Scientists say mice immunized with an experimental SARS (search) vaccine were protected against the deadly respiratory disease that killed nearly 800 people worldwide a year ago.

Results from an accelerated federal research program published in Thursday's issue of the journal Nature show the vaccine prompted an immune response in the mice and dramatically reduced the level of the virus in the lungs of some mice.

But researchers at the National Institute of Allergy and Infectious Diseases (search), which developed the new gene-based vaccine, said more experiments are needed to determine if it will work in humans.

Scientists not connected with the study said the results were encouraging, but should not be overstated. No DNA vaccine has yet been shown to effectively treat any viral disease and the approach is still unproven compared to conventional vaccines.

"I don't think it's a home run," said Dr. Robert Brunham, director of the University of British Columbia Centre for Disease Control in Canada. In Canada last year, more than 40 people in the Toronto area died from SARS infection and the nation's tourism industry suffered a sharp plunge.

Vaccines are normally made from dead or weakened viruses and work by mobilizing the body's immune system to build defenses by showing it what the targeted virus looks like. That approach is used every year, for example, to develop flu shots to combat emerging strains of influenza.

But the SARS vaccine was made from a small piece of genetic material from the virus called a plasmid. It biochemically locks onto a specific protein on the outer surface of the virus. This alerts the body's immune system to launch a counterattack against the invading virus.

Scientists tested two versions of the DNA vaccine in 15 mice over a six-week period. The vaccines differed in how much genetic material scientists removed from the original piece of DNA. Both worked, although one appeared to be more effective than the other.

Dr. Gary Nabel, chief of the NIH Vaccine Research Center (search) and the study's lead author, said the government is working with a San Diego-based biotechnology company, Vical Inc., to make a purified vaccine for human testing pending approval from the Food and Drug Administration.

Last May, two research teams separately published the genetic sequences of the SARS virus in an effort to find drugs to treat SARS or develop a vaccine to prevent it.

Earlier this year, China announced plans to test on humans an experimental SARS vaccine that uses a killed virus. The vaccine had been shown to be effective in animal testing.

SARS, or severe acute respiratory syndrome, is a coronavirus. Similar viruses infect the pigs and other animals. It emerged in southern China in late 2002 and spread to more than two dozen countries on four continents before it was contained last summer.

More than 8,000 people worldwide were sickened, and at least 774 died.

Civet cats and other mongoose-like animals that are sold in live food markets in southern China are suspected of spreading SARS to humans although the original source of the virus has not been determined.