What Are the Phases of a Flu Pandemic?

three sick businessmen
A flu pandemic can make many people sick. Darren Robb/Getty Images

The World Health Organization (WHO) revised its flu pandemic phases in 2013. There are 4 alert levels. This system was created to help nations prepare and respond to influenza outbreaks.

Flu Pandemic Phases

  • Interpandemic Phase - the period between flu pandemics
  • Alert Phase -  a new subtype of influenza has been identified as causing illness in humans. Careful monitoring of the virus and risk assessments at the local, national and global level occur at this point. Risk assessments will determine if the pandemic alert phase needs to be elevated or de-escalated based on the behavior of the virus.
  • Pandemic Phase - active pandemic phase where the new subtype of influenza virus is spreading and causing illness in humans across the globe.
  • Transition Phase - de-escalation of global actions occur as the spread of the pandemic strain of influenza decreases. Governments may reduce their response activities as appropriate based on their own risk assessments. 

Flu pandemics occur randomly and without warning. When the next one occurs, it will be helpful to know what the different pandemic phases are and what they mean for you and your family. 


"Current WHO phase of pandemic alert." Epidemic and Pandemic Alert and Response 2009. World Health Organization. 15 May 09.

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