Understanding WHO Pandemic Alert Phases

With the Spring 2009 H1N1 swine flu alert currently at Phase 5, the danger posed by epidemic influenza has captured the world’s attention. But what do these alert phases actually mean, and what is their utility? Here is an explanation of World Health Organization’s Global Influenza Preparedness Plan and the associated pandemic alert phases.

Who’s WHO?

The World Health Organization (abbreviated WHO) is the public health agency of the United Nations and is responsible for providing global leadership on health issues. This means that they monitor health trends and infectious disease outbreaks, help coordinate the health research agenda, and set public health standards and norms. They also provide support and coordinate the efforts of specialists during global health emergencies.

WHO Pandemic Alert Phases

WHO currently has a six-phase alert system for assessing the threat level and appropriate response to infectious diseases. Phases 1–3 relate to worldwide preparedness for an epidemic, while Phases 4–6 focus on response and efforts to mitigate an active pandemic disease.

• Pandemic Alert Phase 1: There are many different influenza viruses out there in nature circulating among animals and birds in particular. These viruses are potential threats, but only if they genetically change in a way that enables them to infect humans. Alert Phase 1 means that influenza viruses circulating exclusively among animals have not yet been reported to cause illness in human populations.

• Pandemic Alert Phase 2: When Phase 2 is reached, this indicates that an animal influenza virus is known to have caused a new type of influenza infection in humans, and could potentially become a pandemic threat. Viruses can replicate quickly and in large numbers. Because of this, viral mutations are frequent. When the genes of different animal viruses recombine to form a new virus or when a mutation occurs in an existing animal virus that allows it to infect humans for the first time, this presents a brand new type of human influenza that the human immune system has no established resistance to.

• Pandemic Alert Phase 3: At this phase, an animal or human-animal influenza virus has caused small pockets of disease in human populations, but no sustained community-level outbreaks.

• Pandemic Alert Phase 4: This phase is reached when human-to-human transmission of an animal or human-animal influenza virus is able to cause sustained disease outbreaks in a community. The change from Phase 3 to Phase 4 represents a significant increase in the risk for a pandemic.

• Pandemic Alert Phase 5: This alert level means that the virus has spread from human to human in at least two countries within a WHO region. Reaching Phase 5 signifies high risk of an imminent pandemic and that there is little time left to organize, communicate, and implement measures for alleviating the threat.

• Pandemic Alert Phase 6: In this pandemic phase, there are community level outbreaks in another region in addition to the WHO region specified in Phase 5 criteria. Declaration of this phase indicates that a global pandemic is currently underway.

There are two additional periods specified in the WHO alert protocol, Post-Peak and Post-Pandemic. Post-peak, the pandemic disease levels in most countries have dropped below levels observed at the peak of the disease; the pandemic appears to be declining. However, in this stage it is still important to remain vigilant, since additional waves of the disease may yet occur. The Post-Pandemic period is characterized by the influenza outbreak returning to seasonal levels.

For more information on the World Health Organization’s Global Influenza Preparedness Plan, see the official website.

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