Vaccine
- Vaccine is a biological preparation that improves immunity to a particular disease. They are molecules, usually but not necessarily proteins, that elicits an immune response, thereby providing protective immunity against a potential pathogen.
- Vaccine is used to boost the body’s immune system and prevent the serious life threatening diseases.
- Vaccine can be prepared against bacteria or even eukaryotic protozoans, however most successful vaccines have been developed against viruses.
- The first human vaccine was smallpox vaccine, using cowpox as vaccine.
- Rabies was the first virus attenuated in a lab to create a vaccine for humans.
Characteristics of Vaccine:
Following are the properties that an ideal vaccine should possesses;
- Safe: Vaccine must be safe and must not itself causes illness or death
- Protective: vaccine must protect against illness resulting from exposure to live pathogen
- Sustained protection: protection against illness must last for years
- Induce neutralizing antibodies
- Induce protective T cells
- Low cost
- Biological stability
- Ease of administration
- No side effect or Very few side effects
Types of vaccines
1, Live attenuated vaccines:
- Live attenuated vaccines contain living microorganisms that has been weakened in the lab so it can’t cause disease. These vaccine evoke immune system of the host preventing form the diseases.
- Live attenuated vaccines are relatively easy to create for certain viruses.
- Vaccines against measles, mumps, and chickenpox, for example, are made by this method.
- Live attenuated vaccines are more difficult to create for bacteria. Bacteria have thousands of genes and thus are much harder to control. However, this approach has been used to create a vaccine against Vibrio cholerae.
- Live attenuated vaccine use whole organism as vaccine and are prepared from attenuated strains that almost or completely devoid of pathogenicity but are capable of inducing a protective immune response. They multiply in the human host and provide continuous antigenic stimulation over a period of time.
- Examples: BCG, Typhoid vaccine, Measles vaccine, mumps vaccine, Sabin’s polio vaccine, VAR vaccine, Yellow fever vaccine, Rota virus vaccine etc
2. Killed or Inactivated vaccines:
- Disease-causing microorganisms are killed with chemicals, heat, or radiation. Such vaccines are more stable and safer than live vaccines because the dead microorganisms can’t mutate back to their disease-causing state.
- There are the easiest preparations to use. Such vaccines are simply inactivated or killed microorganisms.
- Preparation of killed vaccine may take the route of heat or chemicals. The chemicals used includes formaldehyde or beta-propiolactone. The traditional agent for inactivation of virus is formalin.
- Excessive treatment can destroy immunogenicity whereas insufficient treatment can leave infectious microorganisms capable of causing disease.
- The inactivated vaccines usually don’t require refrigeration, and they can be easily stored and transported in a freeze-dried form, which makes them accessible to people in developing countries.
- Example: Salk polio vaccine, Anthrax vaccine, Cholera vaccine, Purtusis vaccine, Plague vaccine, Influenza vaccine, Hepatitis A vaccine, Rabies vaccine, Rubella vaccine etc
3. Subunit vaccines:
- Instead of the entire microorganisms, subunit vaccines include only the antigens that best stimulate the immune system and used in vaccine preparation.
- Vaccine that consists of specific, purified macromolecules derived from pathogen are known as subunit vaccine.
- The general forms of such vaccine are in current use: Purified capsular polysaccharides, inactivated exotoxin (Toxoid), recombinant microbial antigen, synthetic peptide.
i. Purified capsular polysaccharide vaccine:
- The virulence of some pathogenic bacteria depends primarily on the anti-phagocytic property of their hydrophobic polysaccharide capsule.
- These are generally conjugate vaccine.
- In some gran negative bacteria, LPS is the outermost covering which protect the bacteria from binding with the antibody. So that the immature immune systems of infants and younger children can’t recognize or respond to them. For this conjugate vaccine is used.
- Examples: Hib vaccine (The vaccine that protects against Haemophilus influenzae type B (Hib) is a conjugate vaccine), Vaccine for Streptococcus pneumoniae, vaccine for Neisseria meningitides
ii. Toxoid vaccines:
- For some bacteria that secrete toxins, or harmful chemicals, a toxoid vaccine is made.
- The toxins are inactivated by treating with formalin, such detoxified toxin is known as toxoid, which is used as vaccine.
- Vaccination with toxoid induces anti-toxoid antibodies, which are capable of binding the toxin and neutralizing its effect.
- Conditions for the production of toxoid vaccines must be closely controlled to achieve detoxification without excessive modification of the epitope structure. Sufficient quantities of the purified toxins is prepared by cloning the exotoxin genes and then expressing them in easily grown host cells, purified and subsequently inactivated.
- Vaccines against diphtheria and tetanus are examples of toxoid vaccines.
iii. Recombinant vaccine:
- Recombinant vector vaccines are experimental vaccines similar to DNA vaccines, but they use an attenuated virus or bacterium to introduce microbial DNA to cells of the body.
- Attenuated bacteria also can be used as vectors.
- The gene coding for immunogenic protein is inserted into plasmid vector and then transformed it into suitable host cell such as bacteria, yeast, mammal cell etc.
- In this case, the inserted genetic material causes the bacteria to display the antigens of other microbes on its surface.
- In effect, the harmless bacterium mimics a harmful microbe, provoking an immune response.
- Recombinant hepatitis B vaccine is the only recombinant vaccine licensed at present.
iv. Synthetic peptide vaccine:
- The development of synthetic peptides that might be useful as vaccines depends on the identification of immunogenic sites.
- The best example is Foot and mouth disease where protection was achieved by immunizing animals with a linear sequences of 20 aminoacids.
- Synthetic peptide vaccine would have many advantages. They are stable and relatively cheap to manufacture. Furthermore, less quality assurance is required.
- Synthetic peptides so not readily stimulate t cells. It was generally assume that, because of their small size, peptides would behave like haptens and would therefore require coupling to a protein carrier which is recognized by T cells.
- It is now known that synthetic peptides can be highly immunogenic in their free form provided they contain, in addition to the B cells epitope, T cell epitope recognized by T-helper cells. Such T cell epitope can be provided by carrier protein molecules, foreign antigens or within the synthetic peptide molecule itself.
- Synthetic peptide vaccine is not applicable for all viruses. For example, it is not applicable for Polio virus because important antigenic sites were made up of 2 or more different viral capsid protein.
4. DNA vaccines:
- DNA vaccine is DNA sequence used as vaccine.
- This DNA sequence codes for antigenic protein of pathogen.
- When the genes for a microbe’s antigens are introduced into the body, some cells will take up that DNA. The DNA then instructs those cells to make the antigen molecules. The cells secrete the antigens and display them on their surfaces. In other words, the body’s own cells become vaccine-making factories, creating the antigens necessary to stimulate the immune system.
- As this DNA inserted into cells it is translated to form antigenic protein. As this protein is foreign to cells, immune response raised against this protein. In this way, DNA vaccine provide immunity against that pathogen.
- Recently, encouraging results were reported for DNA vaccines whereas DNA coding for the foreign antigen is directly injected into the animal so that foreign antigen is directly produced by the host cells.
- In theory these vaccines would be extremely safe and devoid of side effects since the foreign antigens would be directly produced by the host animal.
- In addition, DNA is relatively inexpensive and easier to produce than conventional vaccines and thus this technology may one day increase the availability of vaccines to developing countries.
- The time for development of DNA vaccine is relatively short which enable timely immunization against emerging infectious diseases.
- Also, DNA vaccines can theoretically result in more long term production of an antigenic protein when introduced into a relatively non dividing tissue such as muscles.
- Examples, DNA Vaccine Against West Nile Virus, Influenza and Herpes virus