Vaccines overview:
Recently, there have been several attempts to create vaccines against
human Corona virus infections, however, all were limited due to their
wide diversity of sequences(71). Several vaccines and immunotherapies
have been tried during the latest viral epidemics such as Zika, Ebola
and the previous CoVs family infections. Most of these trials have been
investigated firmly to assess their applicability and usefulness in
preventing the current pandemic (72).
Most of the current CoV vaccines attempts are targeting the S protein of
the virus as it is the principal promotor of antibodies development and
T-cell responses making it the perfect candidate in vaccines development
strategies. Examples of these vaccines are involving the full length S
protein or other appropriate parts of it or S1 receptor binding domain
or virus like particles (VLP) , viral vectors or DNA (71), (73), (74)and
(75).
The most efficient vaccine should enhance both the production of
blocking antibodies that target the S1 subunit receptor binding domain
to block the binding of the S1 protein of the virus to its receptor, in
addition to blocking the viral RNA uncoating. Chen et al., has
demonstrated that the C-terminal domain of the S1 subunit of
porcine Deltacoronavirus conatins the immunodominant region that
evolves the strongest blocking effect(76). Furthermore, because of RBD
capability to trigger the formation of neutralizing antibodies, both
recombinant peptides containing RBD and recombinant vectors encoding RBD
may be promising for the production of successful SARS-CoV vaccines(74).
Kim et al has shown that the nasal administration of recombinant
adenovirus-based vaccines that express MERS Spike protein into mice,
enhances the production of IgG and secretory IgA antibodies as well as
inducing the activation of T lymphocytes and development of memory cells
which reside mainly in lungs giving those mice life-long immune response
responses(77).
Moreover, a study which compared the effect of rabies virus as a viral
vector against Gram-positive enhancer matrix (GEM) as a bacterial
vector; both of them expressing MERS Spike protein; they found that the
viral vector vaccine gave the mice markedly stronger cell mediated
immune response and faster humoral immunity(78). Additionally, knowing
that there are much similarities between SARS and MERS, the
applicability of designing one vaccine working against all CoV family
viruses was investigated and they found that there is possible
cross-reactivity among CoVs(79).
Also, since SARS-CoV-2 and SARS exhibit antigenic similarity, vaccines
developed against SARS could be cross-reactive against SARS-CoV-2 (80).
However, when the sequences of the full-length S protein of SARS-CoV-2
was compared with that of SARS, they found that the most variable
regions exist in the S1 subunit of spike protein which is normally the
main target of most developed vaccine(81) which may assume the
difficulty of designing one common efficient vaccine for both viruses
(82).
The nucleocapsid (N) protein and the possible B cell epitopes of MERS E
protein have been recommended as feasible targets that could initiate
cell mediated and humoral immune interactions(83). In addition, reverse
genetic approaches were applied in live-attenuated vaccines to
deactivate the exonuclease effects of non-structural protein 14 (nsp14)
or to eliminate the envelope protein in SARS (71). Avian infectious
bronchitis virus (IBV) is a chicken Corona virus and it was suggested by
Bijlenga that strain H of avian live virus IBV vaccine might be helpful
in protecting against SARS (84). Since this vaccine is depending mainly
on the production of neutralizing antibodies, so it may be suggested as
another effective choice for protection against SARS-CoV-2 after
assessing its efficacy in monkeys(85).
Rocky Mountain Laboratories are cooperating with Oxford University to
grow an adenovirus vector vaccine against SARS-CoV-2 to be tried in
chimpanzees first. In addition, another study is investigating clinical
trials for a new vaccine expressing SARS-CoV-2 S protein in the mRNA
vaccine platform technology (86). Lately, the Coalition for Epidemic
Preparedness Innovations (CEPI) declared the launch of three plans
designed to produce SARS-CoV-2 vaccines using already existing vaccines’
platforms. These platform work through the synthesis of viral surface
proteins which bind to the host cell membrane and clamp the cells into
shape. This could enhance better and faster detection of viral antigens
by the host immunity(87).