The World Health Organization describes the development of a potential pandemic as a series of 6 phases. We have been at Phase 3 for some consideable time, but overnight the level was raised to Phase 4. This is a significant escalation and you should consider carefully whether you need to travel to affected areas, or to surrounding areas. If you have already booked travel, it is unlikely that your insurers will re-imburse you unless your national government specifically advises against travel to your destination. The UK government is now advising against all but essential travel to Mexico, but you will need to check how your travel agent and insurer view this. You should check the FCO website regularly and you can sign up for email alerts on your destination countries. The following description of the phases is taken (with some editing) from the WHO website. Phases 1-3 correlate with preparedness and response planning activities, while Phases 4-6 signal the need for response and mitigation efforts as well as post-pandemic recovery activities. In nature, influenza viruses circulate continuously among animals and birds. Although they have the potential to develop into pandemic viruses, in Phase 1 no viruses circulating among animals are 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 sustained human-to-human transmission. Limited human-to-human transmission may occur when there is close contact between an infected person and an unprotected caregiver.
In Phase 4 there is 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 affected country needs to decide whether 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. 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. Although pandemic activity appears to be decreasing, additional waves may occur and countries need to be prepared for a second wave. Previous pandemics have been characterized by waves of activity spread over months. In the post-pandemic period, influenza disease activity returns to levels normally seen for seasonal influenza. It is expected that the pandemic virus will then behave as a seasonal influenza A virus.