Immunization is a process whereby an individuals immune system mounts an immune response after first exposure to a pathogen and develops the ability to quickly respond to a subsequent exposure to that same pathogen due to cellular response by B-cells and T-cells. Memory cells and antibodies developed as a result of the cellular immune response are responsible for this quick response. Vaccines are one way in which an individual can acquire immunity. A vaccination is defined as a suspension of live or attenuated dead pathogenic micro-organisms or their antigens administered to a host to induce an immune response thereby producing immunity against it.
Vaccination plays an important role in prevention of disease. Child killer diseases such as Measles, Poliomyelitis. Diptheria and Whopping cough are now so uncommon due to the childrenâ€™s vaccination programmes.
According to the World Health Organisation (WHO) a vaccine should be:
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Affordable for everyone
Applicable to a number of diseases
Administrable by a mucosal surface
Suitable for administration in the early stages of life.
Heat stable during storage
A vaccine should also not cause disease, have minimal side effects and should confer long term immunity to the host.
There are different types of vaccines that can be administered. And these include: attenuated live vaccines, killed viral vaccines, subunit viruses- which can be recombinant or non recombinant, DNA based, recombinant, toxoid and conjugate type of vaccines. Each abd every one of these types has advantages and disadvantages as discussed below.
1.Attenuated live vaccines contain viable pathogens that are capable of growing in the host. The pathogen is treated with chemicals such as formalin to make it lose its virulence. Polio, measles and mumps vaccines are examples of attenuated live vaccines.
This type of vaccination is capable of initiating a humoral and cellular immune system.
It results in long term immunity.
Secretory immunity can be conferred if administered via the oral mucosa route.
No adjuvant required for host to mount an immune response.
Can be administered via injection or oral route.
However because the pathogen is still viable it can cause disease in immune-compromised individuals.
If the pathogen acquires a genetic mutation that can convert it back to a virulent form that can cause disease even in healthy individuals.
Live attenuated viruses are not heat stable and should be stored in a refridgerator.
2. Killed pathogen vaccines contain killed pathogens that have been preserved in such a way as to maintain the antigenic structures. Vaccines for flu and cholera are examples of such.
These vaccines have the advantage of being safe as the pathogen is non-viable and can not cause any disease.
They are also easy to store and require no refrigeration during storage.
Can be administered to immune compromised patients
Induces humoral immunity.
Cheaper than live attenuated vaccines.
Because the cells are not infected with the pathogen since it is non-viable, a cellular immune response might not be mounted. Long term s thus not conferred to the host.
Booster immunizations are often required.
Killed pathogen vaccines also require the use of adjuvant to induce an inflammatory response in the host and try and activate the cellular response.
3. Subunit vaccines contain a subunit of the pathogen, which can be an antigen or protein produced by the pathogen. These can be obtained from cultures of the pathogen (non-recombinant) or can be obtained by the use of genetic recombinant DNA techniques (recombinant).
This type of vaccination is safe
Can cause the host to mount a humoral response.
However it does not effectively induce cellular immunity. Therefore does not result in long term immunity.
This mode of vaccination does not mimic the natural route of infections and hence response to the pathogen might be totally different.
4. DNA based vaccines
In this type of vaccine DNA coding for pathogenic proteins are injected intramuscularly into host. The host cells takes up the DNA and codes for mRNA production and produces the protein which is expressed on the cells eliciting an immune response.
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Since the protein is endogenously synthesized it is fully capable of inducing an immune response.
It is safe and will not result in infection even in immune compromised patients
It confers long term immunity.
Use of adjuvant is not necessary.
In this type of vaccine a gene coding for an antigen of a virulent pathogen is incorporated by recombinant genetic technology into a non-pathogenic vector that might not even be related to the original pathogen. When the vector is administered to the host an immune response against the antigen of the virulent pathogen is mounted.
Can result in humoral and cellular immunity.
Secretory immunity can be achieved if the vaccine is administered through the right route.
Results in long term immunity
Can cause disease in immune compromised patients.
If the vector acquires a genetic mutation it can also become pathogenic.
6. Toxoid vaccines
These are vaccines that contain inactivated toxins that are still capable of inducing an immune response without causing infection. These are most commonly used in prevention of tetanus, diptheria and botulism.
Capable of inducing a humoral immune response by production of an anti-toxin.
Most bacteria have their antigens surrounded by a polysaccharide layer and this makes it hard for an immature immune system to recognize them. In conjugate vaccines the antigens are coupled to an antigen or toxoid that the young immune system can recognize.
Can confer humoral immunity.