Antigenic and genetic characteristics of H5N1 viruses and candidate vaccine viruses developed for potential use in human vaccines, February 2009.

Since their reemergence in 2003, H5N1 influenza viruses have become endemic in some countries and continue to cause outbreaks in poultry and sporadic human infections. It is not known if the next influenza pandemic will be caused by H5N1 viruses or, should one occur, which of the clades of H5N1 viruses would be responsible. However, because an H5N1 pandemic is a possibility, and one with potential for an unusually severe outcome, the development of representative H5N1 candidate vaccine viruses from those available for characterization, coordinated by the World Health Organization (WHO), is an essential component of the overall global strategy for pandemic preparedness. This summary provides an update on the characterization of available H5N1 viruses circulating in birds, including those that have caused human infections, and the current status of the development of candidate H5N1 vaccine viruses. This information may be used to guide national decisions on procurement of H5N1 vaccines.