How vaccines are made

Mass manufacture of poliovirus vaccine started in the 1950s

Many people would be surprised at the animal-based ingredients scientists must use to mass-produce vaccines.

The new scare over poliovirus vaccines made using calf foetuses could draw attention to what goes on behind the scenes.

Vaccines have undoubtedly been one of the greatest triumphs of medicine, pushing back feared diseases like smallpox and polio, and threatening to do the same to killers such as meningitis.

Patients see a clear liquid in a syringe - but back in the laboratory a wide variety of complex processes are used to make it, many incorporating cells or body chemicals which originate from the animal kingdom.

But scientists are adamant that a long history of using such materials has produced no health problems as a result.

There are two ways in which animal material is used to make vaccines.

The first is the use of animal cells as "nurseries" for the modified viruses doctors want to use as the main ingredient of a vaccine.

Chicks and monkeys

The scientist has to find a cell in which the virus in question will replicate to produce more copies of itself.

Scientists have "cell lines" - immortal cells that can keep on dividing indefinitely, derived originally from a variety of sources, including monkeys, hamsters, or from human foetuses.

Bovine material is used in vaccine manufacture

Some viruses - such as the flu virus - can be cultured within chick foetus cells.

However, this active process may need a little encouragement, such as supplies of glucose for energy, and other chemicals.

The cells are bathed in a "soup" made up of these ingredients, and frequently include other organic chemicals such as growth factors, which can help the cells to develop.

Although human growth factors can be extracted, these do not provide as reliable results as other factors, such as foetal calf serum, which is widely used.

It is this chemical which is at the centre of the current scare over poliovirus vaccine.

'Bovril for cells'

Dr Deborah Scopes, a spokesman for the British Society of Immunology, said: "This is like Bovril for cells.

"It's a big broth which helps growth."

Once sufficient numbers of the virus have been replicated, the manufacturers use complex filtration and purification processes to try to remove everything but the viruses from the vaccine.

Dr Scopes said: "The quality control mechanisms are amazing. Basically it is a bad idea to have anything but the virus itself in the vaccine.

"You don't want to produce an immune response to cow tissue - you want to produce the biggest immune response possible to the viruses, to make the vaccine effective."

The problems with one brand of poliovirus stem from foetal calf serum which has been sourced in the UK.

The theoretical risk is that if the serum was drawn from a calf foetus carrying the proteins thought to be the infective agent of BSE, some of those proteins might remain in a vaccine given orally to children.

The purification process used to remove everything but the viruses renders this even less likely.