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Vaccine and Vaccination
Department of Microbiology
Introduction
Vaccines may be defined as biological formulations composed of an
infectious agent or its component (s), the administration of which
results in generation of protective immune response.
Vaccines
• Immunization is considered to be the most significant development in
the field of health care that has saved lives of millions.
• After the use of cow pox material for immunization against small pox,
a large number of biological products have been shown to induce
Immunity.
• Presently, we have various types of biological material in use for the
purpose of Immunization in human or animals.
• There are two broad categories of vaccines i.e., a) whole agent vaccine
and b) sub unit vaccine.
• Whole agent vaccines include vaccines that use live and inactivated
organisms whereas sub-unit vaccines include vaccines which only
contain the components of organism not the whole organism.
Live vaccines
• A large number of live organisms are used as vaccines.
Live vaccines have several advantages:
• The immunity induced by live vaccines is often solid and
gives long term protection.
• Live organisms in the vaccine grow in the host and thus
mimic the natural infection. Therefore, a more exacting
type of response is mounted.
• In addition to this, live vaccines generally do not require
adjuvants and booster immunization.
Live vaccines
However, live vaccines suffer from various disadvantages:
• In live attenuated vaccines there are chances of reversal of virulence
(attenuated organism again becoming virulent).
• Moreover, live vaccines require cold chain maintenance to remain
efficacious.
• Because of the chance of killing of live organism during storage
usually the shelf life of live vaccines are less and depends largely on
the storage conditions.
Types of Live vaccines
• This group includes four types of organism;
a) Attenuated live organism,
b) Naturally existing non pathogenic strains,
c) Pathogenic organism in heterologous host,
and
d) Pathogenic organism in homologous host but through
unnatural route.
a) Attenuated live organisms:
• Attenuation: Attenuation is a process of reducing virulence.
• Traditionally, process of attenuation involves adapting organism to
grow in heterologous host or under unusual condition so that they
lost their adaptation to their natural host.
• Bacillus Calmette Gurein (BCG) strain of Mycobacterium bovis was
rendered avirulent by being grown for 13 years on bile saturated
medium.
• Louis Pasteur attenuated Anthrax bacilli by growing it at 420C.
• Fowl Cholera organism got attenuated by the process of aging.
• Brucella abortus strain 19 was grown under conditions in which
there was a shortage of nutrients
a) Attenuated live organisms:
• Viruses have traditionally been attenuated by growth in cells or species to
which they are not naturally adapted.
• For example RP virus, which is normally a pathogen of cattle, was first
attenuated by growth in rabbits.
• Eventually, a tissue culture adapted RP vaccine was developed.
• Alternatively, mammalian viruses may be attenuated by growth in eggs.
Flury strain of rabies was attenuated by prolonged passage in egg and lost
its virulence for normal cats and dogs.
• The most commonly used method of virus attenuation has been prolonged
tissue culture.
b) Natural non–pathogenic strains:
• Some viruses have naturally occurring non pathogenic strains that can be
used for the purpose of vaccination.
• Lentogenic or mesogenic strains of Newcastle disease virus is used for
the purpose of vaccination whereas velogenic strain is highly virulent.
• Lentogenic : F , B1, Lasota
• Mesogenic: R2B
• Herpes Virus of Turkey (HVT) is used for immunization against Marek’s disease.
c) Pathogenic organism in heterologous host
• However, sometimes even antigenically
related pathogenic organism can act like
a vaccine when inoculated in
heterologous hosts.
• For example:
- Measles virus has been used to protect
against Canine distemper, and
- BVD virus can protect swine against
hog cholera.
d) Pathogenic organism in homologous host
through unnatural route.
• Sometimes fully virulent virus can be
used for the purpose of immunization in
the homologous host by inoculating
through non natural route.
• Contagious Ecthyma (ORF) is a viral
disease of lambs that cause massive scab
formation around the mouth.
• Lambs are vaccinated by rubbing dried
infected scab material into scratches
made in the inner aspect of thigh.
• ILT virus given through cloacal route
gives
protection from
infectious
Laryngotracheitis infection in poultry.
Killed or Inactivated vaccines:
• Killed or Inactivated vaccines are usually
safe (as the chance of reversal of virulence is
negligible) and can be easily stored.
• Agents like
- Formaldehyde,
- Ethylene oxide,
- Ethyeneimine and
- β-propiolactone (BPL)
are commonly used as inactivating
agents.
• These agents do not cause much alteration to
the protein structure and thus maintains
antigenicity to a larger extent.
Killed or Inactivated vaccines:
• But, generally killed vaccines are of low
antigenicity and thus induce less
efficacious immunity.
• To overcome this limitation, adjuvants are
added to enhance immune response to
killed vaccines.
• Killed vaccines also induce short term
immunity and thus multiple injections
(boosters) are required to ensure long term
protection.
Live Vaccine Vs Killed Vaccine
Features
Attenuated vaccines
Inactivated whole cell vaccines
Long Lasting
Short lived
Both humoral and Cellular
Humoral
No
Yes
Usually single
Multiple
Parenteral or mucosal
Mostly parenteral
Reversion to virulence
Possible
No
Adventitious contaminants
Possible
No
Stability
Requirement of cold chain
Less
Cold chain is required
More
Cold chain not required
Not recommended
Can be given
Quantity of antigen required
Less
More
Cost of production
Less
More
Duration of Immunity
Types of Immune response
Adjuvant required
Number of Inoculation (requirement
of boosters)
Route of Inoculation
Administration in Pregnant or
immuno-compromised individuals
Toxoid:
• The pathogenic effects of some bacteria are because of exotoxins
produced by them.
• The toxic effect of exotoxins can be reduced by treating them
with 0.5 % formaldehyde while they retain their immunogenicity
and thus can be used for the purpose of Immunization.
• These formaldehyde treated exotoxins are called “Toxoid”.
• A number of toxoid vaccines are used in Veterinary. For eg:
Tetanus toxoid vaccine
• (DPT in Human: An example of Toxoid vaccine)
Sub-unit vaccines:
• Sub-unit vaccines are made up of components
of the virulent organism that are capable of
inducing protective immune response rather
than whole virulent organism.
• The concept of sub unit vaccine is based on
the fact that immune response generated
against whole array of antigen from pathogen
does not contribute equally in protection.
Sub-unit vaccines:
• Some of the antigenic components plays major role in inducing protection
while others do not.
• Thus, it is possible to use only those components of the pathogen that
induces protective immune response instead of using whole organism.
• For example, antibody response generated against Haemagglutinin (H) and /
or Neuraminidase (N) antigen of Influenza virus is protective.
• Thus, sub unit vaccines against Influenza virus contain only H and N
antigens.
• Principally, toxoid vaccines are also a type of subunit vaccine which contains
only detoxified exotoxin not the whole organism.
Modern Vaccine Technology
Modern Vaccine Technology
• It is always a need to make vaccines more effective, cheaper and
safer.
• Thus scientist fraternity keeps trying to develop vaccines which are
more efficacious, cheaper and with negligible safety concerns (side
effects).
• Modern molecular biological techniques are used for the
development of a number of new generation vaccines like:
a) Subunit vaccines,
b)Genetically attenuated organism as vaccines,
c) Live recombinant organisms as vaccines,
d) DNA vaccines
e) Marker vaccines
f) Edible vaccines
Subunit vaccines:
• It is possible to precisely cut any “gene of interest” from an
organism and express that gene in another host.
• For example, by the help of recombinant DNA technology, the
VP 1 gene of FMD virus was cut, cloned into a plasmid and
inserted into bacterium Escherichia coli.
• This leads into the production of VP 1 protein in E. coli. The
VP 1 protein was harvested, purified, adjuvanted and used for
vaccination against FMD.
Genetically attenuated Organism:
• Using molecular techniques, it is possible to delete virulence gene of
the organism.
• Virulence gene deleted organisms are capable of growth in the host
but cannot produce disease.
• These virulence gene deleted organism are good in inducing
efficacious immune response with no risk of reversal of virulence.
• eg: Pseudorabies virus (Herpesvirus) needs enzyme thymidine
kinase for their replication in non dividing cells such as neuron.
• Thymidine kinase deleted pseudorabies virus can infect nerve cells
and thus induce protective immune response.
• However, they cannot replicate in neuron and thus compromises
with the virulence of the virus.
Live recombinant organism:
• It is also possible to clone gene coding for protective antigen
from one organism into another organism which is capable of
growth in host.
• Such recombinant organisms, carrying gene of other pathogen,
will express the antigen in host along with their own genes.
• A number of viruses like poxvirus, adenovirus, herpesvirus and
bacteria like BCG, Salmonella were used for this purpose.
• These organisms can easily be administered through oral, nasal
routes or by rubbing on skin.
Live recombinant organism:
• The advantages with these recombinant organisms are many. Along
with easy administration, these organisms offer other advantages like:
- Expression of protein antigen in the host,
- its proper folding and
- post translational modifications like glycosylations.
• One such example is of Vaccinia virus recombinant which contains G
protein of Rabies virus (antibodies against G protein neutralizes rabies
virus) and thus induces protective immunity.
• This vaccine has been used widely for oral vaccination of wild animals
in bait.
Live recombinant organism:
• Similarly, Newcastle disease virus gene is cloned
in fowl pox virus.
• Recombinant vaccinia or capripox virus carrying
Haemagglutinin (H) or Fusion (F) gene of RP was
also developed.
• RP gene carrying recombinant capripoxvirus gives
protection against both RP as well as lumpy skin
disease.
DNA vaccine:
• Cloning the “Gene of Interest” (gene coding for protective antigen) in
plasmid vector under strong promoter and introduction of such plasmids
directly into host tissue can generate immune response against the gene of
interest.
• These plasmids can be delivered into the host cells by various techniques.
• The simplest one is by injecting them intramuscularly. Better delivery of
these plasmids to intracellular locations can be achieved by shooting plasmids
adsorbed on gold nanoparticles using “Gene Gun”.
• In the host cells, the gene of interest is transcribed, translated, processed and
presented to cell of immune system.
• Such vaccines are called “DNA vaccine”.
• DNA vaccine against West Nile virus infection has been used successfully in
horses.
Marker vaccines:
• Using molecular biological tools, immunogenic but non protective gene
from virulent organism can be deleted.
• The advantage of such vaccines is that immune response generated in
vaccinated animals differs from animals naturally infected with the
organism.
• To differentiate vaccinated animals from naturally infected one, a test
should be developed (for example: ELISA) which specifically detect
immune response against the gene deleted in marker vaccine.
• As expected, the test will be positive in naturally infected animals
where as negative in vaccinated animals.
• Marker vaccines are also called as DIVA vaccines (Differentiation of
Infected from Vaccinated Animals).
Edible vaccine:
• It is also possible to clone gene
of interest in plants like
tobacco, potato and corn.
• The genes of Transmissible
gastro-enteritis and Newcastle
disease, coding for protective
antigen, were cloned in plants
and these plant products were
used for vaccination.
• In US, tobacco based Newcastle
disease vaccine has been
licensed.
ADJUVANTS
ADJUVANTS
• Adjuvants are substance when administered along with antigen
enhances the magnitude of immune response to that antigen.
• In general, inactivated or killed vaccines are poorly immunogenic.
• Thus, these vaccine formulations include adjuvants so that they
could mount a better and efficacious immune response.
• Addition of adjuvants offer several advantages such as
a) lesser amount of antigen is required,
b) Induces better or heightened immune response,
c) reduces the number of doses required,
d) Induces long term immunity.
ADJUVANTS: Mechanism of action
• The mechanism through which adjuvants exert their effect is
not well established.
• However, better antigen presentation by activation of antigen
presenting cells (APCs) and antigen depot formation are the
two most plausible explanations.
• The adjuvant that forms antigen depot is thought to slow down
the degradation of antigen and mediates sustained release of
antigen.
• This led to the prolonged stimuli for the immune system and
thus resulting into a heightened immune response.
Commonly used ADJUVANTS
• The most commonly used adjuvants include aluminum salts
such as:
- Aluminum hydroxide gel,
- Aluminum phosphate and
- Aluminum potassium sulphate (alum) as well as
- Calcium sulphate.
• The main disadvantages associated with Aluminum based
adjuvants are that they augment (promote) humoral immune
response but have very less stimulatory effect on cell mediated
immunity.
Water in oil emulsions
• Water in oil emulsions are other important means of inducing
better immune response.
• Mixing of antigen with mineral or vegetable oil leads to
formation of “water in oil emulsions” which creates antigen
depot upon inoculation.
• This ensures better immune response to the antigen due to
depot effect.
• Moreover, local tissue irritation produced by oil also activates
antigen presenting cells and thus increases the antigen
presentation.
Water in oil emulsions
• Two most important examples of “water in oil” based adjuvants are
- Freund’s complete adjuvant (FCA) and
- Freud’s incomplete adjuvant (FIA).
• The composition of both the adjuvants is same except for the fact that
FIA does not contain the Mycobacterial component which is present in
FCA.
• FCA is considered to be the most potent adjuvant which is used for
purpose of primary vaccination whereas FIA is used for boosters under
mainly experimental circumstances.
• FCA is not used in cattle and other species because its use may leads to
false positive reaction on tuberculin testing.
• Moreover, oil based adjuvants also spoil the quality of meat.
ADJUVANTS
• Generally, in comparison to soluble antigens particulate
antigens are better taken up by the cells of Immune system and
thus induce better immune response.
• Thus, a number of newer adjuvants formulations like Immune
stimulating complexes (ISCOMS), Liposomes are also tried
with success.
• ISCOMs are stable complexes containing
phospholipid, saponin (Quil A) and antigens.
cholesterol,
• Liposomes are lipid based synthetic microparticles
encapsulating antigens. These biodegradable micro-particles
are readily taken up by the phagocytes.
ADJUVANTS
• A number of chemical compounds of microbial origin are
identified which can be used as adjuvants in near future.
• Most notable examples are LPS, Monophosphoryl Lipid A,
flagellin, CpG containing DNA etc.
• These microbial products are reported to act as TLR agonists
and thus they exhibit their adjuvancity by activating APCs.
• Some plant products have also shown immense promise for
their use as adjuvants. One noteworthy example is Saponin
(from bark of Quillaza saponaria).
QUALITY CONTROL OF VACCINES
• As vaccination is perceived as a means of protection, it should itself
not pose any threat to the animals health.
• Therefore, vaccination of animals with high quality vaccines is in
the forefront of any Immunization programme.
• The quality of a vaccine should be tested on various parameters like:
- potency,
- efficacy,
- safety,
- sterility,
- purity etc.
• Quality vaccines should be potent and efficacious, safe and sterile,
pure and provide durable or long term immunity.
Vaccine quality parameters
• Potency: of a vaccine is its relative strength to generate
protective immune response in suitable laboratory animals and
the host species.
• For live vaccines, bacterial or viral count, at any time point
prior to expiry date, should be sufficiently greater than the live
count shown to be protective (efficacious).
• Efficacy: of a vaccine is the quality that confers solid
protection to the host against any challenge (experimental or
natural) by the pathogens.
Vaccine quality parameters
• Safety: safety of a vaccine indicates the absence of associated
risks in the inoculated individuals.
• Purity: Purity of a vaccine indicates the lack of contaminants
in the vaccine formulation i.e., it must contain only the
protective agents and no other organisms. Each batch of
vaccine should be tested for presence of any extraneous
viruses, bacteria, mycoplasma or fungi.
• Sterility: Sterility of a vaccine means lack of residual
virulence in inactivated vaccines and lack of reversion of
attenuated organism to virulent forms in live vaccines. The
term is often confused with “purity” of vaccine.
•
THE END