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Flu Pandemic Phases

What Are the Phases of a Flu Pandemic?


Updated October 04, 2011

The World Health Organization (WHO) revised its flu pandemic phases in 2009. There are 6 different phases and two post pandemic phases. This system was created to help nations prepare and respond to influenza outbreaks.

Flu Pandemic Phases

  • Phase 1
    No viruses circulating among animals have caused infections in humans.

  • Phase 2
    A flu virus that typically causes infection in animals is known to have infected a human. This means that the virus has the potential to cause a flu pandemic.

  • Phase 3
    An animal or human-animal combination flu virus has caused sporadic cases of disease in humans, but has not developed the ability to spread from human to human easily. Occasionally, human-to-human transmission may occur, but only with very close contact and not at the rate necessary to cause a pandemic.

  • Phase 4
    An animal or human-animal combination flu virus has the ability to be transmitted from human to human and cause "community-level outbreaks." The ability of a flu virus to transmit from human to human and cause these types of outbreaks indicates that it is much more likely to be able to cause a flu pandemic, but it is not necessarily a "forgone conclusion."
    Any country that suspects or has verified such an outbreak, should contact the WHO to determine if containment procedures are necessary.

  • Phase 5
    There is confirmed human-to-human transmission of the virus in at least two countries in one WHO region. Although most countries may not be affected, phase 5 indicates that a pandemic is imminent and countries worldwide should finalize plans and preparations.

  • Phase 6
    A global pandemic is underway. There are confirmed community level outbreaks in at least one other country in a different WHO region (in addition to those that were verified in phase 5).

  • Post Peak
    Pandemic disease levels have dropped below peak levels in most countries that have adequate surveillance. This period signifies that disease levels appear to be decreasing, but further waves of disease may still occur, so countries should be prepared for that possibility.

  • Post Pandemic
    Disease levels have returned to levels typical of seasonal influenza. It is still important to maintain surveillance of the virus and plan accordingly. A recovery and evaluation period may be necessary.


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

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