bird flu, H5N1 bird flu, virus, Professor Antonio Lanzavecchia

The discovery could lead to treatments that complement flu vaccines in the event of a human epidemic of the virus.

The H5N1 bird flu virus is estimated to have killed more than 180 people around the world since 2003.

Some countries are already stockpiling vaccines for a possible bird flu outbreak in humans, but no one knows how effective they might be.

'Emergency antidote'

This is because the particular strain of bird flu that might eventually spark a human pandemic is unknown.

But scientists working in Switzerland, Vietnam and the United States say they have isolated antibodies that they hope could offer protection against several different strains of the virus simultaneously.

Antibodies are used by our immune system to neutralise bacteria and viruses - in this case, the scientists have isolated antibodies that bird flu survivors in Vietnam produced to fight off the disease.

Professor Antonio Lanzavecchia, at the Institute for Research in Biomedicine in Switzerland, says the antibodies have already proven effective in the lab and in mice and he is confident that they could be used in humans.

"We in a way exploit the immune response of an individual who has been infected and has survived the infection and of course has made antibodies that neutralise these viruses," he said.

"And using this technique, we can isolate the cells that make these antibodies so that this antibody can now be reproduced in vitro and eventually massively produced to treat other individuals."

The antibodies could be used to protect key workers, such as nurses and doctors, in countries where a bird flu epidemic strikes.

The researchers say it could also be used as an emergency antidote in people who have already been infected with bird flu - if administered within a few days.

It is hoped human trials could begin shortly.