The World Health Organization is watching for signs of a flu pandemic around the world. It responds to situations based on a table of phases, shown below.
MORE ABOUT PHASES:
Phase 1 is when no viruses circulating among animals have been reported to cause infections in humans.
Phase 2 simply cautions that there is a new flu virus in animals, somewhere in the world, that is known to cause infections in humans. It tells health scientists to watch carefully for evidence that people have caught this new flu virus.
Phase 3 is when humans begin to catch this strain of flu that has been in animals. It might pass from human to human, but only rarely. The risks to people are low.
Phase 4 is when the new flu begins to spread from human to human more often. It's still not a big risk, but the danger of a real pandemic is there.
Phase 5 is when there are the first signs that this new flu has become a real threat, and can pass easily from person to person. Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region. 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 indicates that a global pandemic is under way. In addition to the criteria defined in Phase 5, this phase is characterized by sustained community level spread of the new flu strain in several countries in at least two WHO regions. The declaration is based on the spread of the virus, not the severity.
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.
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.
These phases are used for public information, and also help in the creation of plans to respond to an influenza pandemic.