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Influenza Pandemic

An influenza pandemic is different from a seasonal outbreak of influenza. When a new influenza A virus subtype appears in the human population, causes serious illness, and then spreads easily from person to person worldwide, that is considered a pandemic. Such pandemics have occurred throughout the 20th century, and scientists believe it is only a matter of time before there is another one.

What Is an Influenza Pandemic?

An influenza pandemic is a global outbreak of disease that occurs when a new influenza A virus appears or "emerges" in the human population, causes serious illness, and then spreads easily from person to person worldwide.
 

Influenza Pandemic Versus Seasonal Outbreak

An influenza pandemic is different from a seasonal outbreak or "epidemic" of influenza. Seasonal outbreaks are caused by subtypes of influenza viruses that already circulate among people, whereas influenza pandemic outbreaks are caused by:
 
  • New subtypes
  • Subtypes that have never circulated among people
  • Subtypes that have not circulated among people for a long time.
 
Past influenza pandemics have led to high levels of illness, death, social disruption, and economic loss.
 

How Viruses Emerge in a Population

There are many different subtypes of influenza or "flu" viruses. The subtypes differ based upon certain proteins on the surface of the virus, which include the hemagglutinin (HA) protein and the neuraminidase (NA) protein.
 
Pandemic influenza viruses emerge as a result of a process called "antigenic shift," which causes an abrupt or sudden, major change in influenza A viruses. These changes are caused by new combinations of the HA and/or NA proteins on the surface of the virus. Such changes result in a new influenza A virus subtype. The appearance of a new influenza A virus subtype is the first step toward a pandemic (see Pandemic Stages). However, to cause an influenza pandemic, the new virus subtype must also have the capacity to spread easily from person to person.
 
Once a new influenza virus emerges and spreads, it usually becomes established among people and moves around, or "circulates," for many years as seasonal epidemics of influenza.
 
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