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.
There have now been almost 4,000 probable cases of Sars worldwide,
according to the World Health Organization (WHO).
"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
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.
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."
BBC
News
Last Updated: Tuesday, 22 April, 2003
16:18 GMT
The
Beijing Genomics Institute reported that the virus is "expected
to mutate very fast and very easily". 
