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Introduction:

Prophylaxis (a Greek word means “to guard”) is any medical or public health procedure

whose purpose is to prevent, rather than treat or cure a disease. In general term, prophylactic

measures are divided between primary prophylaxis (to prevent the development of a disease) and

secondary prophylaxis (whereby the disease has already developed and the patient is protected

against worsening of this process). Some examples of prophylaxis include many vaccines such as

polio vaccine, small pox vaccine, measles vaccine and others.

Vaccine owes its origin to the Latin word, vacca meaning cow. Edward Jenner did not

introduce the term vaccine, it was Pasteur who did that.

A vaccine is a preparation of bacterial, viral or other pathogenic agents or of their

isolated antigens which is administrated with the objective of stimulating a recipient‟s

protective immunity.

A vaccine is basically an antigen or its component that can induce secondary or adaptive

immunity in the host.

The aims of vaccine is to prevent severe complications of infection by reinforcing or broadening

the defenses by introducing immunological memory.

Properties of an ideal vaccine:

Cause less disease than the natural infection; ideally no clinical side effects.

Be genetically stable.

Stimulate effective and long-lasting immunity in the individual and/or stimulate

sufficient herd immunity to deprive virus of the susceptible needed for its survivl.

Have public support.

Be affordable.

Types of vaccines (Antigens used as vaccine):

There are several types of vaccines that are currently and conventionally used.

A. Natural live vaccines

B. Live attenuated vaccines

C. Killed or inactivated vaccines

D. Toxoid vaccines

E. Polysaccharide vaccines

F. Recombinant vaccines

G. Viral vector vaccines

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H. Bacterial vector vaccines

I. DNA vaccines

A. Natural live vaccines:

Natural live vaccines are prepared from non-pathogenic organisms but it still induce

specific immunity.

Currently natural live vaccines are rarely used.

e.g. cowpox (vaccinia) virus used for immunization against smallpox, simian rotavirus is used in

vaccinations against gastroenteritis.

Disadvantage of natural live vaccines:

These vaccines reside in their ability to mutate and convert into forms that could be pathogenic

to human host.

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B. Live attenuated vaccines:

Attenuation comes from Latin word means to weaken. It refers to the weakening of the

pathogenic bacteria or virus by making it less virulent without altering its immunogenicity.

Live attenuated vaccines are prepared from attenuated microorganism (bacteria or virus).

Attenuation can be achieved by growing pathogenic microorganism (bacteria or virus)

for a long period of time in a foreign host such as embryonated eggs or tissue culture cells.

e.g. BCG; Sabin polio vaccine; MMR; varicella-zoster vaccine (for chicken pox); yellow

fever vaccine.

Advantages of attenuated vaccines:

Attenuated vaccine provide for prolonged exposure of viral antigen to the immune system

resulting in the production of a large numbers of B and T cells and, more importantly,

memory cells.

The immunity induced by this vaccine is generally more durable and more effective.

It enters the gastrointestinal tract and induce the production of secretory IgA as well as

IgG, these antibody serve as an important defence against naturally occurring poliovirus.

Induce quick immunity.

Low cost of production and easy of administration.

Easy to transport.

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Disadvantages of attenuated vaccines:

A major disadvantage of using live attenuated vaccines the possibility of their reversion

to the virulent form.

It cannot be given to persons having immunodeficiency diseases as the immune system of

these patients is severely compromised and these pathogen, even though attenuate or

weakened, can induce some damage.

It can contain adventitious agents.

Can spread to contacts.

Can lose infectivity during storage, transport or use.

Can cause illness directly by losing their attenuation.

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C. Inactivated vaccines:

Inactivated vaccines are prepared by inactivating viruses and bacteria. The inactivation could be

achieved by modifying the antigen in two ways,

1. Chemical inactivation

2. Physical inactivation

Chemical inactivation:

The bacteria and viruses are treated with formaldehyde, phenol, amino acid. This treatment

usually results in the killing of the organisms with almost no change in the antigenic structure. It

gives better results than heat activation.

Chemically inactivated vaccines are associated with some risk. The chemical agent does not kill

all the pathogen present as a result the vaccine contains inactivated pathogen and live pathogen

which cause disease in the vaccine.

e.g. formaldehyde failed to inactivate all of the polioviruses during the preparation of Salk

polio vaccine.

Physical inactivation:

Physical inactivation achieved by heat treatment. Heat inactivation of microbes has one potential

problem. It cause the denaturation of surface proteins, that is, its antigenic determinants and so

their antigenic structure is altered. The antibody/immune response induced by vaccinating these

altered antigens may not be effective against natural pathogen.

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Advantages of Inactivated vaccines:

No mutation or reversion to wild type forms scince pathogenic organism is dead.

Provide sufficient humoral immunity.

Heat stable.

Can be used in immunedeficient patients.

Disadvantages of Inactivated vaccines:

Weak cell-mediated response.

Requires booster stimulation since organism cannot replicate inside host.

Higher cost.

Can cause disease.

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D. Toxoid vaccines:

Toxoid vaccine prepared from toxoids. Toxoid are modified exotoxin that have lost their

toxicity yet are immunogenic. Toxoids are generated either by a chemical modification of toxins

by formaldehyde or by heat treatment.

e.g. Diphtheria vaccine, tetanus vaccine.

Advantages of toxoid vaccines:

It induces antitoxoid antibodies which are capable of binding the toxin and neutralizing

it.

The toxoids are generally mixed with aluminum hydroxide which acts as an adjuvant

resulting in an increased production of specific antibodies, encouraging its removal by

phagocytic cells and hence stimulating T-cell response.

Disadvantages of toxoid vaccines:

Not all organisms produce exotoxin.

The organisms who produce exotoxin in minuscule quantities so it is difficult to purify

them.

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E. Polysaccharide vaccines:

The surface of bacteria is the first thing that is exposed to the immune system. Therefore,

the surface antigens can serve as an excellent vaccine. Moreover, some bacteria have resistance

to phagocytic invasion because these cells have capsular polysaccharides that resists

phagocytosis( for example the polysaccharide capsules of S. pneumoniae, Klebsiella pneumoniae

). The coating of these capsular polysaccharides with antibodies and/or complement components

marks them for destruction by phagocytosis. Those observations have paved the way for

synthesizing vaccines against bacterial-purified polysaccharides.

Advantages of polysaccharide vaccines:

It produces long-lasting immunity.

Polysaccharide-protein conjugate activates TH cells which in turn enables the formation

of memory B cells.

Disadvantages of polysaccharide vaccines:

Polysaccharide vaccine usually do not stimulate T cell.

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F. Recombinant Antigen Vaccines:

With the advent of recombinant DNA technology, virtually any gene encoding

immunogenic protein can be introduced and expressed in yeast , bacterial or even mammalian

cells, using recombinant DNA technology. These cells are then cultured in the laboratory and the

protein produced endogenously is harvested. The genes that are selected for making recombinant

antigen vaccine are usually surface antigen (mostly glycoprotein‟s).

A number of genes coding for various surface antigens have emerged as a better choice

for making these surface antigens as they add and process carbohydrate molecules on the protein

in their golgi bodies in a manner more similar that to mammals and lea care is needed in

comparison with mammalian cells. One such vaccine approved for human use is hepatitis B

vaccine.

Mechanism:

i) A single gene for the major surface antigen of hepatitis B virus is cloned in yeast

cells.

ii) The recombinant yeast cells are grown in fermenters.

iii) HbsAg, the surface antigen is expressed and accumulates inside the yeast cells.

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iv) The yeast cells are then harvested and the burst open by high pressure releasing the

recombinant HbsAg.

v) HbsAg is then purified by a standard biochemical technique such as affinity

chromatography.

vi) The purified antigen has been shown to induce humoral immunity.

Advantage:

a) Inexpensive production of large amount of antigens as well as allowing space for

genetic manipulation of antigens.

b) Genetic manipulation of the gene is possible.

c) Exotoxin of tetanus and diphtheria , which are previously modified chemically.

d) Antigens can be made more immunogenic.

Disadvantage:

a) Do not generate potent Tcyt cell response.

b) Cannot be used to synthesize carbohydrate antigen.

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G. Live Vector Vaccines:

In live vector vaccine, the desired gene coding for target antigens of the virulent pathogen

is put into a vector( attenuated bacteria or virus) and then this vector is infected to the vaccine.

This vector slowly replicates inside the inoculated individual and it serves as a source of antigen,

delivering a large amount of antigen into the system and provoking a strong immune response. A

number of organisms have been used as vectors. For example virus and bacteria.

The most commonly used viral vectors are the vaccinia virus (the smallpox vaccine

virus), adenovirus and canary pox virus (that infect cell but does not replicate into the human).

The most commonly used bacterial vector include attenuated Salmonella typhi(Ty2la),

BCG strain of Mycobacterium bovis as well as the not-so-common but potential vector Vibrio

cholera. In fact all of attenuated viral/bacterial vaccines have been suggested as possible vectors

for their use in live vector vaccines.

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1. Viral vector vaccine:

The most commonly used viral vector vaccine is the vaccinia virus , through genetically

engineered recombinant viral vectors such as pox virus or adenovirus are also deliberated upon.

Genes that have the potential to induce protective immunity are inserted into attenuated live

virus.

Vaccinia, a commonly used virus has a large, double stranded gene (187000 kbp and

approximately 200 genes). Because of its large size , a large number of genes can be replicated

with foreign genes without causing any loss of their capacity to infect host cell. The genetically

manipulated vaccinia virus express high level of gene product and this serve as a potent source

of immunogen inside the host. Experimental viral vector vaccine generated by this method

include those that express influenza virus haemagglutinin, malaria protein, hepatitis virus surface

antigen, herpes simplex virus proteins, among others.

Advantage:

Strong humoral and cell-mediated immune response.

Several segments of DNA encoding antigens from different pathogens can be inserted in

a single virus vector.

Can be targeted to specific tissue due to viral tropism.

No interference in protection produced by other types of vaccine.

Inexpensive and easy to transport.

Disadvantage:

Danger of reversion to virulence is always there.

Some viruses have transforming capabilities making infected cells cancerous.

Immune response to virus infected cells may cause damage to vaccine.

2 . Bacterial vector vaccine:

Like the live viral vectors, some attenuated bacterial strains have been engineered to

carry genes of virulent pathogens. The DNA encoding the antigenic determinants is inserted into

the attenuated bacterial genome. The bacteria then express the antigen along with its own

protein.

The production and expression of antigen by the bacterial vector inside the host body stimulates

the immune system.

An attenuated strain of Salmonella typhimurium (Ty2la), the causative agent of food poisoning,

is being explored as a vector and is currently in human trials.

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Advantage:

Attenuated strain of S.typhimurium, Vibrio cholera, and BCG are easily available and

their genome cal easily be manipulated.

Produce immune response both against the vector as well as the inserted antigen.

Not only produce cell mediated and humoral immunity but also mucosal immunity(IgA

production) since these bact6eria survive in the GI tract.

Disadvantage:

The reversion and emergence of the pathogenic form of bacteria, rejection and

elimination of bacterial vector before it can express the recombinant protein.

Antigens formed inside the bacteria may be proteolysed by endogenous bacterial

enzymes.

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H. DNA vaccines:

The term DNA vaccine is a misnomer. It wrongly implies that the DNA is used as an

antigen and antibodies are formed against DNA.

The DNA vaccine represents a new class of vaccines in which there is a deliberate

introduction of a DNA plasmid usually into the muscle cell of the recipients.

The plasmid consist of a protein coding gene that gets expressed in the cell, leading to

both humoral and cell mediated immune responses. The plasmid can be introduced into the

muscle cell either by infection or by bombarding the skin with DNA-coated gold particles with

fine airgun.

Usually bacterial plasmid is used and a gene coding the antigen is inserted into the

control of a mammalian promoter and this chimeric plasmid is then introduced into the recipient.

The recipient cell is then express the foreign antigenic protein coated by the introduced DNA in

the host body. The immune system then responds to antigen as to any other antigen entering the

body.

Moreover, since proteins are produced inside the muscle in the same way as normal

proteins they processed post transcriptionally the same way as protein of host cells. Memory

cells against the antigen are synthesized, giving lasting immunity.

As plasmids do not replicate, the DNA vaccine is non virulent. The DNA vaccines

stimulate both cell-mediated and humoral responses. Since, there is an expression of antigen for

a long period of time there is also an induction of immunological memory.

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One of the major drawbacks of DNA vaccine is the concern over potential integration of

plasmid into the DNA of cells. Such an integration could lead to insertional mutagenesis and

cause the cell become cancerous. In addition there is a danger of induction of anti-DNA

antibodies causing pathological autoimmune reactions.

Another drawbacks of DNA vaccine is the fact that it can only be formed against protein

antigens but vaccine against polysaccharide antigen cannot be formed.

Advantage:

Easy to manufacture in large amount.

Stable and easy to transport.

DNA sequence and hence antigen can easily be changed.

Mixture of plasmids could be used to form broad-spectrum vaccine.

Absence of components ensures there is no strong immune response against vaccine.

Disadvantage:

Insertion of foreign DNA into host genome may cause cell too become cancerous.

Danger of autoimmune response due to formation of anti-DNA antibodies.

May induce immunologic tolerance by antigen(s) expressed inside host body.

Cannot be formed against polysaccharide antigens.

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Current Vaccine & Modern Approach:

SYNTHETIC VACCINE: The 1st human synthetic vaccine was developed in 1999

against the Heamophillus influenza type b (Hib) bacteria that causes meningitis &

pneumonia in children. If we know the peptide sequence of protein antigens, they can be

chemically synthesized in laboratory. A protein antigen is a large molecule & there are

technical difficulties in synthesizing such large molecules & hence small parts, or

antigenic determinants, are synthesized. These synthesized antigens have one more

problem. Hey are poorly immunogenic (and do not elicit a strong immune response, in

these particular case cell-mediated response). This could be due to the fact that synthetic

antigen may not fold (this is they don‟t have secondary and tertiary structure) as the

negative antigenic determinants because the 3D conformation of antigenic determinants

is determined not only by the leaner amino acid sequence but also by the conformation of

the intact protein, as well as that of the virus/bacteria as a whole.

A synthetic vaccine containing repeats of ~10 amino acids (of HbsAg) are currently in

trial as a potential hepatitis B vaccine.

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MICROENCAPSULATION DELEVERY SYSTEM: In micro encapsulation delivery

system antigen are placed into biodegradable capsule (microspheres, nanospheres) and

introduced inside the body of the recipient. The antigens are released from a capsule

either when the capsule is phagocytosed by macrophages and dendritic cell or via slow

leaching. The polymer capsule is made up of lactic acid and glycolic acid (specifically

poly Lactide-co-Glycolide).

These capsule (~5µm in diameter) enclose the antigen and are delivered orally or nasally.

Animal studies have shown that those polymer capsule release antigens in controlled

phages due to slow polymer degradation, eliminating the need for booster immunization.

Moreover, these capsules can be phagocytosed and hence degraded inside antigen

presenting cells targeting antigens to the lymphoid system.

Animal models are currently being experimented by microsphere-encapsulated influenza

vaccine. As well as with tetanus toxoid And Bortedella pertussis. The major drawbacks

of this delivery system is the lack of cell-mediated immune response to enclosed

antigens. In addition, the polymerization of the capsule around the antigen involves

treatment with organic solvents that may denature some of the protein antigens.

LIPOSOMES AND MICELLES: Liposomes are tiny spheres that contain a

phospholipid bilayer enclosing an aqueous environment, while micelles are amphipathic

molecules (such as detergents) that are arranged with their polar head outwards and non-

polar hydrophobic tail inwards the globular structure. The liposome interior is aqueous

and that can be used to enclose soluble antigens while non-polar/hydrophobic antigens

can be inserted in the membrane. When introduced into the recipient, the membrane of

liposome fuses with the cells, releasing the antigen into the cytosol of the recipient cells.

Liposomes containing hepatitis A antigens were found to induce high titres oof antibody

in the recipient.

ISCOM-(Immuno Stimulating Complex): ISCOMS are liposome-like structures having

an outer coat of naturally occurring lipids mixed with a detergent, and a glycoside such as

Quil A (derived from the bark of tree Quillaia suponaria Molina). These substances form

an outer lipid membrane that encages the antigens inside. ISCOMS can survive the

hostile environment of the GI tract as well as up to several years. ISCOMS have been

shown to stimulate both humoral immunity and Tcyt/TH cells. This property makes

ISCOMS suitable for oral vaccines. HIV antigens incorporated in ISCOMS are being

investigated as potent HIV vaccines in animal studies.

SOLID MATRIX-ANTIBODY-ANTIGEN COMPLEXES (SMAA): Monoclonal

antibodies are first formed against antigenic determinants of pathogens. These antibodies

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are then covalently linked to the solid matrices. The antibody-matrix complex is then

saturated with antigen which binds monoclonal antibodies. A variety of antigens can be

presented on the same matrix by the using mixture of monoclonal antibodies. The

complex which contains solid matrix-antibody-antigen is then introduced into the

recipient. Because of their large size, complexes are easily phagocytosed, and humoral

and cell-mediated immunity elicited. Animal studies have shown positive results after the

administration of SMAA in mice having paramyxovirus infection. Tcyt cells were

primarily stimulated by SMAA.

ANTI-IDIOTYPE VACCINES: The unique amino acid structure of an antigen-binding

site is referred to as idiotype. The antigen-binding site is complementary to the structure

of the antigen. So antibodies raised against the idiotype of the antibody will mimic the

three-dimensional structure of the antigen. This can be used as a vaccine. When the anti-

idiotype antibody (anti-ids) are injected into an individual, the antibodies formed (anti

anti-ids) recognize the original antigen (virus) and can potentially neutralize the antigen.

Efforts are on to develop an anti-idiotype vaccine against hepatitis B antigen. Clinical

trials of anti-idiotypic vaccine developed against metastatic melanoma (I-Mel-2) are in

the phase II stage of testing in human subjects.

ANTIGEN-COCHELATE: Antigen-cochelate consists of a continuous sheet of

phospholipid bilayer-Ca2+ complex-within which antigens are dispersed. The entire sheet

is rolled up like a carpet roll. Because of the folding of the lipid bilayer, few antigens are

exposed, most of them are hidden and protected from degradation. When this complex is

introduced into the recipient, it is slowly degraded exposing the hidden antigen. It

generates both humoral and cell-mediated immunity. This technique is relatively new and

is still in its infancy.

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Adjuvant:

Under some circumstances as in vaccination, it may be desirable to enhance the normal

immune response by administrating an adjuvant with the antigen. Adjuvants enhance the bodies

immune response to the antigen.

A large variety of compounds have been employed as adjuvants. Many adjuvants act by

slowing the release of antigen into the body, but in many cases their mode of action is unknown.

The immune system, being antigen driven, responds to the presence of antigen and terminates

that responds once antigen is eliminated.

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It is possible to slow the rate of antigen release into the tissue by first mixing antigen with

an insoluble adjuvant. Injected into an animal, this forms a focus, or „depot‟. Examples of depot

forming adjuvants include insoluble aluminum salts such as aluminum hydroxide, aluminum

phosphate, and aluminum potassium sulfate. When antigen is mixed with one of these salts and

injected into an animal , a micrograph-rich granuloma forms in the tissues. The antigen within

the granuloma slowly leaks into the body and so provides a prolonged antigenic stimulus.

These depot adjuvants influence only primary immune response and have little effect on

secondary immune responses.

An alternative method of forming depot is to incorporate the antigen in a water-in-oil

emulsion known as Freund‟s incomplete adjuvant. The oilstimulates a local, chronic,

inflammatory response, and as a result a thick layer of macrophages and fibroblasts, called a

granuloma, forms around the site of the inoculums. The antigen is slowly leached from the

aqueous phase of the emulsion. Oil emulsion droplets may also be carried to other sites through

the lymphatic system.

Some bacterial products posses adjuvant activity. For example endotoxins enhance

antibody formation if given about the same time as the antigen. They have no effect on delayed

type hypersensitivity but they can break tolerance, and they have general immunostimulatory

activity, which is reflected ina nonspecific resistance to bacterial infection. Endotoxins act by

stimulating macrophage production of IL-1.

A table of some common adjuvants and their mode of action is listed below-

Type Adjuvants Mode of Action Aluminum salts Aluminum phosphate

Aluminum hydroxide

Alum

Slow release antigen depot

Water-in-oil emulsions Freund‟s incomplete

adjuvant

Slow release antigen depot

Bacterial fractions Anaerobic corynebacteria

BCG

Muramyl dipeptide

Bordetella pertusis

Lipopolysaccharide

Macrophage stimulator

Macrophage stimulator

Macrophage stimulator

Lymphocyte stimulator

Macrophage stimulator

Surface active agents Saponin

Lysolecithin

Stimulates antigen processing

Stimulates antigen processing

Complex carbohydrates Acemannan

Glucans

Dextran sulfate

Macrophage stimulator

Macrophage stimulator

Macrophage stimulator

Mixed adjuvants Freund‟s incomplete

adjuvant

Water-in-oil emulsion plus

mycobacterium

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A major restrictions on adjuvants are that they should not be toxic, and in the case of food

animals they should not be adversely affect the quality of meat.

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Effectiveness and safety of vaccine:

Vaccines are safe and effective. However, they are neither perfectly safe nor perfectly

effective. Consequently, some persons who receive vaccines will be injured as a result, and some

persons who receive vaccines will not be protected. Most adverse events associated with

vaccines are minor and involve local soreness or redness at the injection site or perhaps fever for

a day or so. Rarely, however, vaccine can cause more serious adverse events. Whether an

adverse event that occurs after vaccination was caused by the vaccine or was merely temporally

related and caused by some totally independent (and often unknown or unidentified) factor is

often difficult to ascertain. This is particularly problematic during infancy, when a number of

conditions may occur spontaneously.

Particularly when dealing with rare events, large-scale case–control studies or reviews of

comprehensive records of large numbers of infants may be necessary to ascertain whether those

who received a vaccine had a higher incidence of the event than those who did not.

An important characteristic of most vaccines is that they provide both individual and

community protection. Most of the diseases against which we vaccinate are transmitted from

person to person. When a sufficiently large proportion of individuals in a community is

immunized, those persons serve as a protective barrier against the likelihood of transmission of

the disease in the community, thus indirectly protecting those who are not immunized and those

who received vaccine but are not protected (vaccine failures).

When a community has a high level of vaccination, an individual might decide to not be

vaccinated to avoid the small risk for adverse events while benefitting from the vaccination of

others. Of course, if a sufficient number of individuals make this decision, the protection levels

in the community decline, the herd immunity effect is lost, and the risk of transmission rises.

Measuring safety and effectiveness is not, however, inherently straightforward. Done

correctly, increased confidence and widespread adoption of a vaccine may ensue. Done poorly,

or incorrectly, years of misperceptions and “active disuse” of a vaccine may result. Recent

examples include vaccine safety fears and controversies regarding the possible occurrence of

Guillian–Barre syndrome after pandemic influenza vaccine, or fears about autism spectrum

disorders among intended recipients of MMR vaccines. These controversies started with

anecdotal observations, and proceeded to various types of study designs often of low or

misleading quality.

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References-

1. Essentials of immunology

- Fahim Ali Khan

2. Vaccination Mandates: The Public Health Imperative and Individual Rights - KEVIN M. MALONE AND ALAN R. HINMAN

3. www.elsevier.com/locate/vaccine