In April 2009, the World Health Organization (WHO) redefined their phase descriptions for an influenza pandemic. This corresponded with the start of the H1N1 flu pandemic ("swine flu" pandemic), and was intended to make phases easier to understand, more precise, and based upon observable phenomena.
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Phases 1 - 3 correlate with preparedness, including capacity development and response planning activities.
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Phases 4 - 6 clearly signal the need for response and mitigation efforts.
The H1N1 2009 flu virus: 14 months of pandemic
A new flu virus, which eventually became known as pandemic H1N1, came to the world's attention in March/April 2009. WHO escalated the pandemic phase from Phase 3 to 4 on April 27, when it became evident the new H1N1 virus was spreading between humans in Mexico. It was escalated again two days later, up to Phase 5. That elevation recognized the virus's spread at a community level in both Mexico and the United States. On June 11, 2009, the WHO raised the H1N1 virus to Phase 6 - which means a pandemic is underway. WHO declared the H1N1 pandemic over on August 10, 2010, more than a year after it began. The virus moved from Phase 6 to "post pandemic" phase, which is assigned no number. See an image and explanation from WHO about H1N1's phase.
"Bird flu": avian flu H5N1
Avian flu H5N1 stands at Phase 3 on the WHO Phase scale as of April 2013. It has infected people in small clusters with limited human-to-human transmission. See an image and explanation from WHO about bird flu's phase.
"Bird flu": avian flu H7N9
Avian flu H7N9 stands at Phase 2 to 3 on the WHO Phase scale as of April 2013. It has infected people in small numbers with no evidence of human-to-human transmission.
Current WHO phase of pandemic alert
(Source: World Health Organization, 2013)
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Phase
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Details
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1
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In nature, influenza viruses circulate continuously among animals, especially birds. Even though such viruses might theoretically develop into pandemic viruses, in Phase 1 no viruses circulating among animals have been reported to cause infections in humans.
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Predominantly animal infections: few human infections
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2
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In Phase 2 an animal influenza virus circulating among domesticated or wild animals is known to have caused infection in humans, and is therefore considered a potential pandemic threat.
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3
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In Phase 3, an animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, but has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks. Limited human-to-human transmission may occur under some circumstances, for example, when there is close contact between an infected person and an unprotected caregiver. However, limited transmission under such restricted circumstances does not indicate that the virus has gained the level of transmissibility among humans necessary to cause a pandemic.
The current WHO Phase for H5N1 flu (Avian Flu) is 3. (See an image and explanation from WHO.)
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4
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Phase 4 is characterized by verified human-to-human transmission of an animal or human-animal influenzareassortant virus able to cause "community-level outbreaks." The ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the risk for a pandemic. Any country that suspects or has verified such an event should urgently consult with WHO so that the situation can be jointly assessed and a decision made by the affected country if implementation of a rapid pandemic containment operation is warranted. Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion.
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Sustained human to human transmission
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5
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Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region (Figure 4). While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.
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Widespread human infection
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6
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Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way.
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Post-Peak |
During the post-peak period, pandemic disease levels in most countries with adequate surveillance will have dropped below peak observed levels. The post-peak period signifies that pandemic activity appears to be decreasing; however, it is uncertain if additional waves will occur and countries will need to be prepared for a second wave.
Previous pandemics have been characterized by waves of activity spread over months. Once the level of disease activity drops, a critical communications task will be to balance this information with the possibility of another wave. Pandemic waves can be separated by months and an immediate "at-ease" signal may be premature.
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Possibility of recurrent events
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Post-pandemic
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In the post-pandemic period, influenza disease activity will have returned to levels normally seen for seasonal influenza. It is expected that the pandemic virus will behave as a seasonal influenza A virus. At this stage, it is important to maintain surveillance and update pandemic preparedness and response plans accordingly. An intensive phase of recovery and evaluation may be required.
WHO declared H1N1 flu had entered the post-pandemic period on August 10, 2010. This declaration signaled that the pandemic was over. (See an image and explanation from WHO.)
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Disease activity at seasonal levels
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