A body chemical that could wipe out the virus that causes Sars may have been found by researchers.
The antibodies target the Sars virus
In theory, a drug based on the antibody could be available long before a vaccine could be produced.
Antibodies are created by the body's immune system to launch attacks on foreign invaders such as bacteria.
Experts from a Boston cancer institute looked through a "library" of human antibodies before they found one which worked against Sars virus.
Their findings, reported in the journal Proceedings of the National Academy of Sciences, are a further chapter in a remarkable worldwide scientific effort triggered by international concerns over Sars.
In total, more than 8,000 cases of severe respiratory illness in 2003 were blamed on Sars, and 800 deaths.
There are fears that the virus will return again in 2004, possibly in a more virulent form.
The genetic sequence of the coronavirus that causes Sars has already been de-coded by scientists, and work to produce a vaccine is well underway.
However, this may not be available for some time, and scientists remain keen to find a "first-line" treatment which could be given to unvaccinated patients exposed to the virus.
Following the publication of the Sars genome - only two months after the World Health Organization labelled it a threat - Dr Jianhua Sui, from Dana-Farber Cancer Institute in Boston, began looking for chinks in its armour which might be exploited by an antibody-based drug.
People who have fought off a particular infection hang on to the antibodies they created to help them do it. That is why the same bug cannot make you ill twice, as the antibodies destroy it before symptoms appear.
Spiking the 'spike'
Work focused on a protein which is the building block for the distinctive "spikes" on the surface of coronviruses, and which are important in the viral efforts to enter human cells.
Test tubes coated in this "S1" protein were dipped into the library - a solution containing a staggering 27 billion antibodies culled from blood samples from only 57 human donors.
With some luck, one of the 57 might have at some point fought off a coronavirus infection and have the necessary antibodies in their blood.
The proteins were examined to see if any of the antibodies had "latched on".
Eight had recognised and bound to the protein, and from these, one, labelled 80R, appeared to potently block live Sars viruses from entering human cells in a laboratory dish.
Dr Wayne Marasco, one of the scientists working on the project, said: "It was one of those Eureka experiences."
Mass production of the antibody could mean a drug in production long before safety testing on a vaccine is complete.
Dr Dave Cavanagh, from the Institute for Animal Research in Compton, Berkshire, US, said that it was a "promising" strategy.
He said: "This approach is a good one if you want medicine you can give to patients who actually have the illness.
"It could in theory be available more swiftly than a vaccine."