She was speaking to a news conference after some 150 leading scientists, researchers and public health experts from 30 countries held four days of talks on global vaccine research in the resort town of Montreux in western Switzerland.

The deadly SARS virus infected about 8,000 people worldwide in 2003, killing nearly 800 people. It briefly re-emerged in China last April, killing one person and fueling fears that the disease could surface in an annual cycle similar to human flu.

In the trials, healthy volunteers have been injected with the vaccine, which has previously been successfully tried on Rhesus monkeys, to check for dangerous side effects.

"The (SARS) vaccine has been injected very cautiously in four individuals. Now two weeks after immunization, all are reported to be well," Kieny said.

Three other firms should be ready to begin trials within months, according to the WHO official who gave no details.

Stanley Plotkin, scientific adviser to Franco-German pharmaceutical concern Aventis Pasteur, and who is credited with inventing the rubella or German measles vaccine, said the initial findings from Sinovac were encouraging.

But a SARS vaccine might still not be available for years unless there was some major outbreak which might lead regulatory authorities to accelerate production and distribution, he said.

"You are talking five years, minimum, and that is optimistic," Plotkin said. "It would be foolish of us to give a date because there are many factors and hurdles."

Sinovac has said a second shot is to follow after an initial 28-day period of observation. Observation will continue for a total of 210 days from the start of the trial on May 22.

China's State Drug Administration will analyze results of the first phase and decide whether to approve a second phase of testing on volunteers from a wider demographic range, it added.