This summary describes the current status of the development of new candidate H5N1 vaccine viruses and is meant to provide guidance for national authorities and vaccine companies on the selection of candidate viruses for use in vaccine development. The selection of H5N1 vaccine viruses should consider the geographical spread, epidemiology, and antigenic and genetic properties of recently circulating H5N1 viruses.

Clinical trials using both clade 1 and clade 2 viruses should continue as an essential element in pandemic preparedness.

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