Researchers have discovered that influenza hemagglutinin -- a type of protein found on the surface of influenza viruses that is used to bind to host cells-appears to play an important role in the virus's ability to transmit efficiently among humans.

The findings are published on Thursday on the online early edition of journal Science. Terrence M. Tumpey at U.S. Centers for Disease Control and Prevention and coauthors studied the hemagglutinin protein covering the surface of the 1918 influenza virus that they recreated in 2005. That virus caused a pandemic that claimed the lives of at least 50 million people between 1918 and 1920, and that virus shows genetic sequence similarities to avian influenza viruses.

Researchers changed two amino acids -- basic building blocks of protein -- in the 1918 virus hemagglutinin from a mammalian configuration to an avian configuration and inoculated ferrets with it.

Ferrets are expected to be good predictors of influenza virus transmissibility among humans. Inoculation of ferrets with the 1918 "avian" hemagglutinin virus caused severe disease, but healthy ferrets placed close enough to the sick ferrets to catch it remained healthy.

The findings suggest that for an influenza virus to spread efficiently, its hemagglutinin must prefer attaching to host cells that are found in the human upper airway instead of host cells found predominantly in birds.

The transmission ability among humans is an essential property of a pandemic virus. Understanding flu transmission will assist researchers in their challenge to stop the spread of influenza, especially as concerns mount with the current avian flu epidemic in chickens and the possibility that it will spread to humans.

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