Initial results from the first human tests of a Sars vaccine in China have produced promising results.
Sars caused widespread panic in Asia
The Chinese government said on Wednesday that four volunteers were in "good condition" four days after being inoculated.
The test is part of a drive to try to prevent a repeat of the outbreak which killed 774 people worldwide last year.
Three men and a woman - all students - were injected with the vaccine at a Beijing hospital last Saturday.
Since then they have been subjected to regular blood tests and temperature checks.
The Chinese authorities said that so far there had been no sign of any adverse reactions.
State television has reported that the volunteers have all been released from hospital, but will continue to be monitored for 210 days.
It said researchers would decide after that whether to proceed with the next stage of testing - although it did not say what that would include.
China says it is the first country to reach the stage of testing a Sars vaccine on humans.
The vaccine, developed by the government and a company affiliated to Peking University, is thought to have been made using a "dead" sample of the Sars virus.
In total, 36 volunteers, aged 21 to 40, have been lined up to test the vaccine.
A Sars vaccine has been successfully tested on animals in the US.
But the World Health Organization and other experts say a safe, effective human vaccine is at least a year or two away.
Sars first emerged in China's southern province of Guangdong in late 2002. In total, more than 8,000 people fell ill before the outbreak finally abated in July last year.
China has suffered two outbreaks since then - one in December in Guangdong and another last month in Beijing.