We know that already, corona virus is spreading in our community. So to stop this spreading, everybody is looking for vaccines. Well, what are vaccines, today we will talk about it.


Vaccine is an immuno-biological substance designed to produce specific protection against a given disease. It stimulates the production of protective antibody and other immune mechanisms. Vaccines may be prepared from live modified organisms, inactivated or killed organisms, extracted cellular fractions, toxoids or combination of these.

Over the last century, vaccination has been the most effective medical strategy to control infectious diseases. Smallpox has been eradicated world-wide and poliomyelitis has been almost eradicated. Most viral and bacterial diseases affecting children world-wide are now preventable by vaccines. Vaccination is estimated to save at least 2-3 million lives every year.

A . Live vaccines

Live vaccines (e.g., BCG, measles, oral polio) are prepared from live or wild (generally attenuated) organisms. These organisms have been passed repeatedly in the laboratory in the tissue culture or chick embryos and have lost their capacity to induce full-blown disease but retain their immunogenicity. In general, live vaccines are more potent immunizing agents than killed vaccines. the reason being :

  1. Live organisms multiply in the host and the resulting antigenic dose is larger than what is injected.
  2. Live vaccines have all the major and minor antigenic components.
  3. Live vaccines engage certain tissues of the body, as for example, intestine mucosa by the oral polio vaccine.
  4. There may be other mechanisms such as the persistence of latent of latent virus.

Live vaccines should not be administered to persons with immune deficiency diseases or to persons whose immune response may be suppressed because of leukemia, lymphoma or malignancy or because of therapy with corticosteroids, alkylating agents, antimetabolic agents, or radiation. Pregnancy is another contraindication unless the risk of infection exceeds the risk of infection to the fetus of some live vaccines.

When two live vaccines are required they should be given either simultaneously at different sites or with an interval of at least 3 weeks. In the case of live vaccines, protection is generally achieved with a single dose of vaccine. An additional dose is given to ensure seroconversion, example:- 95 to 98 percent of recipient will respond to single dose of measles vaccines. The second dose is given to ensure that 100 percent of persons are immune. The other exception exception is polio vaccine which needs three or more doses to be given at spaced intervals to produce effective immunity. Live vaccines usually produce a durable immunity, but not always as long as that of natural infection.

B . Inactivated or killed vaccines

Inactivated vaccines are produced by growing virus or bacteria in culture media and then inactivating them with heat or chemicals (usually formalin), when injected into the body they stimulate active immunity. They are usually safe but generally, less efficacious than live vaccines. For example cholera vaccine offers only 50 percent protection. The efficacy of 3 doses of pertussis vaccine is about 80 percent in the first three years, and almost “nil” 12 years after immunization. Killed vaccines usually require a primary series of 2 or 3 doses of vaccines to produce an adequate antibody response, and in most cases “booster” injections are required. The duration of immunity following the use of inactivated vaccines varies from months to many years. Inactivated polio vaccine has been quite an effective vaccine, the widespread use of which in certain countries has led to the elimination of the disease. Killed vaccines are usually administered by subcutaneous or intramuscular route.

Because the vaccine is inactivated, the infective agent can not grow in the vaccinated individual and therefore, can not cause the disease, even in an immunodeficient person.

The absolute contraindication to their administration is a severe local or general reaction to a previous dose. Unlike live antigens are not affected by circulating antibody. They are often more stable than live attenuated vaccines.

C . Subunit vaccines

A vaccines can be made of single or multiple antigenic components of a microorganism that are capable of stimulating a specific immune response sufficient to protect from the relevant pathogen infection or from the clinical manifestation of the disease. Depending on the molecular composition of the purified antigen used to prepare the vaccine and on the techniques applied to obtain the final material used as a vaccine, different types of subunit vaccines can be defined.

1 . Toxoids

Certain organisms produce exotoxins, e.g., diphtheria and tetanus bacilli. The toxins produced by these organisms are detoxicated and used in the preparation of vaccines. The antibodies produced neutralize the toxic moiety produced during infection, rather than act upon the organisms. In general, toxoid preparations are highly efficacious and safe immunizing agents.

2 . Protein vaccines

In case, immunization with a single protein or a combination of proteins from a pathogen is sufficient to to stimulate a protective immune response against that particular microorganism, the approach of a protein-based vaccine is appropriate. Proteins can be purified from in-vitro cultures of a pathogenic microorganism. The resulting vaccine preparations contain different amounts of contaminants depending on the efficiency of the purification process. Licensed acellular pertussis vaccines currently available contain from two to four different proteins purified from Bordetella pertussis and are able to confer protection against whooping cough comparable to that obtained with the whole cell vaccines. One of the most widely used subunit protein vaccines is the influenza vaccine composed of hemagglutinin (HA) and neuraminidase (NA) purified from the inactivated influenza virus.

3 . Recombinant protein vaccines

Development of the recombinant deoxyribonucleic acid (DNA) technology has made possible the expression of protective protein antigens in heterologous expression systems such as Escherichia coli, yeast, mammal cells, or baculovirus. This technology avoids the problems related to growing and manipulating large amounts of a pathogen from which the antigen is purified. Moreover, recombinant proteins are generally better purified from cultured microorganisms resulting in cleaner vaccine preparations with a better safety profile. A drawback of a clean vaccine preparation pure recombinant protein(s) is their reduced immunogenicity that may require the addition of an adjuvant to achieve enhanced efficacy. For example, the hepatitis B vaccine is made by inserting a segment of hepatitis B virus gene into a yeast cell. The modified yeast cell produces large amounts of hepatitis B surface antigen, which is purified and harvested and used to produce the vaccine. The recombinant hepatitis B vaccine is identical to the natural hepatitis B vaccine is identical to the natural hepatitis B surface antigen, but does not contain virus DNA and is unable to produce infection.

4 . Polysaccharide-based vaccines

The surface of many pathogenic bacteria is covered by a capsular shell that is mainly assembled from polymeric glycans. This extensive polysaccharide coat entirely shields the bacteria outer membrane, preventing other surface bacterial components from becoming a target of the immune response. Nevertheless, antibodies to bacterial surface polysaccharides can clear the bacteria from the host by different mechanisms, such as complement-mediated killing and opsonophagocytosis. Hence, stimulation of an antibody response against the surface polysaccharide of pathogenic bacteria is a strategy for the development of vaccines against the capsulated bacteria. The chemical structure or capsular polysaccharides varies not only between different strains within a single species, which are usually differentiated and typed based on their capsular polysaccharides. As a consequence, a limitation of polysaccharide-based vaccine is that the immune responses they elicit are often serotype specific. In addition to Streptococcus pneumoniae, for which a vaccine against 23 serotype is available, polysaccharide- based vaccines have been developed for MenACWY, Haemophilus influenzae and Salmonella typhimurium.

5 . Conjugated vaccines

Children under two years of age, do not respond well to antigens, such as polysaccharides, which produce antibodies vie a T- cell independent mechanism. If these polysaccharide antigens are chemically linked (conjugated) to a protein that t- cell recognize, then these conjugate vaccines can elicit strong immune responses and immune memory in young children. Similar to the polysaccharide-based vaccines, therefore, multivalent formulations are required to achieve protection against multiple serotypes. Examples are Streptococcus pneumonia and meningococcal vaccines.

d . Combinations

If more than one kind of immunizing agent is included in the vaccine, it is called a mixed or combined vaccines is to simplify administration, reduce costs, minimize the number of contacts of the patient with the health system, reducing the storage cost, improving timelines of vaccination, and facilitating the addition of new vaccine into immunization program. No evidence exists that the administration of several antigens in combined vaccines increases the burden on the immune system which is capable of responding to millions of antigens at the time. Combining antigens usually does not increase the risk of adverse reactions and can infect lead to an overall reduction in adverse reactions. The following are some of the well-known combinations:

  • DPT (Diphtheria-pertussis-tetanus)
  • DT (Diphtheria-tetanus)
  • DP (Diphtheria-pertussis)
  • DPT and typhoid vaccine
  • MMR (Measles, mumps and rubella)
  • DPTP (DPT plus inactivated polio)
  • Hepatitis A, and B
  • Hepatitis A, and typhoid.
  • DTwP (Diphtheria, pertussis, tetanus, hepatitis B and haemophilus influenza type B).

The term “polyvalent” is applied to vaccine (e.g., polio, influenza vaccine) which are prepared from two or more strains of the same species. The term “auto” or “autogenous” vaccine is applied when the organism in the vaccine is applied when the organism in the vaccine is obtained from the same patient.

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