From the Overview: "A pandemic influenza (PI) event is defined as a global outbreak of disease in the human population, resulting in serious illness and death. A pandemic event is different from seasonal outbreaks or epidemics, since pandemic outbreaks are caused by new virus subtypes that have never circulated among people, or by subtypes that have not circulated for a long time. When a major change in the virus genetic subtype occurs, usually involving a new hemagglutinin antigen derived from a bird or animal influenza strain, numerous illnesses and deaths result because most humans have no pre-existing immunity."
New York (N.Y.). Office of Chief Medical Examiner
City of New York: https://www.nyc.gov/