London, May 9 (Bloomberg) -- The virus that causes SARS appears to be mutating slowly, calming fears of a disease that will easily resist drugs and increasing chances of a vaccine, according to research that will be published today in the Lancet medical journal.

Samples of the virus taken from 14 patients in Singapore, Hong Kong, Toronto, Beijing and China's Guangdong province showed few genetic changes indicating an ability to quickly elude drugs. The slow rate of change also indicates the virus probably won't mutate into a less harmful form, said Edison Liu of the Genome Institute of Singapore, who led the study.

``This finding may indeed be a double-edged sword,'' University of Ottawa biochemist Earl Brown wrote in an accompanying editorial.

Researchers are racing to understand how to track and treat severe acute respiratory syndrome, which the World Health Organization said has infected 7,053 people and killed 506. More than 90 percent of cases are in China and Hong Kong.

Hong Kong residents may stop wearing the facemasks that have become a symbol of the epidemic, David Heymann, the World Health Organization director for infectious disease, said, as cited by the South China Morning Port. The city is the second-most affected place with 1,661 cases.

``It is time you can probably put down your mask,'' Heymann said, as cited by the newspaper. ``It is not necessary from an epidemiological point of view, although it may reassure people. SARS is not transmitted casually by walking down the street.''

The United Nations health agency told Hong Kong officials that it may lift a travel advisory against the city when the number of new cases reported each day falls to fewer than five. There were seven infections added yesterday, the fewest since Hong Kong began full reporting in March. The city has had 208 deaths.

Singapore, China

In Singapore, the world's third-most-affected place with 204 cases, the number of patients in hospital fell to 23, the fewest since March 18. The city hasn't reported any new cases for three days and the World Health Organization will stop listing it as an affected country if the gap continues to May 18.

The Singapore outbreak is linked to five ``super-spreaders,'' who probably infected at least 144 people, according to a study published by the U.S. Centers for Disease Control and Prevention.

Only in China, which accounts for two-thirds of the world's 7,053 infections, does the disease continue to spread rapidly. The country reported 146 more cases yesterday, for a total of 4,698, and added five deaths for a total of 214.

Elsewhere, Russia closed most of its land border crossing with China after discovering its first probable case of SARS, said Gennady Onishchenko, the country's chief health officer. The patient, Denis Soynikov, is hospitalized in Blagoveshchensk on the Chinese border.

Verification

Indications that the virus mutates slowly will still need to be verified, said Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention.

``The rate of mutation still needs to be determined,'' she said in a telephone call with reporters. An earlier comparison by CDC and Canadian laboratories also showed only small variation between the viruses.

The disease is suspected in people with fevers and pneumonia who have traveled to affected areas, or been exposed to people with SARS. Symptoms, which generally appear up to 10 days after infection, arise when the immune system responds to the virus, according to Malik Peiris, a University of Hong Kong microbiologist.

Immune Response

The body's own response to the infection may cause more damage to the lungs than the virus, Peiris wrote in another study in the Lancet. The most lethal effects ``might not be associated with uncontrolled viral replication, but may in fact be immunopathological in nature,'' he wrote.

Peiris's study of 75 residents of Hong Kong's Amoy Gardens complex who were sickened by the SARS virus found that three- quarters had diarrhea, and the virus was found in the feces of 65 of 67 patients tested. These widespread effects suggest that the SARS virus may have a significant impact outside the respiratory system, said Tomas Aragon, director of the Center for Infectious Disease Preparedness at the University of California, Berkeley.

``Other respiratory tract infections don't lead to this kind of gastrointestinal involvement,'' he said in an interview. ``This coronavirus is unusual.''

Last Updated: May 8, 2003 23:47 EDT