May/June 2009
CDC Update: A Pandemic is Declared
On June 11, 2009, the World Health Organization (WHO) raised the worldwide pandemic alert level to Phase 6 in response to the ongoing global spread of the novel influenza A (H1N1) virus. A Phase 6 designation indicates that a global pandemic is underway.
More than 70 countries are now reporting cases of human infection with novel H1N1 flu. This number has been increasing over the past few weeks, but many of the cases reportedly had links to travel or were localized outbreaks without community spread. The WHO designation of a pandemic alert Phase 6 reflects the fact that there are now ongoing community level outbreaks in multiple parts of world.
WHO’s decision to raise the pandemic alert level to Phase 6 is a reflection of the spread of the virus, not the severity of illness caused by the virus. It’s uncertain at this time how serious or severe this novel H1N1 pandemic will be in terms of how many people infected will develop serious complications or die from novel H1N1 infection. Experience with this virus so far is limited and influenza is unpredictable. However, because novel H1N1 is a new virus, many people may have little or no immunity against it, and illness may be more severe and widespread as a result. In addition, currently there is no vaccine to protect against novel H1N1 virus.
WHO Pandemic Phases
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Interpandemic Period |
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Phase 1 |
No new influenza virus subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. If present in animals, the risk of human infections or disease is considered to be low. |
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Phase 2 |
No new influenza virus subtypes have been detected in humans. However, a circulation animal influenza virus subtype poses a substantial risk of human disease. |
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Pandemic Alert Period |
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Phase 3 |
Human infection(s) with a new subtype. No human to human spread or at most rare instances of spread to a close contact. |
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Phase 4 |
Small clusters(s) with limited human to human transmission. Spread is highly localized, suggesting that the virus is not well adapted to humans. |
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Phase 5 |
Larger cluster(s) but human to human spread still localized, suggesting that the virus is becoming increasingly better adapted to humans, but may not yet by fully transmissible (substantial pandemic risk). |
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Pandemic Period |
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Phase 6 |
Pandemic: increased and sustained transmission in general population. |
In the United States, most people who have become ill with the newly declared pandemic virus have recovered without requiring medical treatment, however, CDC anticipates that there will be more cases, more hospitalizations and more deaths associated with this pandemic in the coming days and weeks. In addition, this virus could cause significant illness with associated hospitalizations and deaths in the fall and winter during the U.S. influenza season.
| More on the WHO Pandemic Declaration |
Background
Novel influenza A
(H1N1) is a new flu virus of swine origin that was first detected in
April, 2009. The virus is infecting people and is spreading from
person-to-person, sparking a growing outbreak of illness in the United
States. An increasing number of cases are being reported internationally
as well.
It’s thought that novel influenza A (H1N1) flu spreads in the same way that regular seasonal influenza viruses spread; mainly through the coughs and sneezes of people who are sick with the virus.
It’s uncertain at this time how severe this novel H1N1 outbreak will be in terms of illness and death compared with other influenza viruses. Because this is a new virus, most people will not have immunity to it, and illness may be more severe and widespread as a result. In addition, currently there is no vaccine to protect against this novel H1N1 virus. CDC anticipates that there will be more cases, more hospitalizations and more deaths associated with this new virus in the coming days and weeks.
Novel influenza A (H1N1) activity is now being detected through CDC’s routine influenza surveillance systems and reported weekly in FluView. CDC tracks U.S. influenza activity through multiple systems across five categories. The fact that novel H1N1 activity is now detected through seasonal surveillance systems is an indication that there are higher levels of influenza-like illness in the United States than is normal for this time of year. Most of thel influenza viruses being detected now are novel H1N1 viruses.
Track Flu Activity (including novel H1N1 and seasonal flu) with CDC's Flu View - www.cdc.gov/flu/weekly
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