As an alternative to antiviral drugs, researchers are attempting to find a way to combat bird flu through passive immunotherapy - treating people with anti-H5N1 antibodies (neutralising antibodies), that can stop H5N1 from infecting cells.

In a recent study, blood from four bird flu survivors in Vietnam was used to extract immune cells that produce antibodies against the H5N1 virus.

Monoclonal antibodies against different strains of the virus were then cultivated, some of which were administered to mice either before or after they were then themselves infected with various lethal strains of bird flu.

"In mice, the antibodies provided protection from infection with the original virus when given a day before or one to three days after infection", the international team of researchers from the US, Switzerland and Vietnam wrote in the study report, published in the Public Library of Science (PLoS) journal Medicine on Tuesday.

The control group of mice that received no antibody treatment all died.

"The antibodies protected mice by limiting viral replication, by lessening the deleterious effects of the virus in the lungs, and by stopping viral spread out of the lungs", the researchers wrote.

This is potentially important because some of the H5N1 viruses are resistant to the antiviral drugs, such as Roche's Tamiflu (oseltamivir phosphate), currently used to treat flu and in the event of a pandemic, there will inevitably be a lag of some months before a vaccine effective against the specific H5N1 strain can be produced in bulk. New, preventative and therapeutic strategies are therefore needed.

"These results indicate that passive immunotherapy with human monoclonal antibodies could help to combat avian H5N1 if (or when) it starts a human pandemic".

However, this research is still in its early stages, and the monoclonal antibodies will have to be tested to see whether they can neutralise not only all the currently circulating H5N1 viruses but also any emerging pandemic versions, which might be antigenically distinct, the researchers noted.

Bird flu does not normally infect humans, but there have been dozens of examples in the last few years of transmission from livestock to people, leading to fears that a strain could mutate that can be transmitted from human-to-human and so spark an international pandemic.

Current drug and vaccine manufacturing capabilities fall billions of doses short of what would be required to protect the six billion people in the world in the event of such a pandemic.