By Pallab Ghosh
BBC Science correspondent
The Sars virus may be mutating
The first detailed study of the spread of the Sars virus in Hong Kong has discovered that the death rate among sufferers was much higher than previously estimated.
The World Health Organization (WHO) initially estimated that the death rate from the virus was 5%.
But the new study - the first to be based on a statistically sound sample of 1,425 cases - puts the death rate at one in five (20%) of those admitted to hospital.
Experts do not yet know whether there have been large numbers of Sars-infected patients who recovered in their homes without ever being admitted to hospital - which would reduce the overall death rate.
Governments need to make sure that patients are encouraged to rapidly report symptoms
One of the scientists leading the study, Professor Roy Anderson of Imperial College, London, said the findings show it is vital that new cases are identified and treated quickly in order to control the spread of the virus.
Scientists fear that the fact that the death rate from Sars appears to be higher than first thought may be a sign that the virus is mutating into more deadly forms.
However, it may just be that the WHO under-estimated the risk.
The study, based on the first nine weeks of the outbreak in Hong Kong, also shows that older people are significantly more at risk from the virus.
The death rate is more than 40% for patients aged 60 or over - but just 13% for those under 60.
Researchers found that, on average, it takes just over six days for clinical symptoms to show after a person is infected.
But, crucially, in Hong Kong the norm was for people to seek treatment between three and five days after those symptoms began to show.
The delay in seeking treatment - especially early on in the epidemic - helps explain why the disease spread so widely in the territory.
Professor Anderson said: "That period is very, very important, because once clinical symptoms become apparent the patient then is excreting or exhaling the virus and is highly infectious to others.
"Governments need to make sure that patients are encouraged to rapidly report symptoms and admit themselves to hospital to arrange for quarantine and testing to see whether they have the Sars virus."
The researchers say that efforts to control the spread of the disease have successfully slowed the spread of the epidemic.
But they warn there may now be a temptation to ease measures such as restrictions on movement - and so prolong the epidemic.
In addition, the coming warm season in Hong Kong brings an increased risk of seasonal dengue fever and influenza outbreaks, which may complicate a rapid diagnosis of Sars.
The new research is published on the website of The Lancet medical journal.