Vaccination is the introduction into the host of antigens from a disease organism in the expectation that the organism will produce a primary immune response and develop the ability to immediately initiate a secondary immune response to any subsequent contact with the antigen. The secondary immune response with its massive release of IgG type antibodies will prevent the development of disease symptoms when the infection occurs. The vaccine may be given orally or by injection depending on the best antibody production response. Vaccination is a form of artificial active immunity.

Types of vaccine

The antigen introduced to the host can be one of a whole range of types depending upon the disease organism concerned.

‘Killed’ organisms: Dead organisms are introduced to stimulate the immune system. The killed description is inapt since viruses aren’t alive anyway! [Influenza,Whooping Cough, Typhus, Cholera]

Living attenuated organisms: The virus or bacterium is damaged in some way, for example culturing at very high temperatures, to cause it to grow slowly or fail to reproduce effectively. The host can then produce antibodies against the organism without the disease developing.[Mumps, Rubella, Poliomyelitis, Yellow fever, Tuberculosis]

Living non-virulent organisms: There are often a range of different strains of organism or even different species which when introduced into the host will provoke the development of antibodies which are also effective against the virulent strains or species. [Typhus, Smallpox, Rubella]

Toxoid: The toxins responsible for the disease effects may be altered slightly, for example by treatment with formaldehyde. The modified toxin will not produce disease symptoms in the host but will stimulate antibody production against the toxoid which also work against the original toxins. [Tetanus, Diphtheria]

Extracted antigen: Only a part of the organism is used for example the polysaccharide coat of typhoid bacteria. [Typhoid]

Genetically engineered antigens: Since antigens are often proteins or protein based then by formulating their DNA code and introducing this into bacteria we can manufacture large quantities of the specific antigen required. [Cholera, Typhoid, Hepatitis B]

The type of vaccine used against a disease depends upon a number of factors, such as can the organism be cultured in large quantities in vitro? If not, then making an attenuated or nonvirulent vaccine is impossible. The earliest vaccine was the use of cowpox virus to stimulate immunity to smallpox virus. The two viruses are extremely similar, one was fatal in humans but the cowpox wasn’t but did stimulate the development of antibodies effective against smallpox.

The immunisation programme in the UK is as follows:

The idea of vaccination is to stop the spread of the diseases by reducing the reservoir of infected individuals in the population. If the numbers of infected people is relatively small then transmission is relatively easy to control. However once a threshold level of infection occurs in the population the rate of transmission rapidly increases to give rise to an epidemic which becomes difficult for the Health Authorities to cope with. By reducing the possible host reservoir in the general population by immunising large numbers of individuals before they contact the disease it becomes less likely that epidemic spread can occur.

Why can’t we vaccinate against all diseases?

The main problem with disease causing organisms is that they exist only to create more of themselves, they are genetically programmed to survive if at all possible. Just as we mutate so do they and the more frequently they do this then the more likely it becomes that new strains will evolve with altered virulence genes or with altered antigens, The HIV virus mutates frequently as does the influenza virus thus a vaccine prepared for these diseases would have to have all possible strains of antigen and then still wouldn’t work if a new strain of virus appears. Unfortunately, it would appear that some disease causing organisms provoke antibody production, but the antibodies are ineffective. Even vaccination is only partially successful in developing true resistance to a disease. Rubella vaccine is 60% successful, which really means 40 out of every 100 people who think they are immune to the disease after being vaccinated just aren’t.

Dangers of Vaccination:

The process of vaccination has it’s attached problems. Some vaccines, especially those made from organisms cultured in egg yolk can induce an anaphylactic shock response in the brains of young infants who are allergic to certain components of egg yolk. This can result in death, or serious brain and sensory damage. Although the chances of such a catastrophe are very small it is still a risk which is rarely discussed with parents before the vaccination occurs. Parents must decide whether the chances of being damaged by the vaccination are greater than that of damage from the disease itself and very few of us have much access to the kind of data needed for a truly objective decision.