PRODUCTION OF VACCINES

The aim of vaccination is to protide sufficient antigenic material to stimulate the body's immune system without causing the disease, i.e. to provoke artificial active immunity. The use of vaccines may already have been discussed as part of the Core syllabus content.

Background


People who are immune through having had the disease have natural active immunity, whilst passive immunity is due to the presence in a child of antibodies which have come from the mother (via the placenta or in her milk - natural passive immunity) or which have been injected to give rapid protection, after extraction from blood of an animal, such as a horse - artificial passive immunIty. Generally, passive immunity lasts for much less time than the active form, because the body's own immune system has not been activated.

Vaccines are of three types, containing either:

1. pathogens which have been killed by treatment with heat or chemicals such as methanal (formaldehyde) or alcohol. The antigens remain, but the pathogen is inactivated. Vaccines against cholera and diphtheria are of this type.

2. live pathogens which have been weakened (attenuated) by culturing them in unusual conditions, for example, as described for Pasteur's attenuated strains of the anthrax bacillus. There is an extremely small risk, with this type of vaccine, of a mutation occurring which might either make the vaccine ineffective or even of restoring the potency of the pathogen so that it can again cause the disease. The BCG, yellow fever and MMR vaccines fall into this category.

3. inactivated bacterial toxins, known as toxoids. For certain diseases, such as tetanus, administration of the toxoid is enough to provoke an immune response.

Manufacture of a vaccine by genetically modified organisms


A single protein on the outside of a pathogen is often sufficient to provoke an immune response. This is true of the hepatitis B virus. The gene responsible for production of one such protein which forms part of the protein coat of this virus has been identified and inserted into yeast cells. When grown in culture, the yeast cells, multiplying rapidly, manufacture the protein in large amounts. It is then separated out, purified and used as a vaccine. Since only protein from the viral coat is involved, there is no risk of the vaccine actually causing the disease.

Background


It is important for students to realise that the aim of vaccination is to protect the community, as well as individuals. If the number of hosts can be reduced to a low enough level, there may not be enough left to sustain the organism, which should then become extinct. This has happened with smallpox and, it is hoped may eventually occur with diseases such as polio.

However, rare cases where vaccination has been associated with adverse effects, such as actually causing the disease rather than protecting against it, combined with public apathy and logistical problems, mean that vaccination programmes do not always progress as rapidly as they might. When a disease becomes rare, it is perceived that the risks associated with vaccination exceed those associated with contracting the disease by infection. In the 1970s, for example, vaccination for whooping cough received extremely adverse publicity following a very few cases of serious side effects. It has always been the case, however, that the risk of death following natural infection is many thousands of times greater than that following vaccination, even in the most controversial cases. In the case of whooping cough, the incidence of the disease in England and Wales fell from well over 150 000 notified cases before the introduction of vaccination , to almost negligible levels in the early 1970s, though the number of cases subsequently rose again, following the bad public perception. It is vital that vaccination programmes continue, as a drop in the proportion of protected people can soon result in an epidemic. These issues provide a worthwhile discussion topic. The controversy which surfaced in March 1998 regarding the use of the MMR vaccine is relevant here.

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