PARIS (AFP) - Researchers in Switzerland have successfully immunized mice against the H5N1 strain of bird flu using human antibodies taken from survivors of the deadly virus, according to a study released Tuesday.

The antibodies, reproduced at the Institute for Research in Biomedicine, also vastly enhanced the survival rate of infected animals, pointing the way to a treatment for people stricken with the often lethal disease, said Antonio Lanzavecchia, co-author of the study and director of the Institute´s immune regulation laboratory.

"We are very confident that this data can be reproduced in humans," he told AFP, saying that the antibodies "provided immediate, short-term immunity" in mice.

The H5N1 bird flu has killed millions of wild and domestic fowl across the world since it first emerged in the late 1990s, and has caused 185 fatalities out of 306 known cases -- most since 2003 -- in humans, according to the

Experts fear that the virus could mutate into a form easily communicable among people as happened during the great flu epidemic of 1918, which caused some 50 million fatalities.

In the experiments, mice were injected with antibodies generated from the blood of avian flu survivors in Vietnam, where more than 40 people have succumbed to the disease since 2003.

The country´s first human case of H5N1 in 18 months, meanwhile, remained in an intensive care unit on Monday in Hanoi.

The mice were then exposed to the same strains that proved so deadly in humans. The treatment provided virtually complete protection, according to the study, published in the open access journal PLoS Medicine.

As important, said Lanzavecchia, was the efficacy of the antibodies in neutralising the virus in mice that had been infected as much as 72 hours earlier.

The antibodies significantly reduced the amount of virus found in the lungs -- by a factor of 10 to 100 -- and almost completely stopped it from reaching the brain or the spleen.

By contrast, none of the untreated mice in a control group survived.

The development of a vaccine against a possible global H5N1 pandemic has been a major focus of scientists in the field, many of whom are gathering at the end of this week in Paris for the second International Conference on Avian Influenza in Humans.

But relatively little attention has been devoted to antibodies, which act differently, Lanzavecchia said.

A vaccine induces a long-term or permanent immune response, but typically takes weeks or months to take effect. Vaccines are also useless to a patient once the disease has struck.

Antibodies, however, work immediately, and are relatively easy to manufacture on an industrial scale. But the protection is only likely to last a few months, he explained.

This could still be critical in saving the lives of those infected, who typically seek medical help only a couple of days after flu-like symptoms appear.

Antibody treatment could also immunize frontline nurses and doctors during a possible pandemic.

Because it is not possible to conduct regular clinical trials due to the lack of cases, regulators in the United States and Europe have authorized a "fast track" approval process for an antibody-based drug, Lanzavecchia said.

If a treatment shows the same results in two animal models, including one on primates, and then passes a safety analysis, it could then go to market. This process typically takes between three and four years, he said.

The research has been funded by Britain´s Wellcome Trust, the second largest medical research charity in the world, as well as the US National Institute for Health and the Swiss