he first major study of the genome of the SARS virus shows that it has not mutated significantly in its spread to different countries.

The findings were encouraging because if the virus remains stable chances are increased that a vaccine might be developed, the authors and other experts said yesterday. That effort is expected to take years.

But the experts said that the findings also meant that SARS, unlike some other new and emerging diseases, had not weakened as it passed through successive generations. Some experts had expressed hope that the virus would cause less severe illness as it spread.

At the same time, the World Health Organization announced yesterday that because of continuing spread, it was extending its warning against nonessential travel to Tianjin municipality and Inner Mongolia in China, and to Taipei, in Taiwan.

Also, in a report on the outbreak in Singapore, the United States Centers for Disease Control and Prevention said that five people had acted as "superspreaders" in passing the virus to 144 other people, while 81 percent of infected people did not transmit to anyone else. C.D.C. defined superspreaders as individuals who transmitted SARS, or severe acute respiratory syndrome, to 10 or more other people. Doctors have described superspreaders in other infections like tuberculosis, rubella and Ebola.

The director of the C.D.C., Dr. Julie L. Gerberding, said her agency was making available to state health departments the biological material needed to perform a diagnostic blood test. The test detects the antibodies that the immune system forms as it fights off infections; the immune system produces a specific antibody against each microbe.

C.D.C. researchers have developed a test like the Elisa, which is used to screen for the AIDS virus and other infectious agents. The SARS test is expected to have limited use because it cannot detect antibodies until three weeks after the onset of illness. A positive result would, however, strongly indicate that someone had been infected with the SARS virus, while a negative test would not necessarily rule out such infection, Dr. Gerberding said.

"Probably the most important lesson that we learned from the Singapore experience" is the need to "remain vigilant," Dr. Gerberding said. "If we were unfortunate to have someone with unrecognized SARS," who could infect many others, "we could have a cascade of transmission established; we are certainly not immune to that."

The genome study, by Dr. Edison T. Liu and his team in Singapore, involved comparing the complete genomes of SARS virus from nine cases there with virus isolated in Canada, China, Hong Kong and Vietnam. It was published in The Lancet, a British medical journal.

"This is the first major analysis of this virus," said Dr. David L. Heymann, the executive director of communicable diseases at the World Health Organization. The agency, which is based in Geneva, has overall responsibility for investigating the epidemic, which has affected 29 countries.

A number of laboratories have also found that the SARS virus has not mutated significantly in the seven weeks since it was detected, Dr. Heymann said.

Dr. Earl G. Brown, a virologist at the University of Ottawa, said: "I hope that SARS will change, but I'm concerned. This virus seems to be happy with the genes it's got."

Dr. Brown, in a commentary on the study in The Lancet, suggested that because the SARS virus had changed relatively little in its first few months, it seemed unlikely to mutate into a milder form.

In an interview, Dr. Brown said that if a new virus was going to evolve into a more benign form, it normally did so in the early months of an outbreak, a pattern followed by the Ebola virus, which causes a deadly hemorrhagic fever.

"Ebola transmits like wildfire in hospitals and among family members for two or three transmissions and then loses the ability to infect people," Dr. Brown said. "It loses the ability to spread and keep its virulence, and burns out on its own."

The World Health Organization has said the SARS virus, which is a newly discovered member of the coronavirus family, is the cause of SARS. But the agency and other scientists agree that more work needs to be done to be certain. Scientists know that other coronaviruses have a high rate of mutation and so they say that it is far too soon to know whether the SARS virus will change to cause milder, or even more severe, illness, or become a seasonal disease, like influenza and other coronaviruses.

The SARS virus does not closely resemble other known human and animal coronaviruses, and scientists do not know its origins.

Dr. Brown said that studying animals in China to trace SARS back to its possible origin would be a formidable task. Countless animals would have to be swabbed for viruses, and the samples would then have to be cultured, sequenced and compared. And since it is also possible that SARS came from a mutated human coronavirus, researchers would have to search for human viruses as well.

Dr. Heymann said that the epidemics of SARS appeared to be waning, though not conquered, in all affected countries except China and Taiwan.

Epidemiologists are trying to determine what conditions would be necessary to declare SARS an endemic disease and give up trying to eradicate it. The World Health Organization is also trying to find all the chains of transmission in China since the disease began, presumably in Guangdong Province last November.

Scientists want to find all possible branches in the transmission chain to see if there are any small changes in the genome that could help trace contact histories. Some small changes have occurred, leading the authors to say that they could distinguish the strains isolated from the individuals who caught SARS at the Hotel Metropole in Hong Kong from those elsewhere in China.

But Dr. Heymann expressed frustration at the difficulties that a World Health Organization team invited by Chinese officials has encountered in obtaining such details.

"We're itching to get hold of this data which is sitting in Guangdong Province and that is key to much of the outbreak," Dr. Heymann said. "We all have to be patient."