Advantages of edible vaccines

A "subunit vaccine" refers to a pathogen-derived protein that cannot cause disease but can elicit a protective immune response against the pathogen.
Conventional subunit vaccines as well as edible vaccines are composed of antigenic proteins and do not contain pathogenic genes(because obviously they use attenuated strains). Thus, they have no way of establishing infection and safety is assured.
However, conventional subunit vaccines are
a. expensive (because they are made in bioreactors)
b. require very advanced technology
c. need purification and
d. require refrigeration.
Also, they must be injected which requires needles and trained medical workers.
In contrast, edible vaccines
a. enhance conformity, especially in children and
b. because of oral administration, eliminate the need for trained medical personnel.
c. 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!
d. 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 immense capital investment of pharmaceutical manufacturing facilities.
e. Because large amounts can be produced indigenously, dependence on foreign supply will also decrease.
f. They exhibit good genetic stability.
g. They are heat-stable and do not require cold-chain maintenance.
h. Since they can be stored near the site of use, long-distance transportation can also be avoided.
i. Since syringes and needles are not used, chances of infection are also less.
j. 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 infect humans.

Edible vaccines activate both mucosal and systemic immunity, as they come in contact with the digestive tract lining which is not possible with sub-unit vaccines which provide poor mucosal response. This dual effect of edible vaccines provides first-line defence against pathogens invading through mucosa: like Mycobacterium tuberculosis and agents causing diarrhea, pneumonia, STDs, HIV, etc.
Other advantages of edible vaccines include:
a. Administration of edible vaccines to mothers to immunize the fetus-in-uterus by trans-placental transfer of maternal antibodies or the infant through breast milk. Edible vaccines have a potential role in protecting infants against diseases like group-B Streptococcus , respiratory syncytial virus (RSV), etc, which are under investigation.
b. 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.

References:
a.University Of Guelph (1999, September 14). Getting A Boost From Edible Vaccines. ScienceDaily. Retrieved May 10, 2008, from http://www.sciencedaily.com­ /releases/1999/09/990913145730.htm
b.Lal P, Ramachandran VG, Goyal R, Sharma R. Edible vaccines: Current status and future. Indian J Med Microbiol [serial online] 2007 [cited 2008 May 2];25:93-102.
Available from: http://www.ijmm.org/text.asp?2007/25/2/93/32713
c.http://whyfiles.org/166plant_vaccines/