WASHINGTON (Reuters) - An antibody being developed by a Dutch drug company chokes off both seasonal flu and the H5N1 avian flu virus and might offer a way to develop better treatments and vaccines, researchers reported on Thursday.

Crucell NV's antibody, a naturally occurring immune system protein, grabs onto a hidden part of flu viruses, stopping them from infecting cells, they reported in the journal Science.

It is the second report in a week to find antibodies that can interfere with a range of strains of flu -- one of the hardest viruses to fight because it mutates so much.

"This is very exciting because it marks the first step toward the Holy Grail of influenza vaccinology -- the development of a durable and cross-protective universal influenza virus vaccine," Ian Wilson, a researcher at he Scripps Research Institute in La Jolla, California, who helped lead the research, said in a statement.

"Such a flu vaccine could be given to a person just once and act as a universal protectant for most subtypes of influenza, even against pandemic viruses."

On Sunday, another research team said they had found a batch of antibodies that do something similar.

Flu vaccines and drugs focus on proteins found on the surface of the flu virus called hemagglutinin and neuraminidase, which give influenza A viruses their names, as in H5N1 or H1N1.

Hemagglutinin is a lollipop-shaped structure with a big, round head. This head is so large that it attracts most of the immune system antibodies -- which then slip off when it mutates.

Because of the mutations, vaccines have to be reformulated every year and the viruses can develop resistance to antiviral drugs.

The antibodies found by Wilson's group and the U.S. team earlier this week attach to the "stick" of the hemagglutinin lollipop. This mutates less than the head, and so provides less of a moving target.

In both studies, the antibody suppressed a range of flu viruses, including H5N1 avian flu and the currently circulating H1N1 seasonal flu virus, although they did not work well against another seasonal flu virus called H3N2.

Wilson's team and Crucell Holland found their antibody, called CR6261, in the blood of people who had been vaccinated with the ordinary seasonal flu vaccine.

Similar antibodies have now also been found in other people, but it is not clear how well they protect people from flu or whether some people's bodies use them more efficiently than others.

Both groups said their antibodies provide a way to treat people infected with flu, as well as a route to designing better drugs and vaccines.

(Reporting by Maggie Fox; Additional reporting by Julie Steenhuysen in Chicago; Editing by Eric Walsh)

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