Vaccine: Introduction, Mechanism, Types, Route of Administration

Vaccine: Vaccine is a substance that is introduced into the body to prevent the disease produced by certain pathogens. Vaccine consists of dead pathogens or subunits (purified protein subunits, polysaccharides) or live but attenuated (artificially weakened) organisms.

Introduction of Vaccine

The vaccine is a substance that is introduced into the body to prevent the disease produced by certain pathogens. The vaccine consists of dead pathogens or subunits (purified protein subunits, polysaccharides) or live but attenuated (artificially weakened) organisms.

The vaccine induces immunity against the pathogen, either by the production of antibodies or by activation of T lymphocytes.

Edward Jenner produced the first live vaccine. He produced the vaccine for smallpox from the cowpox virus.

Nowadays, vaccines are used to prevent many diseases like measles, mumps, poliomyelitis, tuberculosis, smallpox, rubella, yellow fever, rabies, typhoid, influenza, hepatitis B, etc.

Vaccination: The process of distributing and administrating vaccines is referred to as Vaccination.

Immunization

Immunization is defined as the procedure by which the body is prepared to fight against a specific disease. It is used to induce the immune resistance of the body to a specific disease.

Immunization is of two types:

Active immunization
Passive immunization

The Hierarchy of Vaccines

First-generation vaccines are whole-organism vaccines– either live and weakened, or killed forms.
The so-called second-generation vaccines are subunit vaccines, consisting of defined protein antigens (such as tetanus or diphtheria toxoid)or recombinant protein components (such as the hepatitis B surface antigen).
DNA vaccines are third generation vaccines
1796 Jenner: wild type animal-adapted virus
1800’s Pasteur: Attenuated virus
1996 DNA vaccines

The Mechanism of a Vaccine

In an ideal scenario, whenever a vaccine is first administered, it is phagocytized by an antigen-presenting cell.
Recent research suggests that it is particularly important that the vaccine be taken up by a dendritic cell.
This is because dendritic cells play a key role in activating T cells, which become helper T cells.
From there, the activated Th cells go on to activate mature B-cells.
These activated B-cells divide into two cell types, antibody-producing plasma cells and, most importantly, memory B cells.

Importance of Secondary Immune Response

During the secondary immune response, the body mounts a quicker, more robust attack on the pathogen.

Thus, the pathogen is cleared from the body before it has the chance to cause an infection

Adjuvants

An adjuvant is a chemical substance that can be added to a vaccine in order to enhance the immune response to the vaccine.

Types of vaccines

1. Live-attenuated (weakened) vaccines
These vaccines contain modified strains of a pathogen (bacteria or viruses) that have been weakened but are able to multiply within the body and remain antigenic enough to induce a strong immune response.

The varicella-zoster vaccine, oral poliovirus (OPV) vaccines, or yellow fever virus vaccine are some examples of this type of vaccine

2. Heterologous vaccines

Heterologous vaccines are a sub-group of live attenuated vaccines produced from strains that are pathogenic in animals but not in humans.

It is a vaccine that confers protective immunity against a pathogen that shares cross-reacting antigens with the microorganisms in the vaccine. e.g. cowpox virus that protects against smallpox in humans.

3. Killed-inactivated vaccines

To produce this type of vaccine, bacteria or viruses are killed or inactivated by chemical treatment or heat.

This group includes for example the inactivated poliovirus (IPV) vaccine, pertussis vaccine, rabies vaccine, or hepatitis A virus vaccine.

4. Sub-unit vaccines

Instead of the entire microbe, subunit vaccines include only the antigens that best stimulate the immune system.

In some cases, these vaccines use epitopes—the very specific parts of the antigen that binds to antibodies or T cells. Because subunit vaccines contain only the essential antigens and not all the other molecules that make up the microbe, the chances of adverse reactions to the vaccine are lower.

5. DNA Vaccines

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.

6. RECOMBINANT VECTOR VACCINES

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.

“Vector” refers to the virus or bacterium used as the carrier.

TOXOID VACCINES

These vaccines are used when a bacterial toxin is the main cause of illness.
When the immune system receives a vaccine containing a harmless toxoid, it learns how to fight off the natural toxin.
The immune system produces antibodies that block the toxin. e.g. Vaccines against diphtheria and tetanus.

8. Gene deleted vaccines

These are genetically engineered vaccines that involve the removal or mutation of the virulence gene of the pathogen.

9. Peptide vaccines

These are the subunit vaccines prepared by the chemical synthesis of short immunogenic peptides.

Vaccines type Vaccines of this type on U.S. Recommended

Childhood (ages 0-6) Immunization Schedule

Live attenuated: Measles, mumps, rubella (MMR combined vaccine) Varicella (chickenpox) Influenza (nasal spray) Rotavirus
Inactivated/Killed: Polio (IPV) Hepatitis A
Toxoid (inactivated toxin): Diphtheria, tetanus (part of DTaP combined immunization)
Subunit/conjugate: Hepatitis B, Influenza (injection) Haemophilus influenzae type b (Hib) Pertussis (part of DTaP combined immunization) Pneumococcal Meningococcal

Routes of Administration of Vaccines

Deep subcutaneous or intramuscular route (most vaccines)

Oral route (oral BCG vaccine)

Intradermal route (BCG)

Scarification (smallpox)

Intranasal route (live attenuated influenza vaccines)

Note:-Vaccine is usually given in the liquid form.

Scheme of immunization

Primary vaccination

One dose vaccines (BCG, measles, mumps, rubella, yellow fever)
Multiple-dose vaccines (polio, DPT, hepatitis B)
Booster vaccination
To maintain immunity level after it declines after some time has elapsed (DT, MMR)

Boosters

For most vaccines, the immunity against a particular pathogen has a tendency to wear off over time.
In this case, a periodic “booster” administration must be given in order to strengthen and lengthen the duration of immunity.

Periods of maintained immunity due to vaccines

A short period (months): cholera vaccine

Two years: TAB vaccines (typhoid paratyphoid A and B vaccines)
Three to five years: DPT vaccines (diphtheria, pertussis (whooping cough), and tetanus)
Five or more years: BCG vaccines (Bacillus Calmette–Guérin is a vaccine against tuberculosis)
Ten years: yellow fever vaccine

Toxoids

Toxoid is a substance that is normally toxic and has been processed to destroy its toxicity but retains its capacity to induce antibody production by the immune system.
Toxoid consists of weakened components or toxins secreted by the pathogens.
Toxoids are used to develop immunity against diseases like diphtheria, tetanus, cholera, etc.

Origin of Vaccines

1. Bacterial vaccines

Live attenuated: BCG. Anthrax, brucella, Plague

Killed: TAB (enteric fever), cholera, Pertussis

2. Viral vaccines

Live attenuated- Smallpox, measles, influenza, mumps, Sabin (poliomyelitis), yellow fever

Killed –Salk (poliomyelitis), rabies, influenza

3. Toxoid–

Bacterial –e.g. Diphtheria, Tetanus

4. Structural component

Bacteria-Pneumococcal, Hemophilus influenza

Viral-Hepatitis B (HBsAg)

Attenuated Vs inactivated vaccines

Attenuated Inactivated

1. Type of host Immune responses- 1. Mainly produces a humoral response

Produce both cell-mediated and humoral

immune response

2. Tendency to revert: May revert to 2. Does not revert to a virulent strain

original virulent by recombination

virulent form with wild-type strain or reverse mutation.

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