Coronaviruses are large, enveloped, positive-strand RNA viruses. Several coronaviruses
are pathogenic in humans, including severe acute respiratory syndrome coronavirus
(SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV) and this novel
virus, SARS-CoV-2, which has caused the global pandemic currently. Genetic sequencing
suggests that SARS-CoV-2 is related to SARS-CoV and its close relatives (Chen et al.
2020). There are four genera of coronavirus, including Alphacoronavirus, Betacoronavirus,
Gammacoronavirus and Deltacoronavirus. Like other Betacoronaviruses, the genome of
SARS-CoV-2 codes four major structural proteins, including the spike surface glycoprotein
which is called S protein, small envelope protein, matrix protein, and nucleocapsid
protein. S proteins are type I transmembrane proteins with a large ectodomain and
a short endodomain. Coronavirus replication is initiated by the binding of S protein
to the cell surface receptor(s). The S protein is composed of two functional subunits,
S1 (bulb) for receptor binding and S2 (stalk) for membrane fusion. Specific interaction
between S1 and receptor triggers a drastic conformational change in the S2 subunit,
leading to the fusion between the virus envelope and the cellular membrane and release
of the nucleocapsid into the cytoplasm. Receptor binding is the major determinant
of host range and tissue tropism for a coronavirus (Cui et al. 2019). SARS-CoV-2 is
able to use human angiotensin-converting enzyme 2 (ACE2) as an entry receptor in ACE2-expressing
cells, indicating that ACE2 is likely the cell receptor of SARS-CoV-2 (Zhou et al.
2020). McLellan et al. showed biophysical and structural evidence that SARS-CoV-2
S protein binds ACE2 with higher affinity than SARS-CoV S protein (Wrapp et al. 2020).
To date, although some repurposed drugs showed inhibitory effect on SARS-CoV-2, no
drugs have been licensed to be used in the treatment of SARS-CoV-2 infection or other
coronavirus infections in humans, and no vaccines have been licensed to prevent these
infections. Just days after Chinese scientists shared the genetic map of SARS-CoV-2,
scientists around the world, along with a list of biotech and vaccine companies, promptly
launched a race to pursue different types of vaccines to prevent infection with this
novel virus. Up to the end of April 2020, at least 115 SARS-CoV-2 vaccine projects
have been announced to be in development, including vectored vaccines, oral vaccines,
subunit vaccines, inactivated vaccines, peptide-based vaccines, nucleotide-based vaccine,
etc. Several SARS-CoV-2 vaccine candidates have recently been approved for clinical
phase I or II development (Thanh Le et al. 2020).
Among these SARS-CoV-2 vaccine candidates, mRNA vaccines are quite remarkable. What
is mRNA vaccine? Unlike traditional vaccines, an mRNA vaccine consists of an mRNA
sequence encoding a disease-specific antigen. Once delivered into target cells, the
antigen is expressed, processed, presented and recognized by the immune system, and
a strong humoral and T cell immune response will be instigated. Compared to vaccine
production of whole microbes, live attenuated and subunit vaccines, mRNA vaccines
are believed to be faster and less expensive to produce, and they do not involve in
any living stage of the pathogenic virus or bacteria. This manufacturing process makes
mRNA vaccines a promising bioproduct and potentially fills the gap between the desperate
demand for vaccines to control disease outbreaks and epidemics, with the potential
to scale and standardize the vaccine manufacturing process. The process of mRNA vaccine
development is illustrated in Fig. 1, exemplified by the mRNA vaccine coding the S
gene of SARS-CoV-2. Currently, most of the components needed for mRNA vaccine production
are available at the Good Manufacturing Practices (GMP) grade to satisfy commercialization.
Fig. 1
The streamlined process of the creation of an mRNA vaccine coding the S gene of SARS-CoV-2,
including the design, formulation, testing, and approval.
Two forms of mRNA vaccines have been developed, including conventional mRNA vaccines
and self-amplifying mRNA (SAM) vaccines. Conventional mRNA encoding target antigen
can be synthetically produced, including an open reading frame (ORF) for the target
antigen and 5′ and 3′ untranslated regions (UTRs), through a cell-free enzymatic transcription
reaction. The in vitro transcription reaction includes linearized plasmid DNA encoding
the pathogen antigen as a template, recombinant RNA polymerase, and nucleoside triphosphates.
A cap analog and a poly(A) tail are added sequentially to the transcriptional product
following the reaction to form a functional mRNA. After injection, transient and low
levels of antigen expression are driven (Zhang et al. 2019). SAM vaccines are derivatives
of alphavirus RNA replication, with the structural protein sequences replaced with
the antigen gene, generating multiple copies of the antigen-encoding mRNA and expressing
high levels of the heterologous gene. Then, in vitro transcribed (IVT) mRNA is encapsulated
in lipid nanoparticles and delivered into cells. Consequently, humoral and T cell
immune responses are stimulated. The immune response mechanism instigated by mRNA
remains to be elucidated. The dimension of the immune response is dependent on the
quality of the IVT mRNA, delivery vehicle, and administration route.
In the last two decades, mRNA vaccines have been investigated extensively for infectious
disease prevention and cancer immunotherapy. Several mRNA vaccines against infectious
disease have showed a promising future, such as mRNA vaccines against influenza viruses,
rabies virus, HIV, Ebola virus, and Zika virus (Demoulins et al. 2016; Maruggi et
al. 2019). Although clinical trials of mRNA vaccines for infectious disease are still
in their early stage, a plethora of publications have shown that mRNA-based vaccines
are a promising novel platform that is highly flexible, scalable, inexpensive and
cold-chain-free. The growing body of preclinical and clinical results demonstrates
that prophylaxis and therapy with mRNA promises to be useful for the prevention of
infectious disease and that mRNA vaccines are safe and tolerated in animal models
and humans. Despite the great challenges in the creation of new processes to generate
mRNA vaccines, such as delivery and translation efficiency, these processes hopefully
will be streamlined to establish large-scale production. It is just a matter of time
for mRNA vaccines to be used in humans and animals.
Several biopharmaceutical companies have announced promptly the establishment of mRNA
vaccine projects for SARS-CoV-2 since its outbreak. On January 23, 2020, the Coalition
for Epidemic Preparedness Innovations (CEPI) announced the initiation of three programs
to develop vaccines against SARS-CoV-2. Moderna, an mRNA-based biotechnology company,
announced that it has received funding from CEPI and would manufacture an mRNA vaccine
against SARS-CoV-2 using their mRNA vaccine platform technology, and it has partnered
with the National Institute of Allergy and Infectious Diseases (NIAID) to develop
a vaccine for the Wuhan strain of coronavirus. The NIAID will conduct investigational
new drug (IND)-enabling studies and a phase I clinical study in the U.S. In projects
conducted by Moderna, approximately 1000 people have received different versions of
mRNA vaccines so far in seven clinical trials designed to prevent illness from other
respiratory viruses, including a strain of the flu, respiratory syncytial virus (RSV),
and human metapneumovirus (HMPV). mRNA vaccine candidate against SARS-CoV-2 developed
by Moderna is the first one entering phase I clinical trial (Moderna 2020).
In February 2019, CEPI agreed to provide up to $34 million in support of CureVac’s
technology development, which will provide a rapid supply of lipid-nanoparticle-formulated
mRNA vaccine candidates that can target known pathogens and prepare for rapid response
to new and previously unknown pathogens. The principle of CureVac’s proprietary technology
is the use of mRNA as a genetic information carrier to instruct the human body to
produce its own proteins capable of fighting a wide range of diseases. Additionally,
the Federal Ministry of Education and Research has committed a total of 90 million
Euros to support this work (Smith 2020).
Pharmaceutical and biotechnology companies in China have also stepped up to the development
of vaccines of SARS-CoV-2. Four SARS-CoV-2 vaccines have been approved for phase I
or II clinical trial in China at the end of April of 2020. Several biotechnology companies
have announced the development of mRNA vaccines against this mated coronavirus. In
early January 2020, Stermirna Therapeutics, cooperating with Shanghai East Hospital
of Tongji University, proclaimed that no more than 40 days will be needed to produce
vaccine samples based on the new generation of mRNA technology and some preliminary
procedures. The samples would be sent for tests and be brought to clinics as soon
as possible (Xinhua net 2020). On February 12, 2020, the Institute of Advanced Technology
of Shenzhen announced that the first batch of mRNA vaccine against SARS-CoV-2 was
being prepared under GMP standards. The mRNA matching the requirement for the clinical
trial would be ready in April 2020 (SIAT 2020). On February 14, Zhuhai Lifanda Biotechnology
announced that an antibody against antigen encoded by mRNA was detected in mouse serum
at day 12 after the mice had been immunized. Data on rhesus macaques immunized with
mRNA vaccine candidates would be obtained soon. The mRNA vaccine against SARS-CoV-2
would go into clinical testing within 2–3 months. A GMP factory would be put into
use in April, 2020 (cnBeta 2020).
Like other coranaviruses, S protein of SARS-CoV-2 plays an essential role in binding
to receptors on the host cell to initiate an infection and determines host tropism.
It is also the major target of neutralizing antibodies and it may be the main antigen
of development of SARS-CoV-2 vaccines by most related groups. According to the public
information, S protein or receptor binding domain (RBD) sites are main antigens edcoded
by SARS-CoV-2 vaccine candidates. However, due to the limited information and patent
protection on the adaptation of antigens of SARS-CoV-2 vaccines in development, the
antigen information of most SARS-CoV-2 vaccine candidates are unclear. The expected
immune response, such as antibody or T cell mediated immune response, stimulated by
these SARS-CoV-2 vaccines in development are not clear either. Thus accordingly, the
protection against SARS-CoV-2 can not to be predicted.
During the development of SARS-CoV and MRES-CoV vaccines, some groups found that enhanced
lung inflammation following homologous challenge in mice and nonhuman primates was
mediated by S specific IgG (Liu et al. 2019; Wan et al. 2020). Antibody-dependent
enhancement (ADE) was initially found to be induced by dengue vaccine (Katzelnick
et al. 2017). ADE was also observed in other vaccines later, such as influenza vaccine
(Winarski et al. 2019). ADE modulates the immune response and can elicit sustained
inflammation, lymphopenia, and/or cytokine storm, one or all of which have been documented
in severe cases and deaths. ADE also requires prior exposure to similar antigenic
epitopes. Currently, immunologists are worrying that SARS-CoV-2 mRNA vaccines based
on S protein maybe induce ADE, which will blur the adaptation in the prevention and
control of SARS-CoV-2 pandemic. However, in a recent paper, Qin et al. reported inactivated
SARS-CoV-2 vaccine was safe in nonhuman primate and has not induced ADE in macaques
(Gao et al. 2020). The other obstacle to developing a successful SARS-CoV-2 mRNA vaccine
is the abundant glycosylation sites on the S protein which facilitate the escaping
of SARS-CoV-2 from neutralization by the S protein specific antibodies (Vankadari
and Wilce 2020).
As a novel vaccine platform, mRNA vaccine technology is not mature to be used in immune
prophylaxis and therapy. It still needs improvement and has uncertainty in the SARS-CoV-2
mRNA vaccine development. The mechanisms of immunity induction of mRNA vaccine in
vivo are under elucidation. Published results showed mRNA vaccines stimulated potent
immune response, but in human, the immune response is not strong enough with low dose
of antibodies (Pardi et al. 2018b). The delivery tool and administration route determine
the dimension of immune response by mRNA vaccine (Pardi et al. 2018a).
So far, there at least 14 SARS-CoV-2 mRNA vaccine projects had been announced globally
(WHO, 27 May 2020). Regularly, the general stages of the development cycle of a vaccine
include the exploratory stage, the preclinical stage, clinical development, regulatory
review and approval, manufacturing, and quality control. The process of the creation,
testing and production of a vaccine in mass quantities can take many years because
the industry is highly regulated. For mRNA vaccines against SARS-CoV-2 infection,
developers should speed up the process to make them available as soon as possible.
However, until a safe, effective vaccine is commercial available, the best protection
from infection with SARS-CoV-2 is to follow the suggestions of public health experts.