Today majority of peoples are subjected to several infectious diseases. So it is necessary to produce new vaccines that have economic and other advantages over the existing injectable vaccines. In view of this recently plants are being used as the alternative vaccine production system because of its several advantages over animal system. The first human clinical trial of an edible vaccine took place in 1997, when volunteers ate raw potatoes genetically engineered against diarrhoea-causing E coli. Ten of the 11 volunteers who received the vaccine had fourfold rises in serum antibodies. Edible vaccines hold great promise as a cost-effective, easy-to-administer, easy-to-store and socio-culturally readily acceptable vaccine delivery system, especially for the poor developing countries. It involves introduction of selected desired genes into plants and then inducing these altered plants to manufacture the encoded proteins. Introduced as a concept about a decade ago, it has become a reality today. A variety of delivery systems have been developed. Initially thought to be useful only for preventing infectious diseases but now it has also been used in been used in birth control, cancer therapy, etc.
Edible vaccines are currently being developed for a number of human and animal diseases. There is growing acceptance of transgenic crops in both industrial and developing countries. Resistance to genetically modified foods may affect the future of edible vaccines. They have passed the major hurdles in the path of an emerging vaccine technology. Various technical obstacles; regulatory and non-scientific challenges need to be overcome.
Like conventional subunit vaccines, edible vaccines are composed of antigenic proteins and are devoid of pathogenic genes. Thus, they have no way of establishing infection, assuring its safety, especially in immune compromised patients. Conventional subunit vaccines are expensive and technology-intensive, need purification, require refrigeration and produce poor mucosal response. In contrast, edible vaccines would enhance compliance, especially in children and because of oral administration, would eliminate the need for trained medical personnel. Their production is highly efficient and can be easily scaled up. For example, hepatitis-B antigen required to vaccinate whole of China annually, could be grown on a 40-acre plot and all babies in the world each year on just 200 acres of land! They are cheaper, sidestepping demands for purification (single dose of hepatitis-B vaccine would cost approximately 23 paise ), grown locally using standard methods and do not require capital-intensive pharmaceutical manufacturing facilities. Mass-indefinite production would also decrease dependence on foreign supply. They exhibit good genetic stability. They are heat-stable; do not require cold-chain maintenance; can be stored near the site of use, eliminating long-distance transportation. Non-requirement of syringes and needles also decreases chances of infection. Fear of contamination with animal viruses - like the mad cow disease, which is a threat in vaccines manufactured from cultured mammalian cells - is eliminated, because plant viruses do not affect humans. Edible vaccines activate both mucosal and systemic immunity, as they come in contact with the digestive tract lining. This dual effect would provide first-line defense against pathogens invading through mucosa, like Mycobacterium tuberculosis and agents causing diarrhea , pneumonia, STDs, HIV, etc. Scientists place high priority on combating the diarrheal agents - Norwalk virus, Rotavirus, Vibrio cholerae and enterotoxigenic E. coli (ETEC) - responsible for about three million infant deaths/year, mainly in developing countries. Administration of edible vaccines to mothers might be successful in immunizing the fetus-in-utero by transplacental transfer of maternal antibodies or the infant through breast milk. Edible vaccines seroconvert even in the presence of maternal antibodies, thus having a potential role in protecting infants against diseases like group-B Streptococcus , respiratory syncytial virus (RSV), etc, which are under investigation. Edible vaccines would also be suitable against neglected/rare diseases like dengue, hookworm, rabies, etc. They may be integrated with other vaccine approaches and multiple antigens may also be delivered. Various foods under study are banana, potato, tomato, lettuce, rice, etc. Edible vaccines are currently being developed for a number of human and animal diseases, including measles, cholera, foot and mouth disease and hepatitis B, C and E.
Edible plants are very effective as a delivery vehicle for inducing oral immunization:
· Adjuvant for immune response is not necessary.
· Excellent , feasibility of oral administration compared to injection.
· Easy for separation and purification of vaccines from plant materials.
· Effective prevention of pathogenic contamination from animal cells.
· Convenience and safety in storing and transporting vaccines.
· Effective maintenance of vaccine activity by controlling the temperature in plant cultivation.
· Easy for mass production system by breeding compared to an animal system.
· Possible production of vaccines with low costs.
· Reduced need for medical personnel and sterile injection conditions.
· Economical to mass produce and transport.
· Reduced dependence on foreign supply.
· Storage near the site of use.
· Heat stable, eliminating the need for refrigeration.
· Antigen protection through bioencapsulation .
· Subunit vaccine (not attenuated pathogens) means improved safety
Producing stable and reliable amounts of vaccines in plants is complicated by the fact that tomatoes and bananas don’t come in standard sizes! There may also be side-effects due to the interaction between the vaccine and the vehicle. People could ingest too much of the vaccine, which could be toxic, or too little, which could lead to disease outbreaks among populations believed to be immune.
Side Effects of Edible Vaccines:
Development of edible vaccines is a possible high-volume, low-cost delivery system for third-world countries to fight against fatal maladies like AIDS, hepatitis and diarrhea. Research by the National Institute of Allergy and Infectious Diseases (NIAID) and the University of Maryland School of Medicine showed no significant side effects in a small study using genetically-engineered potatoes to make toxin of the Escherichia coli, a diarrhea-causing bacterium.
No Serious Side Effects:
Volunteers reported no serious adverse reactions to genetically-altered potatoes used to deliver edible vaccine toxin, according to the National Institutes of Health. The NIH said 10 to 11 volunteers who ate the raw potato bites developed four times the antibodies against the E. coli bacteria without obvious side effects.
Reduced Anaphylactic Risk:
Reduced risk of anaphylactic side effects from edible vaccine over injection systems is one benefit reported by the Bio-Medicine.org. They report the edible vaccine carries only part of the allergen compared to injection methods that reduce anaphylactic risk.
Long-term reactions to edible vaccines are yet to be determined. Possible delayed reactions not yet discovered are dangers the World Health Organization would like further study on before edible vaccines are put into production
Edible vaccines hold great potential, especially in Third World countries where transportation costs, poor refrigeration and needle use complicate vaccine administration. While research is also being conducted with laboratory animals, diabetics may someday benefit from an edible form of insulin. NSF and other government-agency and industry-funded researchers have developed technologies that permit the introduction of a hybrid gene that produces human insulin in potatoes. For diabetics, insulin-bearing potatoes may help train the body's defenses to stop reacting to insulin as if it were a foreign material.