In recent years a completely new type of medicine has become available for doctors to use in treating patients with a wide variety of problems. Recombinant medicines are made by genetically modified organisms which act as biological factories, producing the chemicals we need. The great advantage of using genetically modified organisms to produce medicines is that large volumes of the chemical can be made relatively cheaply.

One of the best known success stories is the use of bacteria to produce human insulin, which has revolutionised the supply of pure, human insulin to people suffering from diabetes everywhere. Not only has it removed the need for people to inject themselves with insulin from pigs and cows, it has also made sure that the medicine is never in short supply.

Genetically engineered microorganisms are the main source of recombinant medicines, but mammals such as sheep and goats have also been engineered to produce some of the bigger and more complex protein molecules needed by some people to overcome their diseases.

One of the most recent developments is the production of a vaccine against hepatitis B using genetically modified yeast cells.

Hepatitis B is a viral infection which attacks the cells of the liver. It can be very serious, causing chronic liver failure, liver cancer and death. Hepatitis B can be prevented by a vaccination, and in countries like New Zealand where it is relatively common, children are vaccinated against Hepatitis B while they are still at school.

For many years the vaccine was produced by growing the live virus in animals and then inactivating it by chemical treatment. This led to the risk of infection during the manufacture, and in the delivery of the vaccine, as well as raising animal rights issues.

The part of the hepatitis B vaccine which actually triggers the immune response is a specific protein usually found on the surface of the hepatitis B virus. The protein alone is enough to give immunity to the disease.

A strain of yeast has been genetically modified so that when it is cultured it makes copies of the hepatitis B protein. The yeast can be grown in big fermentation tanks in a very similar way to that used for making beer. The big difference is that during the fermentation process the genetically modified yeast produces large quantities of hepatitis B protein. The protein is extracted from the yeast culture, purified and used to make a very safe and relatively cheap vaccine.