Pandemic influenza refers to a worldwide epidemic due to a new, dramatically different strain of influenza virus. A pandemic virus strain can spread rapidly from person to person and, in its most virulent strains, can cause high levels of disease and mortality around the world.
Over time all influenza Type A viruses experience slight changes (antigenic drift) in their genetic structure. This necessitates a change in annual vaccines to protect against seasonal influenza strains. Occasionally influenza viruses undergo a major change in genetic composition or mutation (called reassortment) through the combination of an avian and human virus.
A pandemic can occur when avian influenza viruses acquire the ability to infect and cause disease in humans, and then spread rapidly from person to person.
The creation of a novel virus means that the majority, if not all, people in the world will have never been exposed to the new strain and thus they have no immunity to the disease. It also means that a new vaccine specific to the pandemic virus must be developed and produced once we are in a pandemic
Please see resources provided by the CDC at the following link.