Interactive video: Synthetic viruses

BBC medical correspondent Fergus Walsh explains how British scientists have used a new technique to develop a synthetic virus which heralds a major development in vaccines.

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    What is a virus?


    Viruses are the most common biological unit on Earth, outnumbering all other types put together. About 100 times smaller than human cells, they can be found wherever there are host organisms to infect. A virus consists of genetic material contained in a protein shell which enables them to enter a host cell and replicate, killing the cell and causing illness.

    Some of the most deadly or life-limiting diseases in the world are viral, including HIV, influenza, measles, SARS and polio. Although vaccines and anti-viral drugs have helped in combatting some of these diseases, viruses can mutate, making them difficult to eradicate.

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    A virus consists of genetic material in the form of RNA or DNA which lies within an outer shell called a capsid, which is formed from proteins. Some viruses are also contained within an "envelope" with proteins on its surface. Proteins on the capsid or the envelope help the virus bind to and attack host cells.

    Once inside the cell, the virus hijacks the host's own replication machinery, to make copies of itself. These new copies of the virus burst out of the cell, destroying it, and start to infect many more cells unless tackled by the immune system. The infection can also start to spread to other organisms through sneezing, coughing, vomiting and other methods.

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    Immune system


    The body's immune system reacts when it recognises a viral attack, sending white blood cells called lymphocytes to seek and destroy the invaders. Specialist B lymphocytes produce antibodies: proteins which attach themselves to the shell of the virus. Once the antibodies are locked on to their target, T lymphocytes are signalled to destroy the virus.

    The antibodies remain in the body so the if exposure occurs a second time, the immune system deals with it immediately. Vaccination works by exposing the body to a small amount of virus which is weakened or killed, so that it does not cause illness but does trigger the creation of antibodies.

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    Synthetic virus


    British scientists have created a synthetic version of the foot and mouth virus. But their replica viruses are empty of the genetic material which causes infection. However, as the shells are identical to the real virus, the immune system still recognises them as foreign entities and so creates antibodies against them. This means a vaccine can be made without involving live virus, reducing biosecurity risk in manufacture and thus the cost and complexity.

    As the scientists have also managed to stabilise and strengthen the synthetic shell, the resulting vaccine is much more heat-resistant. This means it does not need constant refrigeration, making it easier to transport. They are now applying similar techniques to the production of a polio vaccine.

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