MELBOURNE (Reuters) - Scientists battling SARS face the challenge of finding a vaccine that does not trigger a harmful immune reaction in the body, a top Australian virologist says.

"We have to be very cautious, and I think that that will then mean any viable vaccine is a number of years down the track," Steve Wesselingh, director of the Burnet Institute, told reporters at a global meet on disaster and emergency medicine.

A treatment for the flu-like virus would also take time, Wesselingh, a specialist in infectious diseases, added.

SARS has killed more than 500 people and infected more than 7,000 worldwide, after emerging in China late last year and being carried round the world by air travellers.

The disease kills as many as 55 percent of SARS sufferers older than 60, a recent study shows, although the figure for victims overall is closer to 10 percent.

Potential vaccines could have the effect of invoking an immune response that could worsen a patient's reaction to SARS, Wesselingh said.

Companies and government labs with antiviral agents are now scrambling to find a drug to treat SARS.

"And one would hope they will find an agent that has some activity against the virus, although there isn't any agent at the moment that has activity against related viruses. So again I think that will take a bit of time," Wesselingh said.

While new cases of SARS appeared to be on the wane in Hong Kong, Singapore, and Toronto, and eliminated in Vietnam, public health officials warned countries like Australia, which has had no confirmed SARS cases, need to watch travellers coming from countries that have been affected, such as China.