Vaccine that contains antigenic parts of the pathogen.
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] Subunit vaccine can be made from dissembled viral particles in cell culture or recombinant DNA expression,[3] in which case it is a recombinant subunit vaccine.
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.[4] 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 vaccines 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, possibly requiring adjuvants and booster shots, and requiring time to examine which antigenic combinations may work best.[2]
After injection, antigens trigger the production of antigen-specific antibodies, which are responsible for recognising and neutralising foreign substances. Basic components of recombinant subunit vaccines include recombinant subunits, adjuvants and carriers. Additionally, recombinant subunit vaccines are popular candidates for the development of vaccines against infectious diseases (e.g. tuberculosis,[9]dengue[10])
Recombinant subunit vaccines are considered to be safe for injection. The chances of adverse effects vary depending on the specific type of vaccine being administered. Minor side effects include injection site pain, fever, and fatigue, and serious adverse effects consist of anaphylaxis and potentially fatal allergic reaction. The contraindications are also vaccine-specific; they are generally not recommended for people with the previous history of anaphylaxis to any component of the vaccines. Advice from medical professionals should be sought before receiving any vaccination.
The first certified subunit vaccine by clinical trials on humans is the hepatitis B vaccine, containing the surface antigens of the hepatitis B virus itself from infected patients and adjusted by newly developed technology aiming to enhance the vaccine safety and eliminate possible contamination through individuals plasma.[11]
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 immunological 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]
A protein subunit is a polypeptide chain or protein molecule that assembles (or "coassembles") with other protein molecules to form a protein complex.[12][13][14] Large assemblies of proteins such as viruses often use a small number of types of protein subunits as building blocks.[15] A key step in creating a recombinant protein vaccine is the identification and isolation of a protein subunit from the pathogen which is likely to trigger a strong and effective immune response, without including the parts of the virus or bacterium that enable the pathogen to reproduce. Parts of the protein shell or capsid of a virus are often suitable. The goal is for the protein subunit to prime the immune system response by mimicking the appearance but not the action of the pathogen.[16] Another protein-based approach involves self‐assembly of multiple protein subunits into a virus-like particle (VLP) or nanoparticle. The purpose of increasing the vaccine's surface similarity to a whole virus particle (but not its ability to spread) is to trigger a stronger immune response.[17][16][18]
Protein-based vaccines are being used for hepatitis B and for human papillomavirus (HPV).[17][16] The approach is being used to try to develop vaccines for difficult-to-vaccinate-against viruses such as ebolavirus and HIV.[21] Protein-based vaccines for COVID-19 tend to target either its spike protein or its receptor binding domain.[17] As of 2021, the most researched vaccine platform for COVID-19 worldwide was reported to be recombinant protein subunit vaccines.[16][22]
Vi capsular polysaccharide vaccine (ViCPS) against typhoid caused by the Typhi serotype of Salmonella enterica.[23] Instead of being a protein, the Vi antigen is a bacterial capsule polysacchide, made up of a long sugar chain linked to a lipid.[24] 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.[25]
A peptide-based subunit vaccine employs a peptide instead of a full protein.[27] Peptide-based subunit vaccine mostly used due to many reasons,such as, it is easy and affordable for massive production. Adding to that, its greatest stability, purity and exposed composition.[28] Three steps occur leading to creation of peptide subunit vaccine;[29]
They contain clearly identified compositions which greatly reduces the possibility of presence of undesired materials within the vaccine.[citation needed]
Their pathogenicities are minimized as only fragments of the pathogen are present in the vaccine which cannot invade and multiply within the human body.[30]
Selection of appropriate cell lines for the cultivation of subunits is time-consuming because microbial proteins can be incompatible to certain expression systems.[34]
Active immunity can be acquired artificially by vaccination as a result of the body's own defense mechanism being triggered by the exposure of a small, controlled amount of pathogenic substances to produce its own antibodies and memory cells without being infected by the real pathogen.[36]
The processes involved in primary immune response are as follows:
Following antigen processes by APCs, antigens will bind to either MHC class I receptors or MHC class II receptors on the cell surface of the cells based on their compositional and structural features to form complexes.[36]
Memory B cells and T cells are formed post-infection.[36] The antigens are memorised by these cells so that subsequent exposure to the same type of antigens will stimulate a secondary response, in which a higher concentration of antibodies specific for the antigens are reproduced rapidly and efficiently in a short time for the elimination of the pathogen.[38]
Under specific circumstances, low doses of vaccines are given initially, followed by additional doses named booster doses. Boosters can effectively maintain the level of memory cells in the human body, hence extending a person's immunity.[33][34][43]
Candidate subunits will be selected primarily by their immunogenicity.[44] To be immunogenic, they should be of foreign nature and of sufficient complexity for the reaction between different components of the immune system and the candidates to occur.[45] Candidates are also selected based on size, nature of function (e.g. signalling) and cellular location (e.g. transmembrane).[44]
Mammalian cells are well known for their ability to perform therapeutically essential post-translational modifications and express properly folded, glycosylated and functionally active proteins.[49][52][53] However, efficacy of mammalian cells may be limited by epigeneticgene silencing and aggresome formation (recombinant protein aggregation).[49] For mammalian cells, synthesised proteins were reported to be secreted into chemically defined media, potentially simplifying protein extraction and purification.[48]
The most prominent example under this class is Chinese Hamster Ovary (CHO) cells utilised for the synthesis of recombinant varicella zoster virus surface glycoprotein (gE) antigen for SHINGRIX.[7]CHO cells are recognised for rapid growth and their ability to offer process versatility. They can also be cultured in suspension-adapted culture in protein-free medium, hence reducing risk of prion-induced contamination.[48][49]
Throughout history, extraction and purification methods have evolved from standard chromatographic methods to the utilisation of affinity tags.[57] However, the final extraction and purification process undertaken highly depends on the chosen expression system. Please refer to subunit expression and synthesis for more insights.[citation needed]
Adjuvants increase the magnitude of adaptive response to the vaccine and guide the activation of the most effective forms of immunity for each specific pathogen (e.g. increasing generation of T cell memory).[58][59][60][61] Addition of adjuvants may confer benefits including dose sparing and stabilisation of final vaccine formulation.[58][61]
Recombinant subunit vaccines are contraindicated to people who have experienced allergic reactions and anaphylaxis to antigens or other components of the vaccines previously.[74][75] Furthermore, precautions should be taken when administering vaccines to people who are in diseased state and during pregnancy,[74] in which their injections should be delayed until their conditions become stable and after childbirth respectively.
Antibody concentration ≥10mIU/mL against HBsAg are recognized as conferring protection against hepatitis B infection.[76][77]
It has been shown that primary 3-dose vaccination of healthy individuals is associated with ≥90% seroprotection rates for ENGERIX-B, despite decreasing with older age. Lower seroprotection rates are also associated with presence of underlying chronic diseases and immunodeficiency. Yet, GSK HepB still has a clinically acceptable safety profile in all studied populations.[78]
Cervarix, GARDASIL and GARDASIL9 are three recombinant subunit vaccines licensed for the protection against HPV infection. They differ in the strains which they protect the patients from as Cervarix confers protection against type 16 and 18,[56]Gardasil confers protection against type 6, 11, 16 and 18,[79] and Gardasil 9 confers protection against type 6, 11, 16, 18, 31, 33, 45, 52, 58[5] respectively. The vaccines contain purified VLP of the major capsid L1 protein produced by recombinant Saccharomyces cerevisiae.[citation needed]
It has been shown in a 2014 systematic quantitative review that the bivalent HPV vaccine (Cervarix) is associated with pain (OR 3.29; 95% CI: 3.00–3.60), swelling (OR 3.14; 95% CI: 2.79–3.53) and redness (OR 2.41; 95% CI: 2.17–2.68) being the most frequently reported adverse effects. For Gardasil, the most frequently reported events were pain (OR 2.88; 95% CI: 2.42–3.43) and swelling (OR 2.65; 95% CI: 2.0–3.44).[80]
Gardasil was discontinued in the U.S. on May 8, 2017, after the introduction of Gardasil 9[81] and Cervarix was also voluntarily withdrawn in the U.S. on August 8, 2016.[82]
Flublok Quadrivalent has a comparable safety profile to traditional trivalent and quadrivalent vaccine equivalents. Flublok is also associated with less local reactions (RR = 0.94, 95% CI 0.90–0.98, three RCTs, FEM, I2 = 0%, low‐ certainty evidence) and higher risk of chills (RR = 1.33, 95% CI 1.03–1.72, three RCTs, FEM, I2 = 14%, low‐certainty evidence).[83]
While the practice of immunisation can be traced back to the 12th century, in which ancient Chinese at that time employed the technique of variolation to confer immunity to smallpox infection,[citation needed] the modern era of vaccination has a short history of around 200 years. It began with the invention of a vaccine by Edward Jenner in 1798 to eradicate smallpox by injecting relatively weaker cowpox virus into the human body.[citation needed]
The middle of the 20th century marked the golden age of vaccine science.[citation needed] Rapid technological advancements during this period of time enabled scientists to cultivate cell culture under controlled environments in laboratories,[87] subsequently giving rise to the production of vaccines against poliomyelitis, measles and various communicable diseases.[citation needed] Conjugated vaccines were also developed using immunologic markers including capsular polysaccharide and proteins.[87] Creation of products targeting common illnesses successfully lowered infection-related mortality and reduced public healthcare burden.
As the manufacturing methods continue to evolve, vaccines with more complex constitutions will inevitably be generated in the future to extend their therapeutic applications to both infectious and non-infectious diseases,[citation needed] in order to safeguard the health of more people.
Subunit vaccines are not only considered effective for SARS-COV-2, but also as candidates for evolving immunizations against malaria, tetanus, salmonella enterica, and other diseases.[11]
Research has been conducted to explore the possibility of developing a heterologous SARS-CoV receptor-binding domain (RBD) recombinant protein as a human vaccine against COVID-19. The theory is supported by evidence that convalescentserum from SARS-CoV patients have the ability to neutralise SARS-CoV-2 (corresponding virus for COVID-19) and that amino acid similarity between SARS-CoV and SARS-CoV-2 spike and RBD protein is high (82%).[90]
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