Subunit vaccine

A subunit vaccine is a vaccine that contains purified parts of the pathogen that are antigenic, or necessary to elicit a protective immune response.^[1][2] A "subunit" vaccine doesn't contain the whole pathogen, unlike live attenuated or inactivated vaccine, but contains only the antigenic parts such as proteins, polysaccharides^[1][2] or peptides. Because the vaccine doesn't contain "live" components of the pathogen, there is no risk of introducing the disease, and is safer and more stable than vaccine containing whole pathogens.^[1] Other advantages include being well-established technology and being suitable for immunocompromised individuals.^[2] Disadvantages include being relatively complex to manufacture compared to some vaccines (such as RNA vaccine), possibly requiring adjuvants and booster shots, and requiring time to examine which antigenic combinations may work best.^[2]

Mechanism[]

Subunit vaccines contain fragments of the pathogen, such as protein or polysaccharide, whose combinations are carefully selected to induce a strong and effective immune response. Because the immune system interacts with the pathogen in a limited way, the risk of side effects is minimal.^[2] An effective vaccine would elicit the immune response to the antigens and form memory that allows quick recognition of the pathogens and quick response to future infections.^[1]

A drawback is that the specific antigens used in a subunit vaccine may lack pathogen-associated molecular patterns which are common to a class of pathogen. These molecular structures may be used by immune cells for danger recognition, so without them, the immune response may be weaker. Another drawback is that the antigens do not infect cells, so the immune response to the subunit vaccines may only be antibody-mediated, not cell-mediated, and as a result, is weaker than those elicited by other types of vaccines. To increase immune response, adjuvants may be used with the subunit vaccines, or booster doses may be required.^[2]

Types[]

Protein subunit[]

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A protein subunit is a single protein molecule that assembles (or "coassembles") with other protein molecules to form a protein complex.^{[citation needed]}

One method of production of protein-based subunits involves isolation of a specific protein from a virus and administering this by itself. A weakness of this technique is that isolated proteins can be denatured.^{[citation needed]} A second method of making a subunit vaccine involves putting an antigen's gene from the targeted virus or bacterium into another virus (virus vector), yeast (yeast vector), as in the case of the hepatitis B vaccine^[3] or attenuated bacterium (bacterial vector) to make a recombinant virus or bacteria to serve as the important component of a recombinant vaccine (called a recombinant subunit vaccine). The recombinant vector that is genomically modified will express the antigen. The antigen (one or more subunits of protein) is extracted from the vector.^[3] Just like the highly successful subunit vaccines, the recombinant-vector-produced antigen will be of little to no risk to the patient. This is the type of vaccine currently in use for hepatitis B,^[3] and it is experimentally popular, being used to try to develop new vaccines for difficult-to-vaccinate-against viruses such as ebolavirus and HIV.^[4]

Polysaccharide subunit[]

Vi capsular polysaccharide vaccine (ViCPS) against typhoid caused by the Typhi serotype of Salmonella enterica.^[5] Instead of being a protein, the Vi antigen is a bacterial capsule polysacchide, made up of a long sugar chain linked to a lipid.^[6] Capsular vaccines like ViCPS tend to be weak at eliciting immune responses in children. Making a conjugate vaccine by linking the polysacchide with a toxoid increases the efficacy.^[7]

Conjugate vaccine[]

A conjugate vaccine is a type of vaccine which combines a weak antigen with a strong antigen as a carrier so that the immune system has a stronger response to the weak antigen.^{[citation needed]}

Peptide subunit[]

A peptide-based subunit vaccine employs a peptide instead of a full protein.^{[citation needed]}

Virus-like particles[]

Virus-like particle (VLP) vaccines use VLPs, which are proteins that mimic real virus particles.^[8] They generally consist of proteins assembled into the natural shape of the virus. VLPs are not infectious as they lack genetic materials to replicate.^[9]

Advantages and disadvantages[]

Advantages[]

• Cannot revert to virulence meaning they cannot cause the disease they aim to protect against^[10][11]
• Safe for immunocompromised patients^[12]
• Can withstand changes in conditions (e.g. temperature, light exposure, humidity)^[10]

Disadvantages[]

• Reduced immunogenicity compared to attenuated vaccines^[11][12]
  • Require adjuvants to improve immunogenicity^[10][11]
  • Often require multiple doses ("booster" doses) to provide long-term immunity^[10][11]
• Can be difficult to isolate the specific antigen(s) which will invoke the necessary immune response^[12]

References[]