Download Different Types of Vaccine Whole virus vaccines. either live or killed

Different Types of Vaccine
Whole virus vaccines. either live or killed, constitute the vast majority of vaccines in use at present.
However, recent advances in molecular biology had provided alternative methods for producing
vaccines. Listed below are the possibilities;1.
2.
3.
4.
5.
6.
Live whole virus vaccines
Killed whole virus vaccines
Subunit vaccines;- purified or recombinant viral antigen
Recombinant virus vaccines
Anti-idiotype antibodies
DNA vaccines
1. Live Vaccines
Live virus vaccines are prepared from attenuated strains that are 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, Primary vaccine
failures are uncommon and are usually the result of inadequate storage or administration. Another
possibility is interference by related viruses as is suspected in the case of oral polio vaccine in
developing countries. Several methods have been used to attenuate viruses for vaccine production.
Use of a related virus from another animal - the earliest example was the use of cowpox to
prevent smallpox. The origin of the vaccinia viruses used for production is uncertain.
Administration of pathogenic or partially attenuated virus by an unnatural route - the
virulence of the virus is often reduced when administered by an unnatural route. This principle is
used in the immunization of military recruits against adult respiratory distress syndrome using
enterically coated live adenovirus type 4, 7 and (21).
Passage of the virus in an "unnatural host" or host cell - the major vaccines used in man and
animals have all been derived this way. After repeated passages, the virus is administered to the
natural host. The initial passages are made in healthy animals or in primary cell cultures. There are
several examples of this approach: the 17D strain of yellow fever was developed by passage in mice
and then in chick embryos. Polioviruses were passaged in monkey kidney cells and measles in
chick embryo fibroblasts. Human diploid cells are now widely used such as the WI-38 and MRC-5.
The molecular basis for host range mutation is now beginning to be understood.
Development of temperature sensitive mutants - this method may be used in conjunction with the
above method.
2. Inactivated whole virus vaccines
These were the easiest preparations to use. The preparation was simply inactivated. The outer virion
coat should be left intact but the replicative function should be destroyed. To be effective, nonreplicating virus vaccines must contain much more antigen than live vaccines that are able to
replicate in the host. Preparation of killed vaccines may take the route of heat or chemicals. The
chemicals used include formaldehyde or beta- propiolactone. The traditional agent for inactivation
of the virus is formalin. Excessive treatment can destroy immunogenicity whereas insufficient
treatment can leave infectious virus capable of causing disease. Soon after the introduction of
inactivated polio vaccine, there was an outbreak of paralytic poliomyelitis in the USA use to the
distribution of inadequately inactivated polio vaccine. This incident led to a review of the formalin
inactivation procedure and other inactivating agents are now available, such as Beta-propiolactone.
Another problem was that SV40 was occasionally found as a contaminant and there were fears of
the potential oncogenic nature of the virus.
Live vs Dead vaccines
Feature
Live
Dead
Dose low high
no. of doses
need for adjuvant
Duration of immunity
single
no
many years
multiple
yes
less
antibody response
IgG,
IgA IgG
CMI
good
poor
Reversion to virulence
possible not
possible
Because live vaccines replicate inside host cells, bits of virus antigen are presented to the cell
surface and recognized by cytotoxic cells
Potential safety problems
Live vaccines
1.
2.
3.
4.
5.
6.
Underattenuation
Mutation leading to reversion to virulence
Preparation instability
Contaminating viruses in cultured cells
Heat lability
Should not be given to immunocompromized or pregnant patients
Killed vaccines
1. Incomplete inactivation
2. Increased risk of allergic reactions due to large amounts of antigen involved
Present problems with vaccine development include
1. Failure to grow large amounts of organisms in laboratory
2. Crude antigen preparations often give poor protection. eg. Key antigen not identified,
ignorance of the nature of the protective or the protective immune response.
3. Live vaccines of certain viruses can (1). induce reactivation, (2) be oncogenic in nature
3._Subunit Vaccines
Originally, non-replicating vaccines were derived from crude preparations of virus from animal
tissues. As the technology for growing viruses to high titres in cell cultures advanced, it became
practicable to purify virus and viral antigens. It is now possible to identify the peptide sites
encompassing the major antigenic sites of viral antigens, from which highly purified subunit
vaccines can be produced. Increasing purification may lead to loss of immunogenicity, and this may
necessitate coupling to an immunogenic carrier protein or adjuvant, such as an aluminum salt.
Examples of purified subunit vaccines include the HA vaccines for influenza A and B, and HBsAg
derived from the plasma of carriers.
4. Recombinant viral proteins
Virus proteins have been expressed in bacteria, yeast, mammalian cells, and viruses. E. Coli cells
were first to be used for this purpose but the expressed proteins were not glycosylated, which was a
major drawback since many of the immunogenic proteins of viruses such as the envelope
glycoproteins, were glycosylated. Nevertheless, in many instances, it was demonstrated that the
non-glycosylated protein backbone was just as immunogenic. Recombinant hepatitis B vaccine is
the only recombinant vaccine licensed at present.
An alternative application of recombinant DNA technology is the production of hybrid virus
vaccines. The best known example is vaccinia; the DNA sequence coding for the foreign gene is
inserted into the plasmid vector along with a vaccinia virus promoter and vaccinia thymidine kinase
sequences. The resultant recombination vector is then introduced into cells infected with vaccinia
virus to generate a virus that expresses the foreign gene. The recombinant virus vaccine can then
multiply in infected cells and produce the antigens of a wide range of viruses. The genes of several
viruses can be inserted, so the potential exists for producing polyvalent live vaccines. HBsAg,
rabies, HSV and other viruses have been expressed in vaccinia.
Hybrid virus vaccines are stable and stimulate both cellular and humoral immunity. They are
relatively cheap and simple to produce. Being live vaccines, smaller quantities are required for
immunization. As yet, there are no accepted laboratory markers of attenuation or virulence of
vaccinia virus for man. Alterations in the genome of vaccinia virus during the selection of
recombinant may alter the virulence of the virus. The use of vaccinia also carries the risk of adverse
reactions associated with the vaccine and the virus may spread to susceptible contacts. At present,
efforts are being made to attenuate vaccinia virus further and the possibility of using other
recombinant vectors is being explored, such as attenuated poliovirus and adenovirus.
5. Synthetic Peptides
The development of synthetic peptides that might be useful as vaccines depends on the
identification of immunogenic sites. Several methods have been used. The best known example is
foot and mouth disease, where protection was achieved by immunizing animals with a linear
sequence of 20 aminoacids. Synthetic peptide vaccines would have many advantages. Their
antigens are precisely defined and free from unnecessary components which may be associated with
side effects. They are stable and relatively cheap to manufacture. Furthermore, less quality
assurance is required. Changes due to natural variation of the virus can be readily accommodated,
which would be a great advantage for unstable viruses such as influenza.
Synthetic peptides do not readily stimulate T cells. It was generally assumed 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 cell epitope, T- cell
epitopes recognized by T-helper cells. Such T-cell epitopes can be provided by carrier protein
molecules, foreign antigens. or within the synthetic peptide molecule itself.
Synthetic peptides are not applicable to all viruses. This approach did not work in the case of
polioviruses because the important antigenic sites were made up of 2 or more different viral capsid
proteins so that it was in a concise 3-D conformation.
Advantages of defined viral antigens or peptides include:
1.
2.
3.
4.
Production and quality control simpler
No NA or other viral or external proteins, therefore less toxic.
Safer in cases where viruses are oncogenic or establish a persistent infection
Feasible even if virus cannot be cultivated
Disadvantages:
1.
2.
3.
4.
May be less immunogenic than conventional inactivated whole-virus vaccines
Requires adjuvant
Requires primary course of injections followed by boosters
Fails to elicit CMI.
6. Anti-idiotype antibodies
The ability of anti-idiotype antibodies to mimic foreign antigens has led to their development as
vaccines to induce immunity against viruses, bacteria and protozoa in experimental animals. Antiidiotypes have many potential uses as viral vaccines, particularly when the antigen is difficult to
grow or hazardous. They have been used to induce immunity against a wide range of viruses,
including HBV, rabies, Newcastle disease virus and FeLV, reoviruses and polioviruses.
7. DNA vaccines
Recently, encouraging results were reported for DNA vaccines whereby DNA coding for the foreign
antigen is directly injected into the animal so that the 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. Moreover, the time for development is
relatively short which may enable timely immunization against emerging infectious diseases. In
addition, DNA vaccines can theoretically result in more long-term production of an antigenic
protein when introduced into a relatively nondividing tissue, such as muscle.
Indeed some observers have already dubbed the new technology the "third revolution" in vaccine
development—on par with Pasteur's ground-breaking work with whole organisms and the
development of subunit vaccines. The first clinical trials using injections of DNA to stimulate an
immune response against a foreign protein began for HIV in 1995. Four other clinical trials using
DNA vaccines against influenza, herpes simplex virus, T-cell lymphoma, and an additional trial for
HIV were started in 1996.
The technique that is being tested in humans involves the direct injection of plasmids - loops of
DNA that contain genes for proteins produced by the organism being targeted for immunity. Once
injected into the host's muscle tissue, the DNA is taken up by host cells, which then start expressing
the foreign protein. The protein serves as an antigen that stimulate an immune responses and
protective immunological memory.
Enthusiasm for DNA vaccination in humans is tempered by the fact that delivery of the DNA to
cells is still not optimal, particularly in larger animals. Another concern is the possibility, which
exists with all gene therapy, that the vaccine's DNA will be integrated into host chromosomes and
will turn on oncogenes or turn off tumor suppressor genes. Another potential downside is that
extended immunostimulation by the foreign antigen could in theory provoke chronic inflammation
or autoantibody production
Presentation of immunogenic proteins and peptides
Proteins separated from virus particles are generally much less immunogenic than the intact
particles. This difference in activity is usually attributed to the change in configuration of a protein
when it is released from the structural requirements of the virus particle. Many attempts have been
made to enhance the immunogenic activity of separated proteins.
Adjuvants
Used to potentiate the immune response
1. Functions to localize and slowly release antigen at or near the site of administration.
2. Functions to activate APCs to achieve effective antigen processing or presentation
Materials that have been used include;1. Aluminum salts
2. Mineral oils
3. Mycobacterial products, eg. Freud's adjuvants
Immunostimulating complexes (ISCOMS)
1.
2.
3.
4.
An alternative vaccine vehicle
The antigen is presented in an accessible, multimeric, physically well defined complex
Composed of adjuvant (Quil A) and antigen held in a cage like structure
Adjuvant is held to the antigen by lipids
5. Can stimulate CMI
6. Mean diameter 35nm
In the most successful procedure, a mixture of the plant glycoside saponin, cholesterol and
phosphatidylcholine provides a vehicle for presentation of several copies of the protein on a cagelike structure. Such a multimeric presentation mimics the natural situation of antigens on
microorganisms. These immunostimulating complexes have activities equivalent to those of the
virus particles from which the proteins are derived, thus holding out great promise for the
presentation of genetically engineered proteins.
Similar considerations apply to the presentation of peptides. It has been shown that by building the
peptide into a framework of lysine residues so that 8 copies instead of 1 copy are present, the
immune response induced was of a much greater magnitude. A novel approach involves the
presentation of the peptide in a polymeric form combined with T cell epitopes. The sequence coding
for the foot and mouth disease virus peptide was expressed as part of a fusion protein with the gene
coding for the Hepatitis B core protein. The hybrid protein, which forms spherical particles 22nm in
diameter, elicited levels of neutralizing antibodies against foot and mouth disease virus that were at
least a hundred times greater than those produced by the monomeric peptide.
Immunization and Herd Immunity
The following questions should be asked when a vaccination policy against a particular virus is
being developed.
1.
2.
3.
4.
What proportion of the population should be immunized to achieve eradication.
What is the best age to immunize?
How is this affected by birth rates and other factors
How does immunization affect the age distribution of susceptible individuals, particularly
those in age-classes most at risk of serious disease?
5. How significant are genetic, social, or spatial heterogeneities in susceptibility to infection?
6. Hoe does this affect herd immunity?
A basic concept is that of the basic rate of the infection R0. for most viral infections, R0 is the
average number of secondary cases produced by a primary case in a wholly susceptible population.
Clearly, an infection cannot maintain itself or spread if R0 is less than 1. R0 can be estimated from
as B/(A-D);B = life expectancy, A = average age at which infection is acquired, D = the
characteristic duration of maternal antibodies.
The larger the value of R0, the harder it is to eradicate the infection from the community in
question. A rough estimate of the level of immunization coverage required can be estimated in the
following manner: eradication will be achieved if the proportion immunized exceeds a critical value
pinc = 1-1/R0. Thus the larger the R0, the higher the coverage is required to eliminate the infection.
Thus the global eradication of measles, with its R0 of 10 to 20 or more, is almost sure to be more
difficult to eradicate than smallpox, with its estimated R0 of 2 to 4. Another example is rubella and
measles immunization in the US. Rubella (A = 9 years) has an Ro roughly half that of measles (A =
5 years) and indeed rubella has been effectively eradicated in the US while the incidence of measles
have declined more slowly.
Why do we not require 100% coverage to eradicate an infection? Immunization has both a direct
and indirect effect. The susceptible host population is reduced by mass immunization so that the
transmission of infection has become correspondingly less efficient and eventually, the infection
will be unable to maintain itself.
Measles
Average age Epidemic
R0
of infection period
4-5
2
15-17
Critical
coverage
92-95
Pertussis
Mumps
Rubella
Diptheria
Polio
4-5
6-7
9-10
11-14
12-15
3-4
3
3-5
4-6
3-5
15-17
10-12
7-8
5-6
5-6
92-95
90-92
85-87
80-85
80-85