The first major study of the genome of the severe acute respiratory syndrome (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 on Thursday. That effort is expected to take years.
But the experts said that the findings also mean that SARS, unlike some other new and emerging diseases, has not weakened as it passed through successive generations. Some experts had expressed hope that the virus would cause less severe illness as it spread.
However, in a report on the outbreak in Singapore, the US Centers for Disease Control and Prevention (CDC) said that five individuals had acted as "super spreaders" in passing the virus to 144 other people, while 81 percent of infected people did not transmit to anyone else. The CDC defined super spreaders as individuals who transmitted SARS to 10 or more other people. Doctors have described super spreaders in other infections like tuberculosis, rubella and Ebola.
CDC director Dr. Julie Gerberding said her agency is making available to state health departments the biological material that is 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.
CDC 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 strongly indicate that an individual has been infected with the SARS virus, but a negative test will not necessarily rule out such infection, Gerberding said.
"Probably the most important lesson that we learned from the Singapore experience" is the need to "remain vigilant," 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 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 and was published in The Lancet.
"This is the first major analysis of this virus," said Dr. David Heymann, the executive director of communicable diseases at the World Health Organization (WHO). The Geneva-based agency has overall responsibility for investigating the epidemic that 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, Heymann said.
Dr. Earl 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."
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 seems unlikely to mutate into a milder form.
In an interview, Brown said that if a new virus is going to evolve into a more benign form, it normally does so in the early months of an outbreak, a pattern followed by the Ebola virus, which causes a deadly hemorrhagic fever.