It's the first bird-flu vaccine that's made from cells grown in the lab, instead of from chicken eggs, showing promise in blocking the deadly virus, researchers said.

The vaccine also doesn't need an adjuvant-- a substance added to a vaccine to make it stronger, as there have been safety concerns about adjuvants, they added.

"I think it is a big leap forward," said Dr. Wilbur Chen, a vaccine researcher at the University of Maryland School of Medicine not involved in the study.

The hunt for a good bird flu vaccine to be used in humans has gone on for some 10 years. So far, more than 100 people have died worldwide from H5N1 infection spread through close contact with birds.