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American Journal of  
Chemistry and Pharmacy (AJCP)

Unveiling the SARS-CoV-2 Spike Protein: A Comparative Analysis of  Vaccine 
Development Approaches and Glycosylation Implications

Leonel C. Mendoza1*

Volume 2 Issue 2, Year 2023
ISSN: 2834-0116 (Online)

DOI: https://doi.org/10.54536/ajcp.v2i2.1634
https://journals.e-palli.com/home/index.php/ajcp

Article Information ABSTRACT

Received: May 05, 2023

Accepted: June 25, 2023

Published: July 03, 2023

In December 2019, a mysterious pneumonia-causing sickness frightened the world. SARS-
CoV-2 caused the acute respiratory illness. Since March 11, 2020, 220,563,227 COVID-19 
cases and 4,565,483 deaths have been reported worldwide as of  October 2021. SARS-
CoV-2, like all coronavirus, appears to have crowns due to its S proteins and enters host 
cells using highly glycosylated spike (S) proteins. S1 and S2 are SARS-CoV-2 spike protein 
subunits. S2 controls transmembrane fusion, while S1 controls receptor binding. Antibody-
mediated neutralization targets SARS-CoV-2 spike (S) proteins, which are essential for viral 
entry and fusion. This paper summarized how S protein was used in newly created and 
distributed SARS-CoV-2 vaccines and the implications for future advancements given the 
emergence of  more lethal SARS-CoV-2 variants in this paper. It also discussed the role 
of  S protein glycosylation in the viral entry and binding mechanism of  SARS-CoV-2 and 
the implications for developing adaptive immunity and vaccines. The review was carried 
out through a deductive search strategy with keywords: COVID-19 vaccines, nCoV-2019 
vaccines, coronavirus, COVID-19 vaccine development, S protein, and protein glycosylation 
using Google Scholar. The emergence of  more transmissible and potentially more lethal 
SARS-CoV-2 variants, such as the Delta variant, highlights the need for continued research 
on vaccine development. Future research should focus on understanding the mechanism of  
the spike protein and how vaccines can effectively target the mutated regions. Continued 
monitoring and adaptation of  vaccination strategies are essential to control the ongoing 
COVID-19 pandemic.

Keywords
Coronavirus, COVID-19 
Vaccines, COVID-19 
Vaccine Development, nCoV-
2019 Vaccines, Protein 
Glycosylation, S Protein

1 College of  Teacher Education, Mindoro State University-Calapan City Campus, Philippines
* Corresponding author’s e-mail: leonel.mendoza@minsu.edu.ph

INTRODUCTION
Coronaviruses are single-stranded RNA viruses 
classified into three genera (alpha, beta, and gamma 
coronaviruses), which correspond to groups 1, 2, and 
3 of  the coronavirinae subfamily, coronaviridae family, 
and nidovirales order or superfamily (Weiss & Leibowitz, 
2011). Coronaviruses infect a wide range of  animals, 
both domestic and wild, and humans (Pedersen & Ho, 
2020). Six strains, namely, HCoV‐229E, HCoV‐OC43, 
HCoV‐NL63, HKU1, SARS-CoV, and MERS-CoV, are 
known to cause respiratory diseases (Hasöksüz et al., 
2020). SARS-CoV and MERS-CoV variants infect the 
lower respiratory tract and are therefore more harmful 
(McIntosh & Peiris, 2009); the others, on the other hand, 
infect the upper respiratory tract but only in moderate 
cases (Fehr et al., 2015). 
In December 2019, the globe was alarmed by reports of  
an emergent illness of  unknown origin, characterized by 
pneumonia, and epidemiologically related to a seafood 
market in the Chinese city of  Wuhan (Zhu et al., 2019). A 
novel coronavirus (SARS-CoV-2) has been identified as 
the source of  the acute respiratory infection by employing 
a “pneumonia of  unknown etiology” surveillance system 
built in the aftermath of  the 2003 SARS outbreak to allow 
early detection of  new infections (Li et al., 2020a; Zhu 
et al., 2019). COVID-19 was declared a “public health 
emergency of  international concern” (Li et al., 2020b) 
by the World Health Organization (WHO) on the 30th 
of  January 2020, and then a pandemic on the 11th of  
March 2020. (Olson et al., 2020). On September 7, 2021, 

it was reported that there were 220,563,227 confirmed 
COVID-19 cases and 4,565,483 confirmed deaths 
globally (WHO, 2021).
SARS-CoV-2 is a member of  the betacoronavirus genus 
(Perlman & Netland, 2009; Schoeman & Fielding, 2019; 
Vlasova et al., 2007), which is one of  the genera of  the 
Orthocoronavirinae subfamily (Hasöksüz et al., 2020). 
It is quite similar to the SARS-CoV virus, which caused 
a worldwide epidemic in 2003. SARS-CoV-2 share the 
identical tropism and mode of  entry as SARS-CoV– 
about 80 percent (Zhou et al., 2020)– because they use the 
same cellular receptor, angiotensin-converting enzyme 2 
(ACE2) (Yan et a., 2020). Like all coronaviruses, SARS-
CoV-2 appears to have crowns because of  the S proteins 
that protrude on its surface (Zhao et al., 2021). SARS-
CoV-2 uses these highly glycosylated spike (S) proteins 
(Wrapp et al., 2020).
The purpose of  this research is to describe how the 
SARS-CoV-2 S protein was employed in the development 
of  vaccines given its critical function in receptor binding 
and membrane fusion in the SARS-CoV-2 virus and 
discuss the glycosylation of  its proteins. Specifically, 
this paper compared each approach on which S protein 
is given focus in vaccine development. By doing so, this 
paper can yield implications for future studies.

LITERATURE REVIEW
Structure
The S protein is a class I fusion protein that helps the 
virus connect to the angiotensin-converting enzyme 2 

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(ACE2) receptor on the host cell surface, causing the virus 
and cell membrane to fuse (Pardi et al., 2015; Hoffman 
et al., 2020; Rauch et al., 2018). The overall length of  S 
protein in SARS-CoV-2 is 1,273 amino acids, including a 
signal peptide at the N-terminus (Zhao et al., 2021). Two 
subunits, namely, S1 and S2, composed the S protein of  
SARS-CoV-2. These subunits are cleaved from a furin site 
in the virus’ S protein, containing multiple essential amino 
acids. The S1 subunit is apical V-shaped and harbors one 
ACE2-recognition motif  per monomer, the receptor-
binding domain (RBD) (Sterberg & Naujokat, 2020).
SARS-CoV-2 uses a spike (S) protein that is highly 
glycosylated to gain entry to host cells, as previously 
stated (Wrapp et al., 2020). The S protein is glycosylated 
through the secretory pathway and is glycosylated by the 
host cellular glycosylation mechanism (Zhao et al., 2021). 
With this process comes two benefits for the virus itself. 
To begin with, the mannose residues within these glycans 
are crucial components in interacting with cell surface 
attachment proteins (Li et al., 2017; Tortorici et al., 2019; 
Robson, 2020) before attaching to ACE2. Second, it 
helps protect the virus from host antibodies by hiding 
the underlying polypeptide epitopes (Doores, 2015; 
Bagdonaite & Wandall, 2018).

Function
As mentioned, the S protein in SARS-CoV-2 is 
responsible for receptor identification and membrane 
fusion (Gallagher & Buchmeier, 2001), which are carried 
out by functional domains near the S1 and S2 termini 
(Hasöksüz et al., 2020). The S2 subunit is in charge of  
transmembrane fusion, whereas the S1 subunit controls 
receptor binding (Chen & Guo, 2020; Hasoksuz et al., 
2002). The trimeric S protein is cleaved into S1 and S2 
subunits after viral infection and S1 subunits are released 
during the transition to the postfusion conformation 
(Song et al., 2018; Simmons et al., 2013; Belouzard et al., 
2009; Simmons et al., 2004).
Because it contains the receptor-binding domain (RBD), 
which directly connects with ACE2, a peptidase domain, 
the S1 subunit can bind to receptors (Li et al., 2005). To 
engage a host cell receptor, S1’s receptor-binding domain 
(RBD) undergoes hinge-like conformational movements, 
briefly hiding or exposing the receptor binding 
determinants (Wrapp et al., 2020). When S1 connects to 
the ACE2 receptor on the host, another cleavage site on 
S2 is revealed and cleaved by host proteases, a step that is 
essential for viral infection (Belouzard et al., 2009; Millet 
& Whittaker, 2015; Simmons et al., 2005). The polybasic 
cleavage site of  S may contribute to SARS-CoV-2’s 
high virulence, as furin and furin-like proteases, which 
are necessary for S’s proteolytic activation, are widely 
expressed in humans, allowing SARS-CoV-2 to infect a 
broader range of  tissues.
Furthermore, multiple investigations have suggested that 
the S protein of  SARS-CoV-2 may employ ACE2 to infect 
the host (Zhou et al., 2020; Hoffman et al., 2020; Kuba 
et al., 2005; Li et al., 2003). ACE2 is important because 

it aids in the maturation of  angiotensin, a hormone that 
regulates blood pressure and vasoconstriction (Yan et al., 
2020). Cardiovascular illnesses are very likely when ACE2 
expression in the lungs, heart, kidneys, and intestines 
is reduced (Crackower et al., 2002; Zisman et al., 2002; 
Raizada et al., 2007). The S protein is a target for antibody-
mediated neutralization because of  its essential function. 
Furthermore, studying the prefusion S structure can help 
with vaccine development.

MATERIALS AND METHODS
The review was carried out through a deductive search 
strategy with keywords: COVID-19 vaccines, nCoV-2019 
vaccines, coronavirus, COVID-19 vaccine development, 
S protein, and protein glycosylation using Google 
Scholar. Articles were included if  they were published 
in English, appeared in peer-reviewed publications, and 
were related to the issue of  the SARS-CoV-2 virus and 
vaccine development. Statistics from the World Health 
organization were also included. The flowchart outlining 
the search technique is presented in the figure below.

Figure 1: Search strategy employed

RESULTS AND DISCUSSION
Glycosylation of  SARS-CoV-2 S Protein
Viral protein glycosylation is a successful virus strategy 
for modifying its proteins using the host-cell machinery. 
As a result, glycans play critical roles in viral infection and 
immune response (Zhao et al., 2021). Glycans on viral 
surface proteins play a role in viral entry, fusion, epitope 
shielding, viral protein folding, stability, and protection. 
For instance, by shielding surface antigens with glycan 
envelopes, glycosylation on the surface proteins of  
viruses can prevent antibodies from binding, which plays 
a significant role in viral infection (Shajahan et al., 2021). 
As mentioned, the S protein of  SARS-CoV-2 is highly 
glycosylated. SARS-CoV-2 has 22 N-glycosylation sites 
and several O-glycosylation sites on each protomer of  
the transmembrane homotrimeric protein (Watanabe 
et al., 2020). SARS-CoV-2 infection is influenced by the 
glycosylation process of  both the virus and the target 
cells on several levels, according to recent reports (Reis 

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et al., 2020). Given this, it’s worth examining how this 
process works and how it affects SARS-CoV-2 because it 
could lead to vaccine development breakthroughs.
For replication and protein glycosylation, SARS-CoV-2 
uses the machinery of  the host cell. As a result, the viral 
surface glycans are composed of  host glycans that the 
immune system recognizes as self, suppressing the anti-
carbohydrate immune response (Crispin et al., 2015). 
Moreover, T cell activation and cytokine production are 
affected by differential hemagglutinin N-glycosylation, 

which poses a challenge for vaccine development 
(Hütter et al., 2013). Furthermore, as shown in the case 
of  influenza viruses, mutations in protein sequences 
can alter glycosylation by generating new or removing 
existing glycosylation sites (Altman et al., 2019; Zost et 
al., 2017). This could lead to the emergence of  new virus 
strains (Altman et al., 2019). This paper summarizes how 
protein glycosylation plays a role in SARS-CoV-2 virus 
replication, binding and entry, immune response, therapy, 
and vaccine development in the table below.

Table 1: Biologic and pathologic effects of  glycosylation of  SARS-CoV-2 S protein and other insights from literature
Aspect Role of  glycosylation in biology and pathogenesis of  SARS-CoV-2 S protein and 

additional insights
Reference/s

Viral entry Glycosaminoglycan heparan sulfate of  the cellular glycocalyx is required for 
SARS-CoV-2 infection of  these target cells. Heparan sulfate was found to interact 
with the receptor-binding domain of  the SARS-CoV-2 spike glycoprotein, which 
is located next to ACE-2, causing the spike structure to open up and allow ACE-2 
to bind to it.

Clausen et al. 
(2020)

The conformation of  the S protein’s receptor-binding domain has been shown to 
be modulated by glycans at N165 and N234 of  the protein.

Casalino et al. 
(2020)

A sialic acid-binding pocket was discovered at the N-terminus of  the S protein 
from SARS-CoV-2, suggesting that it may play a role in viral binding.

Verma (2020)

Human lectins galectin-3, 7 and 8, Siglec-10, macrophage galactose lectin (MGL), 
and dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin 
(DC-SIGN) bind to two N-glycans of  the RBD of  the S protein at N331 and 
N343, which may provide insight into SARS-CoV-2 tropism and binding.

Lenza et al. 
(2020)

Adaptive 
immunity

When compared to other blood groups, the proportion of  seropositives in group 
O individuals was significantly lower, implying that group O individuals have a 
lower risk of  infection.

Gallian et al. 
(2020)

Sulfate glycans on the SARS-CoV-2 S protein could act as selectin ligands and play 
a role in immune regulation.

Lowe (2002)

Variations in virus and host glycosylation are likely to influence tissue tropism 
and individual infection susceptibility; differences in virus infectivity and patient 
susceptibility may be caused by SARS-CoV-2 and ACE 2 mutations that alter 
N-glycosylation sites.

Reis et al. 
(2021)

Vaccine 
development

SARS-CoV-2 glycosylation can affect each of  the current vaccine strategies in 
different ways (i.e., vector-based vaccine, mRNA-based vaccines).

Reis et al. 
(2021)

Suitable glycosylation of  the recombinant protein will at least partly determine 
how immunogenic polypeptide epitopes of  vaccine glycoproteins fold.

Reis et al. 
(2020)

The presence of  glycans on protein antigens influences cellular uptake, proteolytic 
processing, MHC presentation, and subsequent T-cell priming, providing insight 
into how proper folding of  recombinant vaccine glycoprotein can affect the 
development of  an adaptive immune response

Wolfert & 
Boons (2013)

The presence of  non-human glycans on recombinant therapeutic glycoproteins 
may induce antibodies directed against non-human glycan epitopes.

Zhou & Qiu 
(2019)

Vaccines Based on the Sars-Cov-2 S Protein
Because the S protein is involved in receptor binding and 
membrane fusion, vaccinations may induce antibodies 
that prevent virus binding and fusion or neutralize virus 
infection. S protein is the major antigenic component of  
SARS-CoV. It is essential to generate host immunological 

responses, neutralizing antibodies, and protective immunity 
against virus infection among all structural proteins (Du et 
al., 2009). This paper reviewed and compared each approach 
on which S protein is given focus in vaccine development. 
Some of  these vaccines are already being used and distributed 
worldwide. Table 1 summarizes the findings.

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mRNA-based
Three mRNA-based vaccines, BioNTech/Pfizer 
(BNT162b2 mRNA), Moderna (mRNA-1273), and 
CureVac (CVnCoV), have been reported (Sadarangani et 
al., 2021; Kowalzik et al., 2021). This type of  vaccine is 
said to be different from the traditional kind of  vaccine. 
In an mRNA vaccine, a transcript encoding one or more 
immunogens is delivered into the host cell’s cytoplasm, 
which is translated into immunogenic proteins (Kowalzik 
et al., 2021). mRNA vaccine manufacture does not entail 
infectious elements or the risk of  stable integration into 
the host cell genome; instead, the vaccine RNA strand is 
destroyed (Sadarangani et al., 2021). Because of  this, this 
type of  vaccine is safer.
In the case of  BioNTech/Pfizer, a prefusion stabilized, 
membrane-anchored SARSCoV-2 full-length spike 
protein is encoded (Polack et al., 2020; Walsh et al., 2020). 
Two proline mutations modify this encoding S protein 
to lock protein in the pre-fusion conformation (Pardi et 
al., 2015; Kariko et al., 2005; Wrapp et al., 2020). mRNA-
based vaccines, particularly the BioNTech/Pfizer vaccine, 
elicit high SARS-CoV-2 neutralizing antibody titers and 
robust antigen-specific Th1-type CD4+ and CD8+ T-cell 
responses, similar to DNA-based vaccines developed 
for SARS-CoV, which induce neutralizing antibody and 
T-cell responses and protective immunity (Wang et al., 
2005; Huang et al., 2007; Liu et (Sadarangani et al., 2021). 
The S1-binding antibody is also present after the first of  

Table 2: Summary of  key findings
Vaccine 
Developer

Nature of  the 
vaccine

S protein utilization approach Reference/s

BioNTech/
Pfizer

mRNA-based prefusion stabilized, membrane-anchored SARSCoV-2 
full-length spike protein is encoded

Polack et al., 2020; 
Walsh et al., 2020

Moderna mRNA-based encodes the SARS-CoV-2 stabilized prefusion spike 
glycoprotein trimer

Baden et al., 2021

University 
of  Oxford/
Astra-Zeneca

Viral vector-based formulated with simian adenovirus vector encoding the 
full-length S protein with a tPA leader sequence that is 
recombinant and replication-deficient

Folegatti et al., 
2020

Gamaleya 
Research 
Institute

Viral vector-based made up of  full-length S protein-expressing recombinant 
and replication-deficient human adenovirus 26 (dose 1) 
and human adenovirus 5 (dose 2)

Logunov et al., 
2020

Janssen Viral vector-based formulation was characterized by a recombinant, 
replication-deficient human adenovirus 26 producing 
full-length S protein with two amino acid alterations in 
the S1/S2 junction that eliminate the furin cleavage site 
and two proline substitutions in the hinge region that 
keep the protein in the pre-fusion conformation

Bos et al., 2020

Novavax Protein subunit uses a recombinant nanoparticle of  full-length S protein 
with protease resistance mutations at the S1/S2 cleavage 
sites and two proline substitutions to stabilize protein 
in a pre-fusion conformation, together with a saponin-
based adjuvant (Matrix- M1)

Keech et al., 2020

Sinovac 
Biotech

Whole cell 
inactivated virus

the attenuated or inactivated whole SARS-CoV-2 virus 
is administered to individuals to elicit the immune 
responses, which target the SARS-CoV-2 S protein.

Li et al., 2021

two doses (Walsh et al., 2020).
Like BioNTech/Pfizer’s the Moderna vaccine encodes 
the SARS-CoV-2 stabilized prefusion spike glycoprotein 
trimer, which is necessary for host cell attachment and 
viral entry (Baden et al., 2021). S-binding antibody was 
also detected 14 days after the first dose, with levels 
increasing marginally by 28 days and significantly after 
the second dose (Jackson et al., 2020). After the second 
treatment, CD4+ T cells secreting TH1 cytokines 
(TNF>IL-2>IFNγ) increased significantly (Jackson et al., 
2020).

Viral vector-based
Incorporating immunogenic full-length or truncated viral 
surface proteins into viral expression has been a common 
strategy for vaccine development against infectious 
agents, primarily viruses (Lundstrom, 2020). Various viral 
vector expression techniques have been used to target 
viral surface proteins. Adenoviruses (Ads), alphaviruses, 
flaviviruses, measles viruses (MVs), rhabdoviruses, 
retroviruses (RVs), lentiviruses (LVs), and poxviruses are 
all examples of  expression vectors (SM Wold & Toth, 
2013; Lundstrom, 2019). Four viral vector-based vaccines, 
namely, University of  Oxford/Astra-Zeneca (ChAdOx1 
nCoV-19), Gamaleya Research Institute (Gam-COVID-
Vac), Janssen (Ad26.COV2.S), and CanSino Biologics 
(Ad5- nCoV), have been reported (Sadarangani et al., 
2021; Knoll & Wonodi, 2021).

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The University of  Oxford/Astra-Zeneca vaccine is 
formulated with a simian adenovirus vector encoding 
the full-length S protein with a tPA leader sequence that 
is recombinant and replication-deficient (Folegatti et al., 
2020). When mice and rhesus macaques were immunized 
with the chimpanzee Ad vector ChAdOx1 nCoV-19, 
which was designed to produce the SARS-CoV-2 S 
protein, it induced robust humoral and cellular immune 
responses and prevented pneumonia in macaques (van 
Doremalen et al., 2020; Folegatti et al., 2020). In humans, 
S-binding antibody was detected 14 days after the first 
dosage, and levels increased by 28 days; there was a 
significant rise after the second dose, with a peak at 14 
days. Also, T cell responses peaked 14 days following 
the first dosage but were slightly greater 28 days later 
(Folegatti et al., 2020).
Meanwhile, the Gamaleya Research Institute vaccine is 
made up of  full-length S protein-expressing recombinant 
and replication-deficient human adenovirus 26 (dose 1) 
and human adenovirus 5 (dose 2) (Logunov et al., 2020). 
S-binding antibody was found in 85–89% of  people 14 
days after the initial dose (Logunov et al., 2020). Fourteen 
days after the second dose, S antibody levels are boosted 
(Logunov et al., 2021). Based on proliferation assays and 
antigen-specific IFN production, CD4+ and CD8+ T 
cell responses were seen 14 days after the initial dosage 
(Lugonov et al., 2021).
For the Janssen vaccine, the formulation was characterized 
by a recombinant, replication-deficient human adenovirus 
26 producing full-length S protein with two amino 
acid alterations in the S1/S2 junction that eliminate 
the furin cleavage site and two proline substitutions 
in the hinge region that keep the protein in the pre-
fusion conformation (Bos et al., 2020). After stabilizing 
substitutions were inserted, the ratio of  neutralizing vs. 
non-neutralizing antibody binding increased, indicating 
that the S protein was in a prefusion conformation, 
as observed in its experimental stages. S-binding and 
neutralizing antibodies were detectable in 99 percent of  
people 28 days after immunization, and antibody levels 
were maintained for at least 84 days (Janssen Biotech, 
2021; Sadoff  et al., 2021).

Protein Subunit
To elicit an immune response, protein subunit-based 
vaccinations combine a protein (or part of  a protein) 
from the targeted virus with an immune-boosting 
chemical termed an adjuvant (Wadman, 2020). Adjuvants 
are frequently required for this type of  vaccine to increase 
the immune response and improve vaccine efficacy (Brito 
et al., 2013). When a protein-based vaccination is taken up 
and processed into several epitopes by cells, it provides 
a more focused response to a particular antigen (Li et al., 
2021). The antigen in most of  these vaccinations is either 
the S protein or its receptor-binding domain (RBD).
The Novavax vaccine, one of  the vaccines that utilize 
protein subunit, uses a recombinant nanoparticle of  full-
length S protein with protease resistance mutations at 

the S1/S2 cleavage sites and two proline substitutions to 
stabilize protein in a pre-fusion conformation, together 
with a saponin-based adjuvant (Matrix- M1) (Keech et al., 
2020). Antibody to S-binding was identified 21 days after 
the first treatment, and significantly increased after the 
second dose. Meanwhile, based on IFN, IL-2, and TNF 
production in response to S protein stimulation, CD4+ 
T cell responses were detectable seven days after the 
second dosage, with a substantial bias towards a TH1 cell 
phenotype (Keech et a., 2020).

Whole Cell Inactivated Virus
Sinovac Biotech (CoronaVac), Sinoparm (BBIBP- CorV; 
WIBP- CorV), and Bharat Biotech are three SARS-
CoV-2 vaccines that use the whole-cell inactivated virus 
method (BBV152) (Sadarangani et al., 2021). Inactivated 
vaccines, also known as killed vaccines, are made by 
growing the virus in a culture medium and then treating it 
with chemicals, heat, or radiation to inactivate it (Li et al., 
2021). The majority of  these vaccines contain aluminum 
hydroxide (Flanagan et al., 2020; WHO, 2021). The Sinovac 
Biotech vaccine, for example, is made from SARS-CoV-2 
produced in Vero cells, inactivated using β-propiolactone, 
and adsorbed onto aluminum hydroxide. In the case of  
vaccines developed against COVID-19, the attenuated 
or inactivated whole SARS-CoV-2 virus is administered 
to individuals to elicit immune responses (Li et al., 2021). 
These immune responses target the SARS-CoV-2 S 
protein. Inactivated vaccines have a long history of  use and 
can benefit many people, even individuals with advanced 
immunological senescence (Iversen & Bavari, 2021).

CONCLUSION
SARS-CoV-2 mutations have been on the rise recently. The 
Delta variant, for example, has been linked to a surge in 
cases in India and has now been found worldwide, including 
a significant increase in cases in the United Kingdom 
(Bernal et al., 2021). The spike protein mutations T19R, 
157-158, L452R, T478K, D614G, P681R, and D950N 
distinguish the delta variant (ECDPC, 2021). Several of  
these mutations may affect immune responses directed at 
the receptor-binding protein’s key antigenic regions (452 
and 478) and the deletion of  a portion of  the N-terminal 
domain (Li et a., 2020). P681R is located at the S1–S2 
cleavage site, and it appears that strains with mutations 
there has greater replication, resulting in higher viral loads 
and transmission (Johnson et al., 2020). This advent of  
more lethal SARS-CoV-2 variations raised questions about 
the efficiency of  existing vaccines, which were meant to 
combat the initial SARS-CoV-2 strain. As a result, future 
vaccine development research must focus on how vaccines 
can effectively battle these developing strains, focusing on 
the mechanism of  the SARS-CoV-2 S proteins, which is 
the site of  mutations in the Delta variant.

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