Analyzing what it said was more complete data, the World Health Organization (WHO) said Wednesday that the estimated overall death rate from SARS was about 15 percent -- double its previous estimate.
The new rate is still lower than the nearly 20 percent reported Tuesday by researchers who studied the course of the epidemic of severe acute respiratory syndrome in Hong Kong. Those scientists used different statistical techniques.
Even so, the revised death rate indicates that as new information from the evolving epidemic comes in, SARS is proving much more serious than first thought. Early estimates of the death rate had ranged down to about 2 percent.
The health agency said its new figures were based on the latest detailed data from Canada, China, Hong Kong, Singapore and Vietnam. The death rates varied by age: more than 50 percent in people 65 and older; 15 percent in those 45 to 64; 6 percent among those 25 to 44; and 1 percent for those 24 and younger.
The new mortality rates, like the old ones, are tentative because the epidemic is thought likely to change as more information comes in.
The WHO did not report the higher figures earlier partly because it did not receive the needed data until recent days, said Dr. Michael Ryan, a medical officer at the agency. He added that the WHO also did not want to "panic the world" by releasing death rates that could turn out to be significant overestimates.
Calculating death rates for new diseases like SARS is notoriously difficult for epidemiologists, particularly if there is no definitive diagnostic test, as is the case with SARS. The final death rate will not be known until the epidemic has run its course. Moreover, death rates can vary from one set of patients to another, depending on factors like the amount of virus transmitted, where it entered the body, and the functioning of an individual's immune system.
Another statistic that has been hard to calculate for SARS is the incubation period -- the interval from the time of exposure to the onset of symptoms -- because individuals may have been exposed to the virus on a number of occasions.
On Tuesday, the same study that found a higher death rate in Hong Kong suggested that the incubation period for SARS might be as long as 14 days. But that analysis, published in the medical journal Lancet, was based on only 57 cases among a much larger group.
On Wednesday, the WHO stood by its 10-day estimate. That figure was determined by reviewing cases in which the patient had a single documented exposure to a known SARS patient in Canada, Europe or Singapore, the agency said.
The incubation period has major implications for control of the epidemic.
A 14-day period could mean longer isolation periods for patients and longer quarantines for people who have had contact with them. It could require health workers to trace four more days' worth of patient contacts. And it could lengthen by eight days, to 28, the period health officials use to declare an area free of SARS.
Ryan said the Lancet study held important additional information that needs to be analyzed further.