Vaccination

There are two main types of immunity, either passive or active immunity. Passive immunity being that derived naturally from a mother. A young animal will gain antibodies from its mother either during birth via the placenta or shortly after birth when the animal consumes colostrum from its mother. It is also possible to gain passive immunity by artificial means, this involves injecting an individual with an antisera (containing specific immunoglobulins). There are advantages and disadvantages of this artificial passive immunisation, the main benefit being that the resulting immunisation is immediately effective. The disadvantages include:

• Temporary effect, lasting only until the Ig proteins are metabolised (a few weeks)
• Only has an effect with diseases where an antibody response is the principle method of protection (as opposed to cell-mediated responses)
• There is the possibility that hypersensitivity may arise from use of serums which have been obtained from foreign species
• Induction of an active acquired immunity is blocked

The other type of immunisation is active immunisation, this is actively acquired immunity derived from either a natural infection or from artificial immunisation (inoculation with a weakened or dead organism).

Active Immunisation

The primary principle behind active immunisation and the use of vaccinations is to induce an anamnestic response (an enhanced reaction to previously encountered antigens). When an individual initially encounters an antigen, the response towards that antigen is slow and not that strong, so the individual gets sick and develops symptoms associated with the disease. The second time the same individual is infected with that antigen; an anamnestic response will be invoked meaning that the immune system will act faster and with more force. Typically as a result the individual will develop mild symptoms if any at all as the antigen is removed from the body rapidly.

By inducing an initial response with a weakened form of an antigen such as a weakened virus, the individual is able to deal with the antigen much easier and as the weakened antigen is typically not pathogenic no symptoms or sickness develops (in some vaccinations there may be side effects associated). This procedure results in the individual being able to produce an anamnestic response to an antigen without having to have been exposed to the full pathogenic form.

This process is also known as immunological memory, as the immune system is able to ‘remember’ a previous antigen and so its response to that antigen when next encountered is heightened (the anamnestic response).

Vaccination

Properties of a Vaccine:

• A vaccine stimulates antigen present presenting cells
• It stimulates both T and B lymphocytes
• Helper and effector cell responses are generated to several epitopes
• However for this to be possible the vaccination antigen must remain at the appropriate site in lymphoid tissue.

Although vaccinations are able to provide the immune system with a way to develop immunological memory to a variety of diseases, there are situations when vaccinations may be undesirable:

• It may be that the infection that the vaccine has been developed for naturally doesn’t generate a good active immune response, so the vaccination is also unable to produce a decent immune response. Possible resolutions to this may be the use of purified antigens or special techniques of administration.
• It is also possible for immunity to prevent the illness but not the infection, thereby making an individual appear asymptomatic. This can be of concern because in effect, symptomless carriers of a disease are able to spread the infection.
• It is possible than the responses generated by a vaccine may interfere with serodiagnosis, which is when a diagnosis is made based on the study of the blood. If there are antibodies present due to the vaccine, it may be hard to distinguish these from other infection-induced responses.

An ideal vaccine would be one which:

• Provides the body with a prolonged and strong immunity
• Shares the develop immunity with any foetus which it may be carrying
• Free of side effects
• Cheap to make, biologically stable and easy to apply
• Produced an immunity which is distinguishable from natural infections

Live and Dead Vaccines

Vaccination can become risky due to certain factors, for example, depending on the type of vaccination used it may be possible to cause the infection which is trying to be prevented by vaccination. It may also be possible to cause other undesirable reactions so it is important to weigh the risks with the benefits.

Depending on whether an alive or dead vaccine is used can play apart in deciding whether it is appropriate to administer a vaccine. Living vaccines consist of an actual disease organism so can pose a slight danger. They have been modified to reduce pathogenicity by means of attenuation (the reduction in virulence of the organism), attenuation can be achieved by means of heat or chemical treatment, culturing over time which causes a reduction of virulence or the culturing of the organism in an unfamiliar host. It is possible for live vaccines to produce an immediate response as they act like a regular pathogen, thus inducing the release of interferons. Occasionally it is possible to use a similar pathogen to the one you are vaccinating for, which is naturally less virulent in the vaccine. For example, the cowpox virus will protect humans against the smallpox virus and it is possible for the measles virus to protect dogs against distemper.

The benefits of using a live vaccine are that the immunity developed lasts much longer, so booster vaccinations do not have to be given as often. The disadvantages as explained above are; attuned vaccines may be able to revert back to a pathogenic state or cause unwanted reactions in the host, they can become contaminated with other pathogens and they are harder to preserve as the organism must be kept alive

The alternative to using living vaccines is the use of dead ones; the immediate benefits are that they can be preserved for longer and that they are safer. However, as a result they are less immunogenic meaning booster vaccinations must be administered more often. Unlike live vaccines they can’t be administered through natural pathways. In terms of responses, antibody response is stronger than cell mediated. Dead vaccines are often accompanied with adjuvants; these are additives that increase the immunogenicity of the vaccine.

It is possible to purify the antigens of an organism for use in a vaccine instead of using the organism in its entirety. Antigens can be purified by means of chemicals, genetics or by manufacture. It is also possible to genetically attenuate organisms (e.g. by removing pathogenic genes).

Administration of Vaccines

Routes:

• Subcutaneous (applied under the skin)/epidermal
• Intramuscular (administered into a muscle)
• Intranasal (administered through the nasal passageway)
• Aerolisation (vaporisation)
• Drinking water

Adjuvants (Additives):

• Aluminium salts
• Water/oil emulsions
• Bacterial components
• Surface-active agents
• A mix of the above

Vaccination Schedules:

• Avoid use in early life
• Could vaccinate the mother to induce passive immunity
• Many vaccines do not need to be administered shorter than yearly intervals
• Some vaccines can be administered before expected/seasonal outbreaks
• Efficacy can monitored by assaying antibodies in the blood

Failures of Vaccination

Vaccination only works when administered to an individual with a working immune system, this is why new born mammals are unsuitable vaccination subjects – Their lymphoid systems are under reactive, but they also have their passive immunity which they developed from their mother. By administering a vaccine to a new-born with maternal antibodies, it is possible that the antigens in the vaccine could bind with the maternal antibodies that would prevent an active immunisation response.

It is possible for pure antigens to be poorly immunogenic (which could be due to their inability to stimulate T-helper cells). There has been a remedy created for this however, by producing an immunostimulating complex. This involves the combination of the purified antigen components, lipids and a detergent. As a result of this combination, there is a definite increase in the potency of the antigen, compared to the purified antigen alone.

Side effects as a direct response to a vaccination are very rare, however one noted example is the formation of feline post-vaccinal sarcoma. This is the formation of a malignant tumour of connective or other non-epithelial tissue at the site of injection caused by adjuvants in the vaccine.