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Last Updated: Tuesday, 22 April, 2003, 16:18 GMT 17:18 UK
Sars virus 'mutating rapidly'
Coronavirus: Sars culprit, say scientists
The virus thought to cause Sars is constantly changing form, say scientists - which will make developing a vaccine difficult.

The Beijing Genomics Institute reported that the virus is "expected to mutate very fast and very easily".

Other experts have warned that, once established, it could be particularly hard to stop the Sars virus causing problems.

Sars appears to be caused by a new strain of a coronavirus which may have "jumped" from animals to humans in the southern Chinese province of Guangdong.

So far the strain has killed more than 200 people, mainly in China, Hong Kong, Canada and Singapore.

The number of new cases in China continues to rise, with the authorities admitting over the weekend that the virus is more widespread than previously acknowledged.

Teams of inspectors are now being sent into remote regions to aid prevention efforts.

Chinese authorities are installing thermal imaging equipment to check the temperatures of travellers moving across the southern border between Shenzhen and Hong Kong.

Officials have also announced plans to crackdown on profiteering in areas hit by the virus. It follows reports of sharp rises in the price of medical drugs and equipment, herbal remedies and basic foods in some regions.

You only get one chance to eradicate something like this - once it's established, you've got a real problem
Dr Adrian Mockett, vaccine specialist

There have now been almost 4,000 probable cases of Sars worldwide, according to the World Health Organization (WHO).

Scientific teams are racing to produce a vaccine against the new strain, but have warned that this may take years. Experts say that a vaccine may only offer limited - and temporary protection.

Other strains of coronavirus can cause "common cold"-like infections in humans.

They are also a significant cause of illness in various types of animals, particularly poultry.

Dangerous changes

Every virus is capable of mutating. Although the virus contains a large amount of genetic information, every time it is "replicated" inside a cell, tiny genetic "mistakes" are made.

Some of these may harm the success of a virus, leave it unchanged - or make it better at infecting and replicating in humans.

Hong Kong: 99
China: 97
Canada: 14
Singapore: 16
Vietnam: 5
Thailand: 2
Malaysia: 2
Natural selection means that "mistakes" that end up benefiting the virus will lead to the creation of strains that are more virulent, or more easily transmitted from human to human.

Dr Adrian Mockett, who has helped developed coronavirus vaccines for use in veterinary medicine, told BBC News Online the virus had particular characteristics that could prove a problem in humans.

"The ability of the virus to mutate has been a real problem in poultry vaccines.

"The virus has the ability to change quite quickly - a vaccine might be suitable for a while, but not forever."

Water problem

He said that because it was likely the new strain of coronavirus had only just "jumped" to humans, newer versions better suited to living in humans were possible.

He said that other coronaviruses in animals had mutated so that the infection could be spread not only through coughs and sneezes, but also through faeces - raising the possibility that a future outbreak could be transmitted through tainted water supplies or contaminated food.

Scientists believe that the current strain is transmitted through droplets coughed out of the lungs, but are still not certain about other possible routes of transmission.

Even if a vaccine works at first, said Dr Mockett, he said that the "duration of immunity" had yet to be determined.

He added: "You only get one chance to eradicate something like this - once it's established, you've got a real problem."

The characteristics of coronavirus and the way it infected humans meant that future vaccines were likely to work in the same way as flu vaccines - with different components needed to tackle a variety of common strains.

However, he said that if different strains evolved in north America and the far East, a jab that worked in one place would offer no protection in the other.

WHO defence

Meanwhile, the WHO has dismissed criticism that it has exaggerated the threat posed by Sars.

Last month it took the unusual decision to issue a worldwide warning about the disease.

Spokesman Dick Thompson, speaking to the BBC on Tuesday, said: "The personal risk to any particular individual is minimal, but the risk to the public health system from this disease is enormous.

"We needed people to be aware of the signs and symptoms of this disease. We did not want them to walk into a health clinic and not alert physicians that they had travelled from one of the outbreak sites."

Adam Brookes reports from Beijing
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