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SF Journal of Medicine and Research
2020 | Volume 1 | Edition 3 | Article 1012
ScienceForecast Publications LLC., | https://scienceforecastoa.com/ 1
1
Edible Vaccines and Oral Immunization against Viruses:
Prospects, Promises and Pitfalls
OPEN ACCESS
*Correspondence:
Aryadeep Roychoudhury, Department
of Biotechnology, St. Xavier’s College
(Autonomous), 30, Mother Teresa
Sarani, Kolkata, West Bengal, India
E-mail: aryadeep.rc@gmail.com
Received Date: 11 Jul 2020
Accepted Date: 31 Aug 2020
Published Date: 04 Sep 2020
Citation: Aryadeep Roychoudhury.
Edible Vaccines and Oral Immunization
against Viruses: Prospects, Promises
and Pitfalls. SF J Med Res. 2020; 1(3):
1012.
Copyright © 2020 Aryadeep
Roychoudhury. This is an open access
article distributed under the Creative
Commons Attribution License, which
permits unrestricted use, distribution,
and reproduction in any medium,
provided the original work is properly
cited.
Review Article
Published: 04 Sep, 2020
Abstract
e lethality of infectious diseases can be largely minimized via implementation of crucial sanitary
procedures such as vaccination. Vaccine is a biological preparation used to establish or improve
immunity to a particular disease. Vaccines, made of killed or attenuated forms of microbes, are
the main modes of defense and protection against several bacterial, viral and parasitic diseases.
However, the process of production and purication makes them expensive and unaordable to
many developing nations. e usual traditional vaccines are generally microsomal preparations
which can be sometimes harmful and not tolerated easily by newborn babies. Another major hurdle
to oral immunization is the digestion of macromolecular antigenic protein within the stomach
due to extremely acidic pH. Dr. Charles Arntzen of Arizona State University rst put forward the
idea of edible vaccines which pose an interesting alternative in overcoming some of the constraints
of traditional vaccines. ey oer cost-eective, easy to administer, storable, widely-acceptable
and bio-friendly vaccine delivery system, particularly in developing countries. Edible vaccines
are obtained by incorporating a particular gene of interest into the edible parts of plant, which
produces the desirable encoded protein. ey are specic to provide mucosal activity along with
systemic immunity against various viral diseases like measles, hepatitis B, hepatitis C, anthrax,
Human Immunodeciency Virus (HIV), etc, as well bacterial diseases like cholera and diarrhoea.
Various foods that are genetically engineered to serve as alternatives to injectable vaccines include
cereals (wheat, rice, corn), fruits (banana) and vegetables (lettuce, potato, tomato). Plant-based
oral vaccines can be grown locally, reducing the cost and complications of transportation, while
the stability of proteins in intact plants removes the need for refrigeration. e edible nature of
vaccines eliminates the need for syringe-based delivery, saving money and reducing the risk of
infection. However, various technical obstacles, regulatory and non-scientic challenges, including
the control of antigen dosage seem surmountable and need to be addressed or overcome. e aim
of this review is to present and critically examine the potentiality of edible vaccines as an option for
global immunization against pathogenic diseases with special emphasis on viral infections.
Keywords: Edible vaccines; Virus; Antigen; Infectious disease; Immunization
Aryadeep Roychoudhury*
Department of Biotechnology, St. Xavier’s College (Autonomous), 30, Mother Teresa Sarani, Kolkata, West Bengal,
India
Introduction
Application of vaccines stands out as an eective measure to prevent infectious diseases,
accounting for more than 54% of total mortality in developing countries. More than one million
people die annually from infectious diseases, with 50% of these diseases caused by pathogens
infecting the mucosal membrane of the mammalian host. Outbreaks of pathogens in dierent
parts of the globe are chiey connected with increase in population density and overcrowding, and
relaxation to follow proper hygiene controls, leading to perpetuation of several infectious diseases,
increasing morbidity and jeopardizing the global health systems [1]. is can be very well felt in
the present scenario when the entire world is essentially crippled due to life-threatening pandemic
infection caused by Severe Acute Respiratory Syndrome-Coronovirus 2 (SARS-CoV2). e
incidence of this pandemic has very well pointed out that infectious diseases can cause devastations
in not only countries lacking health infrastructural facilities, but also in those with strengthened and
consolidated health systems, demonstrating the need of sound protective measures to retard the
global emergence of newer pandemics, without forgetting the fact that till date, only smallpox virus
has been managed to be completely eradicated. Incidentally, the concept of vaccination was rst
put forth by Edward Jenner in 1796 for small pox. Vaccination technique was introduced almost
200 years back with the purpose of delivering a minimum dose of a disease-causing pathogen to
induce a humoral or cellular immune reaction without actually subjecting the individual to the
Aryadeep Roychoudhury SF Journal of Medicine and Research
2020 | Volume 1 | Edition 3 | Article 1012
ScienceForecast Publications LLC., | https://scienceforecastoa.com/ 2
risk of true infection. Vaccination is the process by which the body
is made ready to face and ght o new infections by improving
immunity for a long duration through antibody production, so that
future infections can be warded o [2]. e immune system gets
ready to combat the invading pathogens very quickly, before they get
the chance to spread throughout the body while causing discomfort
to the host. Several diseases like tuberculosis, cholera, typhoid and
especially poliomyelitis have been successfully regulated through
massive vaccination campaigns. Vaccine development involves an
extremely dynamic phase, and routine immunization programs
through vaccination now go beyond the traditional six childhood
vaccines against diphtheria, tetanus, whooping cough, measles, polio
and tuberculosis, and include additional vaccines to prevent hepatitis
B, rubella, pneumococcal disease and rotavirus [3]. With regard to
the present situations, it appears that almost every country all over
the globe urgently feels the need of designing a new vaccine against
SARS-CoV2 as fast as possible to save human life. e challenge in
the present world is therefore to nd unique and innovative vaccines
that can target various newly-evolving pathogens and infections at
various stages.
e evolution of vaccines has led to the discovery of new forms of
vaccination that are eective and cover a wider array of diseases. Live-
attenuated vaccines are considered as the original vaccines, where
a living infectious pathogen is directly used as vaccine. In case of
inactivated vaccines, the debris of the dead pathogen is used as vaccine.
Toxoid vaccines involve utilization of the toxin produced by the
organism as vaccines. Biosynthetic vaccines are synthetically designed
having similar shape and properties to the infectious organism. e
process of using DNA vaccines to prevent or slow down the spread
of disease is also known as polynucleotide immunization. DNA that
is injected into the subject undergoes transcription and translation
which yield protein, making specic T and B cells to dierentiate and
proliferate. Recombinant vaccines involve cloning the gene encoding
the antigen in a recombinant plasmid, expressing it in bacteria and
purifying the protein to be used as vaccine [4].
Challenges Imposed by Conventional
Vaccines
Despite worldwide immunization of children against the six
devastating diseases, 20% of infants are still le un-immunized;
responsible for approximately two million unnecessary deaths every
year, especially in the remote and impoverished parts of the globe.
Despite the advantages of vaccination, there are certain ip-sides
and limitations of traditional vaccines that restrict their use [5]. e
main problem to be solved is the boundaries on the production of
vaccine, supply and distribution. One hundred percent coverage is
desirable, because un-immunized populations in remote areas can
spread infections and epidemics in the immunized safe areas, which
have comparatively low herd immunity. For some infectious diseases,
immunizations either do not exist or they are unreliable or very
expensive [6]. e standard oral vaccines should fulll some criteria
like (i) desired antigens should be present in sucient quantities; (ii)
expressed antigens should be stable at room temperature for a long
time; (iii) protective immunity must be induced by the vaccine; (iv)
vaccines should withstand enzymatic degradation in the stomach [7].
Industrial production of vaccines makes them expensive commodities
and inaccessible in developing countries. All these processes involve
the use of sophisticated and expensive sterile fermentation technology
followed by purication processes. Immunization via DNA vaccines
is quite expensive with some undesirable immune responses. All
pathogenic agents cannot be cultivated in an exogenous medium.
Conventional vaccines produced from attenuated pathogens involve
the synthesis of antigenic proteins via mammalian cell culture which
is easily prone to contamination with harmful pathogens. Vaccine
production through microbial system invites possibility of endotoxin
contamination. When cell culture and transgenic animals are used
to make vaccines, the contamination possibly arises with viruses,
prions and oncogenic DNA. Hence, biosecurity and biosafety
issues need to be addressed to cultivate these pathogenic agents [8].
Although bacterial or viral attenuation for vaccine production is a
controlled process, reversion of these attenuated micro-organisms to
their virulence or pathogenic forms also restricts their use. Vaccine
construction in several cases has been hampered because of varying
or mutating strains of the pathogen, antigen dri, antigenic shi and
other unrevealed mechanisms. Such regulations make it dicult to
select the actual peptide sequence to prime the immune system, since
the peptide sequences of the individual strains will all be dierent.
Even if successfully produced, these commercial vaccines have specic
expiry period and need refrigeration facilities, thereby enhancing
control, storage, transportation and distribution costs. Vaccine
degradation aer acid digestion in the stomach is another concern
[9,10]. In the present-day world, due to the existing and continuously
emerging diseases, together with the safety and ecacy problems,
associated with the available vaccines, researchers have tried to
nd an alternative approach for production of widely aordable,
economic and reliable vaccines that are safe, acceptable and can be
easily administered by simple feeding without the requirement of any
trained personnel. Moreover, vaccines should be such that there are
no refrigeration requirements, are easily transportable and could be
smoothly delivered in the developing countries. Most of the currently
available vaccines are associated with several problems including
safety due to vaccine-associated side eects in human subjects. Edible
vaccines are hence considered as ideal replacements for conventional
vaccines [10].
Edible Vaccines and Their Advantages
e term ‘edible vaccine’ was coined in 1992 by Charles Arntzen
and co-workers by introducing the concept of transgenic plants as
a production and delivery system for subunit vaccines. However,
the development of edible vaccines still remains in its infancy due to
the emergence of various medical, legal, ethical and environmental
uncertainties [11]. e World Health Organization in 1990’s
presented a task of nding cheap methods of oral vaccine production
that do not need refrigeration requirement. Edible vaccines are
generally antigen-expressing plants, where the edible part of a plant
is genetically modied to express the antigens, thereby eliciting an
immune response upon consumption, and serving as a factory
for vaccine manufacturing [12]. e process of purication and
downstream processing, which make conventional vaccines costly,
are eliminated in case of edible vaccines, so that they are inexpensive
and more aordable and accessible. Plant-derived vaccines can be
designed to contain numerous antigens for various diseases. ese
multi-component vaccines are called ‘second generation vaccines’ that
provide immunization against many diseases in a single dose. Plant-
based systems only need greenhouses and not fermentation tanks for
cell culture. Besides, purication from plant extract is simpler because
plants are not carriers of viruses that could possibly be detrimental
to humans [13]. e advantages of edible vaccines include lack of
contamination occurring during traditional mammalian culture
cells, no involvement of syringes or needles and easy to deliver to
Aryadeep Roychoudhury SF Journal of Medicine and Research
2020 | Volume 1 | Edition 3 | Article 1012
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the body. e specic antigen is protected by the cell wall of plant
cells, cannot be damaged by gastric enzymes and can easily reach the
blood stream where it activates the mucosal and systemic immunity.
Edible vaccines reduce the need for skilled personnel to administer
injections and negate the concerns regarding the reuse of syringes
or needles. Any recipient of plant-derived vaccine is only exposed to
non-infectious and non-toxic bit of protein. Vaccines generated in
plants have an element of plant sugars that markedly elicits greater
immune response. ough sugars also get attached to antibodies in
case of animal-sourced vaccines, that is not reckoned to be benecial
[14]. Heat stability eliminates the need for refrigeration. Plant cells are
capable to correctly fold and assemble not only antibody fragments
and single chain peptides, but also full-length multimeric proteins.
An edible vaccine provides higher safety of individual as compared
to traditional vaccine since they are subunit preparation and do not
involve attenuated pathogens which sometimes causes disease when
administered as vaccine. ey can be ingested by eating the plant/
part of the plant. So, the need to process and purify does not arise.
Expression of antigen in plant seeds provides a convenient system of
vaccine storage for long duration of time, thus reducing storage and
shipping costs under ambient conditions. If the local/native crop of
a particular area is engineered to produce the vaccine, then the need
for transportation and distribution can be eliminated [15]. erefore,
plants are considered for ecient production systems for vaccines
as an alternative to circumvent the problems of traditional vaccines.
Transgenic plants are also convenient for producing antigens in
large amounts (kg) for using in parenteral and oral applications.
Immunization by directly eating the transgenic plant parts (fruit,
tuber, seed) which produce vaccine antigens are thought to reduce
the high production costs like purication, storing and transportation
costs. As plants are a safe biological system and harbor no pathogen
of human infections, there are no ethical problems associated with
plant edible vaccines, unlike the vaccines produced from animal cell
cultures and transgenic animals.
Mechanism of action
e antigens in transgenic plants are delivered through
bioencapsulation, i.e., tough outer plant cell wall that ensure
protection against gastric secretions. Administration of edible
vaccines lead to the degradation of majority of the plant cells in the
intestine by the action of digestive and bacterial enzymes, resulting in
the release of antigens present in the plant product. Edible vaccines
are mucosal-targeted vaccines that stimulate both the systematic
and mucosal immune network, activating the rst line of defense of
human body through mucosa. e mucosal surfaces are found lining
the digestive tract, respiratory tract and urinoreproductive tract.
Mucosal immune system is induced aer recognition of an antigen
by specialized cells called M-cells, which are localized in the mucosal
membranes of lymphoid tissues such as Peyer’s patches within the
small intestines. Upon oral administration, the released antigens are
taken up by the intestinal M cells, passed on to macrophages, other
antigen-presenting cells and local lymphocyte population which
generate antiserum responses. e M cells especially present the
antigen to the Antigen Presenting Cell (APC) surface and activate B
cells with the co-operation of helper T cells to secrete immunoglobulin
A (IgA) [16]. On passing through the mucosal epithelial layer towards
the lumen, the IgA molecules form complex with membrane-bound
secretory components to form secretary IgA (sIgA), which in turn
interacts with specic antigenic epitopes and neutralize the invading
pathogen. e common problem of most oral vaccines/ therapeutics
is the tolerance towards the vaccine in the gut. is problem can
be overcome by immune suppression using triamcinolone in small
amounts, increasing the dosage of the vaccine signicantly and
applying multiple doses over a specic period of time.
Production of edible vaccines
ere are a number of factors that make plants an ideal candidate
for edible vaccine production. ey should have a long shelf life so
that they can be stored for a sucient period without degradation
(e.g., rice, wheat), should have fast luxuriant growth (e.g., tomato,
tobacco) and should be easily transformable. Edible vaccines can
be produced by incorporation of transgene in the edible parts of
the selected plant species, where the antigen producing gene(s)
are introduced in plant genome through dierent transformation
techniques like direct gene delivery method (biolistic or gene gun),
cloning in a vector by indirect gene delivery method (Agrobacterium
tumefaciens-mediated transformation) or utilizing chimeric plant
viral expression systems [17]. New or multiple transgenes can be
introduced by sexual crossing of plants, thus creating novel vaccines
against multiple diseases. Moreover, plant cells are able to perform
complex posttranslational modication of recombinant proteins,
such as glycosylation and disulde bridging that are oen essential
for biological activity of many mammalian proteins, allowing for
the retention of native biological activity. Potato tubers are widely
consumed all around the world and quite aordable. A large amount
of data on genetic manipulation is available in potato, thus making
optimized protocols available. However, one major disadvantage of
using potato is that it requires cooking before consumption which
can denature the antigen [1]. e main reason why rice and maize are
attractive as candidate edible vaccines is because they can be stored
without refrigeration for a very long period of time. However, the
problem with the cereals is that they take relatively long periods of
time and require perfect conditions to grow. e major disadvantage
with tomato is that it undergoes spoilage rapidly aer ripening.
Edible Vaccines Against Viruses
Edible vaccines targeted against viruses have been a large and
exciting eld of research almost from its inception, for viral pathogens
ranging from hepatitis B or C to foot and mouth disease viruses, and
human papillomavirus, to mention just a few. Successful expression
of antigens in plants was achieved for rabies virus G-protein in
tomato, Norwalk virus capsid protein in tobacco and potato, hepatitis
B virus surface antigen in tobacco and potato, etc [8, 18,19]. Table
1 summarizes the edible vaccines generated against dierent human
viruses.
Can Edible Vaccine Approach be Targeted
Against SARS-CoV2?
e 2019-2020 pandemic caused by SARS-CoV2 all over the
globe has raised severe concerns and is evidencing the lack of
substantial clinical treatments till date. us, there is an urgent need
for having scalable production platforms to generate SARS-CoV2
vaccines. Plant-made vaccines oer the possibility of performing the
oral delivery of formulations made with freeze-dried plant biomass,
which avoids the costs derived from purication and parenteral
delivery. As an initial step toward provision of an oral vaccine against
SARS-CoV, which resulted in a worldwide outbreak in 2003 spanning
almost 20 countries, Li et al. (2006) [20] expressed a partial spike (S)
protein in the cytosol of nuclear-transformed and in the chloroplast
of plastid-transformed tobacco and lettuce plants. e S protein is
Aryadeep Roychoudhury SF Journal of Medicine and Research
2020 | Volume 1 | Edition 3 | Article 1012
ScienceForecast Publications LLC., | https://scienceforecastoa.com/ 4
a major virion structural protein. S (471-503), a peptide located at
the Receptor Binding Domain (RBD) of SARS-CoV S1 subunit, could
specically block the binding between the RBD and angiotensin-
converting enzyme 2, resulting in the inhibition of SARS-CoV
entrance into host cells in vitro. is study provided the possibility of
establishing a safe and inexpensive vaccination strategy against SARS-
CoV. In another study, the N-terminal fragment of SARS-CoV S
protein (S1) was expressed in tomato and low-nicotine tobacco plants
and systemic and mucosal immune response was evaluated in mice
aer oral ingestion of tomato fruits. Sera of mice parenterally primed
with tobacco-derived S1 protein revealed the presence of SARS-CoV-
specic IgG as detected by Western blot and ELISA analysis [21].
e 2014-2015 Ebola virus outbreak in West Africa (Liberia, Sierra
Leone, Mali, Nigeria and Senegal) created an urge among scientists
to nd out a novel approved vaccine against this virus. Monreal-
Escalante et al. [22] successfully overexpressed the VP40 antigen
from the Ebola virus in tobacco plants, reaching accumulation levels
up to 2.6 µg g-1 fresh weight of leaf tissues. e antigenicity of the
plant-made VP40 (viral protein) antigen was evidenced by Western
blot and an initial immunogenicity assessment in test animals, that
revealed the induction of immune responses in mice, following three
weekly oral or subcutaneous immunizations at very low doses (125
and 25 ng respectively) without the inclusion of accessory adjuvants.
erefore, this plant-based vaccination prototype is proposed as an
attractive platform for the production of vaccines in the ght against
Ebola virus disease outbreaks. ese success stories make us to believe
that identication of antigen(s), through genomic, proteomic and
bioinformatic tools, most likely to induce an immune response will
enable us to overexpress the antigen in plants. Within the COVID-19
vaccine race, the strategy of molecular farming in generating edible
vaccines has already been initiated by the Canadian Company,
Medicago, which claims to produce 10 million doses per month if
their production method and clinical trials obtain approval from US
Food and Drug Administration (FDA). Another American company,
Kentucky Bioprocessing which has raised transgenic tobacco and
is conducting pre-clinical tests has claimed to manufacture up to
three million doses per week. e University of California, San
Diego, is working on an innovative collaborative project to develop
a microneedle patch-vaccine that uses proteins grown in genetically-
modied plants. e Center for Research in Agricultural Genomics
(CRAG) of Spain, will also develop antigens for SARS-CoV2 in
genetically modied lettuce and tobacco, and the international project
NEWCOTIANA, which works on the development of medicines
and vaccines in plants with funding from the European Union, has
released the complete genetic sequence of Nicotiana benthamiana in
order to accelerate the development of a plant-based vaccine.
Pitfalls of Edible Vaccines
e three main challenges of edible vaccine production are: (i)
the selection of antigen and plant expression host, which will ensure
the safeness of the vaccine produced and its thermostability, (ii)
consistency of dosage, and (iii) manufacturing of vaccines according
to Good Manufacturing Practice (GMP) procedures [23]. erefore,
the challenges facing plant-based-vaccine development include
technical, regulatory and economic aspects, and public perception.
Among technical challenges to be considered, the crop should
provide ample biomass for accumulation of a sucient quantity of
the antigenic protein. e expression of antigens in plants is a major
regulatory concern. Targeting transgene expression via a tissue-
specic promoter may reduce regulatory concerns. Not all vaccine
candidate proteins are highly immunogenic in plant tissues and
secondary metabolites found in plants may compromise the ability
of the candidate vaccine protein to induce immunity [24]. Among
regulatory challenges, issues relevant to any genetically modied
crop that have to gain regulatory approval from the USDA, FDA
and/or EPA apply equally to vaccines generated in edible plant
parts. Allergenic reactions to plant protein glycans and other plant
antigens are a challenging issue. It has been suggested that plant-
Virus Antigen targeted Transgenic plant Effect
Hepatitis B S form of HbsAg Potato, Tomato HEV-E2 gene correctly expressed and antigen showed
normal immune-reactivity in mice
Hepatitis C
Synthetic hypervariable region 1 (HVR1)-derived
peptide called R9, a potential neutralizing epitope of
HCV derived from the envelope protein E2
Nicotiana
benthamiana Intranasal immunization of mice elicited anti-HVR1 serum
antibody
Inuenza Haemagglutinin (HA) protein N. benthamiana Grams of cGMP-grade plant-made H7N9 vaccine were
produced in the form of HA-only VLPs, in response to the
viral outbreak in humans
Cervical Human Papilloma
Virus like particles (HPV) HPV type 11 L1 major capsid protein Potato Oral immunization induced anti-VLP immune response in
mice
Human rotavirus HRV-VP7 Potato Elicited serum IgG and mucosal IgA by oral delivery to mice
Measles Loop forming B cell epitope (H386-400) Carrot Intraperitoneal immunization of mice with carrot extracts
induced high titers of antibodies
Respiratory
Syncytial Virus
(RSV)
RSV-F protein Tomato Oral immunization of mice induced both serum and
mucosal RSV-F specic antibodies
Gastroenteritis Glycoprotein S (N-gS) from transmissible
gastroenteritis coronavirus (TGEV) Potato
Intraperitonal immunization in mice resulted in serum IgG
specic for TGEV, oral immunization through direct feeding
developed serum antibodies specic for gS protein
Foot and mouth disease virus VP1 capsid protein Arabidopsis
thaliana
Mice injected intraperitoneally with the partially puried
VP1 protein were totally resistant to a challenge with a
virulent strain of the virus
HIV-induced Acquired
Immunodeciency Syndrome
(AIDS)
Stable chimeric CPMV particles that express
epitopes derived from human rhinovirus 14 and
HIV-1
Tomato Expressed protein was detected in different parts of the
plant, including the ripe fruit
Norwalk virus Capsid protein (NVCP) Tobacco and
potato
Stimulated serum IgG and gut IgA specic for NVCP when
fed to mice
Rabies Rabies Virus (RV) glycoprotein Tomato
Induction of neutralizing antibodies in mice that were
parenterally vaccinated with a peptide of RV glycoprotein
fused to a plant virus coat protein; antigen-capsid fusion.
Table 1: Edible vaccines generated against different human viruses.
Aryadeep Roychoudhury SF Journal of Medicine and Research
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derived recombinant proteins or antibodies may have increased
immunogenicity or allergenicity, as compared to mammalian
counterparts, along with few side eects such as toxicity on central
nervous system, cytokine-induced sickness and autoimmune diseases.
Another problem is that the glycosylation of trans-proteins in plants
diers slightly from those produced in transgenic animals or animal
cells in vitro. A signicant dierence with trans-protein production in
plants is their inability to add sialic acid to glycoproteins [24]. ere
are chances of contamination of recombinant proteins by pesticides,
herbicides, mycotoxins or potentially toxic factors due to the fact
that some plant species contain numerous toxic alkaloids and other
secondary metabolites. All these factors invite regulatory constraints
and uncertainties for approval as human drug. e stability of
vaccines in fruits is uncharacterized. Moreover, there is a possibility
of weakening of the medicinal property and denaturation of vaccine
proteins in case of cooked foods. Evaluation of dosage requirement
and proper maintenance of dosage are dicult, since consistency of
dosage may vary within the plants of the same species, from fruit to
fruit and from generation to generation due to the size and ripeness
of the fruits or plants [25]. Fruits like tomato and banana do not
appear in market in xed or standard sizes, so that it is quite dicult
to optimize the dosage. It is also quite dicult to evaluate the required
dosage for every patient. e levels of innate and adaptive immune
responses generated in dierent individuals may vary, based on the
types of antigens being exposed in the body. Between two patients
with dierent body weight as well as their age, the dosage of plant-
based vaccine required will be dierent. If this issue is not monitored
carefully, an immunological tolerance will be induced when the patient
is overdosed, while reduction in antibody production will occur when
the patient is under dosed. Development of immune-tolerance to the
vaccine protein or peptide is therefore a major concern [1]. Fruit
vaccines should be easily identiable to avoid the misadministration
of the vaccine, which may lead to complications such as immune-
tolerance. e oral vaccines are complicated by the need to protect the
antigen from the eects of the acidic and proteolytic environment of
the gut. Hence, to be eective, oral subunit vaccines generally require
higher doses than oral replicating vaccines. Variable conditions for
edible vaccine are also a major problem. Potato-containing vaccine
to be stored at 4°C could be stored for longer time, while a tomato
does not last long. us, these vaccines need to be properly stored
to avoid infection through microbial spoilage [26,27]. ough the
plant edible vaccines are a lucrative option in the eld of vaccination,
several potential issues need to be addressed via exhaustive research
and development to use this area of health care for greater benets.
Conclusion and Future Prospects
Vaccines are capable of reducing the use of antibiotic and can play
a vital role in an era where antibiotic resistance is gradually becoming
a major challenge. Edible vaccine discovery can be considered as
one of the major breakthroughs in the area of biotechnology, which
successfully embraced the obstacles encountered in rising vaccine
technology. When compared to the traditional vaccines, edible
vaccines do not require sophisticated equipments and machines for
vaccine production in order to stimulate both systemic and mucosal
responses, without any requirement for sterile injection conditions
and adequate cold-storage facilities, and may be derived directly by
simply consuming a fruit [28]. However, relevant technical, social,
environmental, ethical and policy issues concerning plant-based
edible vaccines need to be addressed before they become product-
ready. Future researches should overcome several limitations like low
expression, immune-tolerance, glycosylation, immunogenicity and
stability of the trans-proteins. e development and improvement
of suitable gene delivery methods for ecient and optimum vaccine
production should continue. More insights into the heterologous
gene regulation and expression patterns in transgenic plants could
answer some of the unknown mechanisms that make heterologous
gene expression a tough task [24]. It is very dicult to establish a
stable antigenic protein concentration in plant tissues, and there
is no certainty that the expressed antigen will produce an immune
response. Further studies are anticipated in overcoming the
problem, related to dose variability in the transgenic plants. Some
food-processing techniques (batch-processing and freeze drying)
could maintain the normal conformation and native antigenicity
in transgenic plants such as tomato, potato and Arabidopsis, thus
standardizing the concentration of antigens in the plants. Plant-made
oral vaccines might induce allergenicity during post-translational
modications and oral tolerance when co-administered with
oral adjuvants. is may activate the mucosal immune system by
provoking hypersensitive responses to other proteins contained
in the daily food. In other words, recurrent delivery of plant-made
edible oral vaccines can boost regulatory T-cell stimulation, contrary
to vaccine antigen, causing hypersensitivity in case of pollen allergy
or food allergy [17]. As a drug that is contained in a plant or its fruit, it
should be stringently evaluated, authorized and supervised by Public
Health Institutes or a similar Human Health Organization in each
country. It is anticipated that regulatory approval will be granted
ultimately to help in the wake of global disease control. One very
important point is the proper coordination between academia and
industry to help these vaccines ultimately reach people. It will also
be a challenge to create a positive public perception regarding safety
and ecacy of these vaccines, in the midst of enormous fuss created
over the safety issues of transgenic crops. e societal acceptance will
largely depend on awareness campaigns on the use and benets of
edible vaccines [29,30]. In the face of global SARS-CoV2 pandemic
ravaging the entire world at present, edible vaccines represent a ray
of hope and valuable bio-friendly alternative, having undeniable
potential to mitigate infection and supporting public health programs
to reduce the COVID-19 outbreak.
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