Pandemic Alert Levels Explained

I had to look this up, so to save others the trouble of finding it,
here is the WHO
href="http://www.who.int/csr/disease/avian_influenza/phase/en/index.html">explanation
of pandemic alert levels:

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.



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.



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.



Phase 4 is characterized by
verified human-to-human transmission of an animal or human-animal
influenza reassortant 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.



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.



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.



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.



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.