The vaccine prevented Sars infection

Researchers from the US National Institute of Allergy and Infectious Diseases said it reduced levels of the virus in mice's lungs.

The vaccine uses a small piece of the virus's DNA to stimulate a protective response from the body's immune system.

Writing in the journal Nature, the scientists say they hope to begin human trials of the vaccine soon.

Figures from the World Health Organization show Sars infected 8,098 people and killed 774 worldwide in the outbreak between November 2002 and July 2003.

The US researchers created the DNA vaccine by taking a small piece of DNA that codes for a coat protein normally found on the outer surface of the virus.

It represents a critical first step towards developing an effective human Sars vaccine Dr Gary Nabel, National Institute of Allergy and Infectious Diseases

The protein helps the Sars virus gain access to living cells, where it can cause infection.

The DNA in the vaccine is unable to cause infection itself because it only contains the code for the protein.

Immune defence

The vaccine directs the body's cells to make proteins very similar to those on the surface of the virus.

These then trigger the immune system to mount a defence which can recognise and neutralise any subsequent encounters with the actual Sars virus.

Other vaccines are being developed, but they tend to use killed or weakened forms of a whole virus or bacteria to protect against Sars.

The US researchers gave three doses of the vaccine to mice over a six-week period. Another group received an inactive vaccine.

A month later, all the mice were exposed to the Sars virus. After two days, the scientists measured levels of the Sars virus in their lungs.

The mice given the active vaccine had one million times fewer virus particles than those who were not vaccinated.

The scientists found the vaccine prompted the body's immune system to produce antibodies which render the virus harmless.

Researchers now plan to carry out further experiments to evaluate the vaccine's safety and test it in humans.

'Critical step'

Dr Gary Nabel, director of the Vaccine Research Centre at the institute, said: "This vaccine dramatically reduced the level of virus in the lungs of infected mice, more than a million-fold.

"It represents a critical first step towards developing an effective human Sars vaccine."

Dr Dave Cavanagh, an expert in coronaviruses at the UK's Institute for Animal Health, told BBC News Online: "This is very promising. It shows that the vaccine could work.

"It also shows that the DNA approach works. These are safer than the conventional vaccines because those use the infectious agent.

"And as they found that it was the antibodies which were protective, they could also be given to people as an inoculation."