Avian Flu - WHO changes H5N1 strains for pandemic vaccines, raising concern over virus evolution.

Michael T. Osterholm, PhD, MPH, a leading pandemic preparedness expert, said recognition of the three subclades demonstrates how diverse the virus is and how dynamically it is evolving. He said the WHO notice is more important for the questions it raises than for the vaccine guidance it contains. "Does that mean H5N1 is closer to becoming an agent that can readily transmit human-to-human? That's the billion dollar question,"

Many experts who follow the ongoing analysis of the H5N1 virus sequences are alarmed at how fast the virus is evolving into an increasingly more complex network of clades and subclades, Osterholm said. The evolving nature of the virus complicates vaccine planning. He said if an avian influenza pandemic emerges, a strain-specific vaccine will need to be developed to treat the disease.

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