Influenza pandemics are unpredictable but recurring events that can have severe consequences on societies worldwide. This revised WHO guidance publication on pandemic influenza preparedness and response has been developed to guide WHO Member States and others responsible for public health and pandemic influenza preparedness activities at the national level. It is the product of expert opinion, derived from several international consultations which included examination of available information and modeling studies, input from public health experts on lessons learned from SARS and both animal an