For the past decade, scientists have been monitoring with alarm the slow spread of H5N1 influenza around the world. Many fear that this particularly lethal form of flu (known as bird or avian flu) could become the next worldwide pandemic, resulting in millions of deaths.

Researchers are working on possible vaccines. But this week, an international group of medical scientists published a study showing that antibodies extracted from the blood of people who have already recovered from H5N1 flu could be used for treatment.

Doctor Kanta Subbarao, from the U.S. National Institute of Allergy and Infectious Diseases, explains that generating antibodies that could then be administered to someone else is referred to as passive immunization. "That's different from vaccinating somebody with a dose of vaccine and having the person make antibodies to the vaccine that then protect them from the infection. That's active immunization. The approach we're talking about here is passive immunization."

Passive immunization has been used in treatments for other viral diseases -- for example, rabies, chicken pox and hepatitis A. Scientists take antibodies against a disease and clone them, creating monoclonal antibodies.

Subbarao says her team gave human monoclonal antibodies to mice and then proceeded to infect the mice with the bird flu virus. The mice resisted getting the flu, and when they did get sick, they didn't get as sick as mice that had not gotten the antibodies.

Then, Subbarao says, they tested the antibodies to see if they could be a treatment for mice they had infected with H5N1. "The next day, we gave them the monoclonal antibodies, or we delayed the administration of the monoclonal antibodies up to 48 or 72 hours after infection and we found the antibodies gave complete protection from lethality even when they administered at 72 hours post infection."

The antibodies Subbarao's team used were from a virus circulating in 2004. She says they worried that virus strains had changed since then. "The good news was that some of these antibodies worked even against strains that had appeared after 2004."

Subbarao says this approach could be an important therapy for people who become ill with H5N1, especially in the absence of a vaccine against the disease. She says the next step will be to test the antibodies in other animals and begin testing them in humans. Subbarao's study is being published in the open-access on-line journal, Public Library of Science: Medicine.