April 29th, 2009
07:56 PM ET

What Pandemic Level 5 means

Caleb Hellerman
CNN Medical Unit Senior Producer

The current Pandemic Phase system of the WHO has only been in existence for five years. It was created in response to concern about the emergence of the H5N1 avian flu virus in Asia. Until this week, the Phase has never been higher than 3. (H5N1, while it carries an alarming fatality rate, does not spread easily from person to person.)

It’s safe to say that SARS would have rated at least a 4, and possibly a 5. More context: there have been four significant influenza pandemics since these things have been tracked: 1889, 1918, 1957 and 1968. Because of this pattern, many experts believe – generally – that we have been due for another one.

One thing concerning health officials is that in each of these four pandemics, a wave of relatively mild illness in the spring was followed by a wave of more virulent illness a few months later. Here are some casualty figures of past pandemics, from the CDC’s pandemicflu website.

1918: approximately 675,000 deaths from the flu occurred in the U.S. alone. 35-100 million worldwide. 1/3 world infected, 28 percent of U.S.

1957: 69,800 people in the U.S. died.

1968: The number of deaths between September 1968 and March 1969 for this pandemic was 33,800, making it the mildest pandemic in the 20th century.

And here's some information from the WHO web site:

A Chart from the WHO:

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.

This morning, Keiji Fukuda of WHO said they were waiting to be sure that the virus is established – is circulating within – more than two countries. (as opposed to popping up in travelers just off the plane)

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. 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.

Filed under: 360° Radar • 360º Follow • Health Care • Public Health
soundoff (7 Responses)
  1. Ann

    Swine Flu be related to indiviuals that have Diabetes? because theres so many people that has diabetes in mexico so it might be related, if it is then that means midkine is the most effective vaccine...

    April 29, 2009 at 10:04 pm |
  2. James

    How safe is it to travel by plane to another country like australia if the virus is treatable and there has been no cases in the area you are going or coming from? And can this said virus be transmitted by a mosquito?

    April 29, 2009 at 9:31 pm |
  3. Larry

    Its very important for CNN to provide perspective to this crisis and though I am thankful CNN airs all appropriate news conferences, the headlines and banners, particularly those on The Situation Room, are excessive, full of hyperbole and irresponsible.
    There was a headline this afternoon that suggested "humanity" was in peril as if someone had just set off a nuclear bomb over one of our cities. Everything is not breaking news all of the time. Please use restraint.

    April 29, 2009 at 9:30 pm |
  4. Chantal Meek

    If it is now at level 5 even though many cities have no virus, can we just keep our children out of school? What is the point of taking a chance in exposing children to this deadly virus? Also how did H1N1 come about? The only thing Swine, bird and human would have in common would be manure.

    April 29, 2009 at 9:08 pm |
  5. Annie Kate

    If the pandemic levels only go to 6 and we are at 5 what is the meaning of each level? Seems kind of early on to be at level 5 especially when you compare the numbers affected so far now with the numbers from these earlier pandemics.

    April 29, 2009 at 8:24 pm |
  6. Bryan

    History has shown that pandemics usually occur during times of stress and, in particular, civil hardship. Case-in-point, look at the impact of the wide-spread Eurasian conflict preceeding the Black Death, the Anglo-Dutch wars the 1665 London plague, and the effects of World War I on the Spanish Influenza of 1919. Given these historical examples, what possible outcome could the current global economy and ongoing border violence have on the possibility of this progressing into a pandemic?

    April 29, 2009 at 8:23 pm |
  7. Vera

    Could the Swine Flu be related to indiviuals that have Diabetes? In Mexico City 2.18 million have diabetes. Known fact people with Diabetes contract infections,it takes their body longer to heal. Could there be a connection?

    Thank You

    April 29, 2009 at 8:06 pm |