New research in The Lancet medical journal shows Sars is killing one in five of those infected - a death rate of 20%, as opposed to the 6-10% estimated by the World Health Organisation (WHO).

The results are based on statistical analysis of 1,425 suspected Sars patients who were admitted to Hong Kong hospitals between February 20 and April 15.

The Lancet study, conducted by scientists from Imperial College in London, the University of Hong Kong and the Hong Kong health authorities, confirmed that the elderly are particularly vulnerable.

It estimated that the death rate could be as high as 55% in people over the age of 60, while in those under 60 the death rate could be as low as 6.8%.

Roy Anderson, professor of infectious disease epidemiology at London's Imperial College and one of the scientists leading the research, said: "That's sadly still very high for a respiratory infection. In other common respiratory infections (such as influenza) it is much less than 1% in the vulnerable elderly."

The study also found that the incubation period - the time from infection to displaying symptoms - averaged six days.

Last week the WHO warned that the Sars death rate was likely to rise, but these latest estimates are much higher than the previous 6-10% figures.

Professor Christl Donnelly, another of the scientists working on the latest study, said: "Although this study shows the fatality rate from Sars is higher than previously thought, we now know that public health interventions have been successfully reducing the spread of the disease."

Prof Donnelly said that in a growing epidemic, early death rates often underestimate the true mortality rate. "We have no evidence to show that the death rate has gone up over time. Instead we've gained more information about it and this is using the most statistical robust analysis technique to date," she said.

But WHO officials stressed that the death rate is lower in places where the outbreak has ended or is nearing an end. "We know that in a real situation where the outbreak has completed itself from beginning to end - in Hanoi, Vietnam - they had an 8% case fatality rate," said WHO spokesman Dick Thompson.

And in Canada, where patients have been older, the death rate is 15%, he said. "What we do see is that in people under 40 the death rate is generally lower and in people over 60 the numbers are much higher."

The publication of the study came as China announced five more people had died from Sars and another 159 were infected, taking the death toll in the country to 219 and the number of cases to 4,560.