"This rapid evolution is like that of a murderer who is trying to change his fingerprints or even his appearance to try to escape detection," said Dr. Dennis Lo, a chemical pathologist at the Chinese University of Hong Kong.
Researchers at the university said they have determined the genetic sequences of virus samples taken from 11 SARS patients, and that by late March there were two forms of the virus present in Hong Kong.
Lo said one strain was detected in a woman whose illness was linked to an outbreak caused by a mainland Chinese man who spread SARS to others at a Hong Kong hotel. The other strain came from a Hong Kong man believed to have caught it in the mainland border city of Shenzhen.
"We have shown that the SARS coronavirus is undergoing rapid evolution in our population," Lo said. But he noted that more work is needed before researchers can say whether the virus has become more infectious and lethal.
Researchers also need to find out whether people who get SARS from one strain can develop immunity to other strains, he said. If not, finding ways to better diagnose it and to develop a vaccine could be more difficult.
The World Health Organization says there's no evidence that the mutations have any effect on the disease itself. WHO scientists also say it's not surprising the SARS bug shows genetic changes, because the coronavirus family is prone to mutations.
CBS News Correspondetn Elizabeth Kaledin reports there are various questions about the disease that are making it hard to mobilize an effective defense. The disease is unusually contagious, yet its spread thus far is restricted geographically, found in one place but not another.
"It's a new virus to humans and it's not very well adapted to humans,'' says Dr. Martin Blaser with the NYU School of Medicine.
A U.S. coronavirus expert, David Brian, at the University of Tennessee in Knoxville, agreed that a rapidly mutating SARS virus could complicate work toward a vaccine and reliable diagnostic tests.
The crucial question is where the mutations occur in the SARS virus genome, he said. If they affect the shape of an outer protein on the virus, it could hamper vaccines, which rely on training the immune system to recognize particular protein shapes, he said.
As for diagnosis, it is based on specific features of the bug's genetic sequence. So if one of the crucial features is removed by mutation, the detection kit becomes less sensitive to recognizing the virus, he said.
Hong Kong scientists are also concerned that the virus may survive in an infected person's body for at least a month after recovery. Doctors are urging patients to avoid personal contact such as hugging and kissing when they go home.
"The virus still exists in the patients' urine and stool after they were discharged. It will persist for at least another month or maybe even longer," said Dr. Joseph Sung, head of the Department of Medicine at the Chinese University of Hong Kong.
What's more, scientists here also fear that 12 people may have relapsed. The new findings raise questions as to how doctors can tell whether a patient has fully recovered, underscoring the difficulty health authorities face in tackling this new disease.