London ? The overall death rate from SARS worldwide is about 15 percent — double previous estimates — according to the first in-depth analysis by the World Health Organization.

Officials at the U.N. health agency said Thursday the calculation indicates the disease is more dangerous, particularly in the elderly, than experts had thought. More than half of SARS patients over 65 are dying.

In addition, a study to be published today suggests the SARS virus is surprisingly stable and not mutating significantly — a finding that could indicate the virus may have been in humans for a while.

Until now, WHO had estimated the death rate to be between 6 percent and 10 percent. The new estimate was based on more complete and detailed data from China, Canada, Hong Kong, Singapore and Vietnam.

A study earlier this week estimated the death rate in Hong Kong to be about 20 percent overall, but about 50 percent in patients older than 60.

“The global estimate we had previously was always clearly an underestimate,” said Mike Ryan, who is coordinating SARS response at the World Health Organization.

“We always had a figure in our heads that was a range, but now we see that it’s toward the upper end of that range,” he said.