Sars has spread to many countries
The Sars virus could pose a threat to humans for many years to come, research suggests.
Scientists have compared samples of the Severe Acute Respiratory Syndrome virus from Singapore with samples from other countries where it has struck.
They have found that the main components of the virus have remained unchanged as infection has spread across different countries.
Scientists say the finding suggests that it is well adapted to resisting attack by the human immune system - and so does not need to evolve rapidly.
But the stability of the virus also means that any vaccine that is developed may remain effective against Sars for a long period.
Scientists have identified the virus that causes Sars as a new member of the Corona family. It has been dubbed Sars-CoV.
Usually human coronaviruses have a high rate of genetic mutation which can lead to new strains.
Researchers led by Dr Edison Liu, from Singapore's Genome Institute, studied the genetic make-up of cultured Sars viruses from five different sources.
They found only a handful of mutation differences between the samples - and it was thought these probably resulted from the virus adapting to laboratory conditions.
The virus could be more resistant than previously thought
The findings were published in an on-line report from the Lancet medical journal.
BBC science correspondent Richard Black says that if the human immune system is good at fighting a viral infection, that usually produces changes over time in the genes of that particular virus.
But that is not happening in the Sars coronavirus - suggesting our immune systems are not very effective at producing defences to it.
The World Health Organization (WHO) has welcomed the study, but says it is too early to tell whether significant mutations are occurring.
Other scientists comment that if the virus is relatively stable, that may make it easier to design a vaccine.
However, they also warn that vaccines against animal coronavirus diseases have often been unsuccessful.
In other developments:
- China reports six new Sars deaths and 118 new cases on Friday - bringing the nationwide death toll to 230 and the overall number of cases to 4,805
- Taiwan's state oil company suspends an exploration project with its Chinese counterpart because Sars is preventing its officials from travelling to China
- Russia takes additional measures to prevent to spread of Sars, including ordering airlines not to sell tickets for China and closing some border checkpoints
- The WHO's director general nominee, Jong-Wook Lee, is in China for talks on Sars with Foreign Minister Li Zhaoxing
- Authorities in Beijing will go ahead with next month's university entrance exams for 80,000 students, as officials say the number of new Sars cases is declining.
Meanwhile another study - also published on The Lancet website - concludes that much of the lung damage associated with Sars is in fact caused by the body's own response to infection.
Researchers from the University of Hong Kong found a common pattern of illness among 75 patients who were admitted to hospital following an outbreak of Sars at Amoy Garden, a Hong Kong high-rise housing estate.
For the first week after admission, symptoms gradually improved, but a deterioration set in during the second week of the hospital stay.
Some 85% of the patients developed recurrent fever after nine days.
This delayed deterioration suggests that damage is being inflicted on the lungs not by the continued spread of the virus, but by an overblown immune response.