Conjugate vaccines (which can also be known as subunit-conjugated vaccines) are developed by linking or joining the polysaccharide component of the causative agent to a protein carrier molecule that augment the immunogenicity and/or antigenicity of the microbe’s polysaccharide molecule when used as a vaccine candidate. Conjugate vaccines are primarily developed against capsulated bacteria (i.e. pathogenic bacteria that forms capsules as microbial resistant forms). Certain pathogenic bacteria have polysaccharide outer coats that are poorly immunogenic. These polysaccharide outer coats are chemically linked to a carrier protein molecule to form a combination molecule that is antigenic and one that can generate immunity against a given infection in the individual being vaccinated. Examples of conjugate vaccines include Haemophilus influenzae Type B vaccine, Meningococcus A, C, Y, W135 and pneumococcus vaccine.

Recombinant vaccines are genetically generated vaccines that are prepared using recombinant DNA technology or genetic engineering techniques. In the preparation of recombinant vaccines, the genes for desired antigens of a pathogenic organism are inserted into a vector.  This method is complex and capital intensive. An example of a recombinant vaccine is the Hepatitis B vaccine (HBV) used against Hepatitis B virus (HBV) infection. Hepatitis B surface antigen is produced from a gene transfected into yeast cells (particularly Saccharomyces cerevisiae) and genetically purified for injection into a host.

Edible vaccines are vaccines currently being developed and are prepared by introducing the genes responsible for the antigenic determinant of the virus into crops (especially cereals like maize and rice) that can be eaten or consumed by humans. Eating these crops is known to induce some of form of immunity in the host. In edible vaccines, the antigenic protein molecule is engineered into an edible plant; and after ingestion, the protein is uncloaked and recognized by the host’s immune system which produces antibodies in advance. Edible vaccine development is still under development, and it holds potential in revolutionizing the technology vaccination in the future.

Subunit vaccines contain parts or fragments of the target pathogen instead of the whole organisms – as is applicable in live attenuated vaccines and killed inactivated vaccines. They are usually made by isolating a specific protein from a pathogen and presenting it as an antigen on its own. Subunit vaccines generally contain the purified portions of the microbe being vaccinated against. The antigenic properties of the pathogen of interest (usually in the form of protein molecules and carbohydrate molecules) isolated in their pure forms are used in the development of subunit vaccines; and these molecules are expected to stimulate the immune system of the host in advance. Unlike in live attenuated vaccine in which there is possibility of reactivation of the microbe into a pathogenic organism, there is no risk that subunit vaccine (usually known as toxoids) can provoke the disease in the vaccinated individual. One of the major drawbacks in the development of subunit vaccine is the difficulty in identifying potential protective or antigenic molecules out of the complex protective molecules in pathogens – that could be used as starting materials for the development of the vaccines. Examples of subunit vaccine are Diphtheria toxoid, Tetanus toxoid, Pertussis toxoid and Hepatitis B vaccine.


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