Germany and Fosun Pharma in Shangai, China. It promotes the synthesis of SARS-CoV-2 S protein. A higher dose of RNA is required for mRNA than for delf-reoplicating RNA, which amplifies itself [69].

CanSino Biologics developed the Ad5-nCoV vaccine (a China-based company), using human adenovirus serotype five vectors to deliver the message that codes for COVID-19 virus full-length S protein into human cells. An HRB26M mouse-adapted SARS-CoV-2 virus was used to challenge the immune mice. Immunized mice showed no signs of viral replication in their lungs or any other obvious viral symptoms. Immunized ferrets were subjected to wild-type SARS-CoV-2, and the results of vaccination immunogenicity and viral challenge protection were repeated [70]. Oxford University’s AZD1222 is a viral vectored vaccine, while AstraZeneca is another contender. This vaccine from Oxford University was the first to enter clinical trials using a weakened chimp adenovirus (ChAdOx1) platform. Because very few humans have had previous interaction with a simian virus, this was done to avoid the issue of pre-existing immunity to the vector. Another, Russian Research Institute, Gamelaya produced Sputnik V, which is the only heterologous prime-boost coronavirus vaccine. This vaccine candidate can circumvent the challenge of reduced immunogenicity due to immunoglobulins built against the viral vector after the initial immunization [71]. Here the adenoviral vector serotype used for the prime vaccination is more diverse from the adenoviral serotype used as a booster, thus giving an imperative and stronger immunity in the first shot itself. In the list of vaccine candidates, Janssen Pharmaceuticals developed JNJ-78436735. This comes under Johnson & Johnson Pharmaceuticals’ vaccine development division. This vaccine is made up of a replication-defective adenovirus 26-based vector that expresses the COVID-19 virus’s stabilized pre-fusion S protein, which was developed by Beth Israel Deaconess Medical Center researchers in Boston a decade ago [71].

A Maryland based pharma company developed an inactivated, protein subunit vaccine—NVX-CoV2373. It comprises a pre-fusion full-length recombinant COVID-19 glycoprotein nanoparticle that was exposed in a baculovirus/Sf9 system and treated with Matrix M1 adjuvant. This saponin-based Matrix M1 adjuvant-based RBD-dimeric antigen, developed by Anhui Zhifei Longcom Biopharmaceutical and the Chinese Academy of Medical Sciences’ Institute of Microbiology, investigates the many forms of the whole S protein or its RBD. As a result, when these moieties engage with the ACE2 receptor on the host cell, the pre-fusion conformation changes to a highly stable post-fusion conformation. This strengthens the viral-host cell bond. These pre-fused viral proteins are more immunogenic, thus are explored well as lucrative vaccine candidates. This ZF2001 is the most recent subunit vaccine candidate in phase 3 clinical trials that was launched in December 2020 [69, 70].

Virus like vaccines [virus-like particles (VLP’s)] is basically made of empty virus particles lacking genetic material. These VLP’s like CoVLP represent many numbers of copies of the same antigen on their surface making it more immunogenic, thus helping to trigger an efficient immune response. It allows rapid uptake by APCs followed by phagocytosis and presentation by the DCs [71]. They are much more efficient than attenuated vaccines but because of their complexity in development and assembly, makes their production technically challenging. The first trials were done by a Canadian Pharmaceutical firm Medicago Inc. 180 participants were enrolled in the first phase of clinical trials (age 18–55). Adjuvants like CpG1018/As03 (DynaVac) helped in the induction of robust cellular and humoral immune response [72, 73]. Study results on CoVLP indicated that administration was well tolerated with only mild to moderate transient adverse events [74].

The coronaVac vaccine was initially launched under PiCoVacc, which is a purified, inactivated viral vaccine with alum-adjuvant. This potential vaccine was created by propiolactone-activation of the CN2 strain of SARS-CoV-2 using a broncho-alveolar lavage of a hospitalized SARS-CoV-2 patient [71]. Sinopharm collaborated with Beijing Biological Products to create BBIBP-CorV, the second inactivated viral vaccine candidate. BBIBP-CorV was created by inactivating the 19nCoV-CDC-Tan-HB02 strain SARS-CoV-2 in Vero cells using propiolactone [72]. Adjuvant, aluminum hydroxide in this vaccine stimulates the receptor subunit Nod-like receptor family pyrin domain containing 3 (NLRP3) from the inflammasome and inflammasome-derived IL-1β and IL-18 release is observed at a marked increased level. These cytokines thus stimulate the proinflammatory cells of the immune system [73]. Bharat Biotech an India-based pharmaceutical company that produced Covaxin/BBV15 is a whole virion inactivated vaccine candidate approved by the Indian Council of Medical Research. This vaccine was created utilizing a new coronavirus that was identified by the Indian National Institute of Virology and propagated on Vero CCL81 cells before being inactivated with propiolactone.

The DNA-based platforms offer great versatility while handling the coded antigen and great potential to advance. Till date no single DNA vaccine has been registered for human use; however, they are commonly used in veterinary medication. Certain vaccines are stable and can quickly be produced in large quantities in bacteria. Once inserted by the myocyte pathway, DNA plasmids penetrate human cells, and a small local electrical pulse may boost their ability to do so (electroporation) [75]. Once entered by the APC pathway (Figure 2), plasmid DNA causes the cell to produce the target protein briefly. This way, DNA vaccination stimulates the APC and enhances the production of antibodies. A similar scenario can also be seen in gut-associated lymphoid tissues (GALTs) such as the Peyer’s patches [76].

The till date vaccine candidates available or which are under different phases of development are summarized in Table 3.