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Pandemic:
A Worldwide Outbreak of Influenza
An
influenza pandemic is a global outbreak of disease that
occurs when a new influenza A virus appears or “emerges”
in the human population, causes serious illness, and
then spreads easily from person to person worldwide.
Pandemics are different from seasonal outbreaks or “epidemics”
of influenza. Seasonal outbreaks are caused by subtypes
of influenza viruses that already circulate among people,
whereas pandemic outbreaks are caused by new subtypes,
by subtypes that have never circulated among people,
or by subtypes that have not circulated among people
for a long time. Past influenza pandemics have led to
high levels of illness, death, social disruption, and
economic loss.
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Appearance (Emergence) of Pandemic Influenza Viruses
There
are many different subtypes of Influenza or “flu”
viruses. The subtypes differ based upon certain proteins
on the surface of the virus (the hemagglutinin or “HA”
protein and the neuraminidase or the “NA”
protein).
Pandemic
viruses emerge as a result of a process called "antigenic
shift,” which causes an abrupt or sudden, major
change in influenza A viruses. These changes are caused
by new combinations of the HA and/or NA proteins on
the surface of the virus. Changes results in a new influenza
A virus subtype. The appearance of a new influenza A
virus subtype is the first step toward a pandemic; however,
to cause a pandemic, the new virus subtype also must
have the capacity to spread easily from person to person.
Once a new pandemic influenza virus emerges and spreads,
it usually becomes established among people and moves
around or “circulates” for many years as
seasonal epidemics of influenza. The U.S. Centers for
Disease Control and Prevention (CDC) and the World Health
Organization (WHO) have large surveillance programs
to monitor and detect influenza activity around the
world, including the emergence of possible pandemic
strains of influenza virus.
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Influenza Pandemics during the 20th Century
During
the 20th century, the emergence of several new influenza
A virus subtypes caused three pandemics, all of which
spread around the world within a year of being detected.
- 1918-19,
"Spanish flu," [A (H1N1)], caused the highest
number of known influenza deaths. (However, the actual
influenza virus subtype was not detected in the 1918-19
pandemic). More than 500,000 people died in the United
States , and up to 50 million people may have died
worldwide. Many people died within the first few days
after infection, and others died of secondary complications.
Nearly half of those who died were young, healthy
adults. Influenza A (H1N1) viruses still circulate
today after being introduced again into the human
population in 1977.
- 1957-58,
"Asian flu," [A (H2N2)], caused about 70,000
deaths in the United States . First identified in
China in late February 1957, the Asian flu spread
to the United States by June 1957.
- 1968-69,
" Hong Kong flu," [A (H3N2)], caused about
34,000 deaths in the United States . This virus was
first detected in Hong Kong in early 1968 and spread
to the United States later that year. Influenza A
(H3N2) viruses still circulate today.
Both the 1957-58 and 1968-69 pandemics were caused
by viruses containing a combination of genes from
a human influenza virus and an avian influenza virus.
The 1918-19 pandemic virus appears to have an avian
origin.
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Stages of a Pandemic
WHO
has developed a global influenza preparedness plan , which
defines the stages of a pandemic, outlines the role of WHO,
and makes recommendations for national measures before and
during a pandemic. The phases are:
Interpandemic
period
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 infection or disease is considered to
be low.
Phase
2: No new influenza virus subtypes have been detected
in humans. However, a circulating animal influenza virus
subtype poses a substantial risk of human disease.
Pandemic
alert period
Phase
3: Human infection(s) with a new subtype, but no
human-to-human spread, or at most rare instances of spread
to a close contact.
Phase
4: Small cluster(s) with limited human-to-human
transmission but spread is highly localized, suggesting
that the virus is not well adapted to humans.
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 be fully transmissible
(substantial pandemic risk).
Pandemic
period
Phase
6: Pandemic: increased and sustained transmission
in general population.
Notes:
The distinction between phases 1 and 2 is based on the risk
of human infection or disease resulting from circulating strains
in animals. The distinction is based on various factors and
their relative importance according to current scientific
knowledge. Factors may include pathogenicity in animals and
humans, occurrence in domesticated animals and livestock or
only in wildlife, whether the virus is enzootic or epizootic,
geographically localized or widespread, and other scientific
parameters.
The
distinction among phases 3 , 4, and is based on an assessment
of the risk of a pandemic. Various factors and their relative
importance according to current scientific knowledge may be
considered. Factors may include rate of transmission, geographical
location and spread, severity of illness, presence of genes
from human strains (if derived from an animal strain), and
other scientific parameters.
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Vaccines to Protect Against Pandemic Influenza Viruses
A
vaccine probably would not be available in the early stages
of a pandemic. When a new vaccine against an influenza virus
is being developed, scientists around the world work together
to select the virus strain that will offer the best protection
against that virus. Manufacturers then use the selected strain
to develop a vaccine. Once a potential pandemic strain of
influenza virus is identified, it takes several months before
a vaccine will be widely available. If a pandemic occurs,
the U.S. government will work with many partner groups to
make recommendations guiding the early use of available vaccine.
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Antiviral Medications to Prevent and Treat Pandemic
Influenza
Four
different influenza antiviral medications (amantadine, rimantadine,
oseltamivir, and zanamivir) are approved by the U.S. Food
and Drug Administration (FDA) for the treatment and/or prevention
of influenza. All four usually work against influenza A viruses.
However, the drugs may not always work, because influenza
virus strains can become resistant to one or more of these
medications. For example, the influenza A (H5N1) viruses identified
in human in Asia in 2004 and 2005 have been resistant to amantadine
and rimantadine. Monitoring of avian viruses for resistance
to influenza antiviral medications continues.
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Preparing for the Next Pandemic
Many
scientists believe it is only a matter of time until the next
influenza pandemic occurs. The severity of the next pandemic
cannot be predicted, but modeling studies suggest that the
impact of a pandemic on the United States could be substantial.
In the absence of any control measures (vaccination or drugs),
it has been estimated that in the United States a “medium–level”
pandemic could cause 89,000 to 207,000 deaths, 314,000 and
734,000 hospitalizations, 18 to 42 million outpatient visits,
and another 20 to 47 million people being sick. Between 15%
and 35% of the U.S. population could be affected by an influenza
pandemic, and the economic impact could range between $71.3
and $166.5 billion.
Influenza
pandemics are different from many of the threats for which
public health and health-care systems are currently planning:
- A
pandemic will last much longer than most public health emergencies
and may include “waves” of influenza activity
separated by months (in 20th century pandemics, a second
wave of influenza activity occurred 3 to 12 months after
the first wave).
- The
numbers of health-care workers and first responders available
to work can be expected to be reduced. They will be at high
risk of illness through exposure in the community and in
health-care settings, and some may have to miss work to
care for ill family members.
- Resources
in many locations could be limited, depending on the severity
and spread of an influenza pandemic.
Because
of these differences and the expected size of an influenza
pandemic, it is important to plan preparedness activities
that will permit a prompt and effective public health response.
The U.S. Department of Health and Human Services (HHS) supports
pandemic influenza activities in the areas of surveillance
(detection), vaccine development and production, strategic
stockpiling of antiviral medications, research, and risk communications.
In May 2005, the U.S. Secretary of HHS created a multi-agency
National Influenza Pandemic Preparedness and Response Task
Group. This unified initiative involves CDC and many other
agencies (international, national, state, local and private)
in planning for a potential pandemic. Its responsibility includes
revision of a U.S. National Pandemic Influenza Response and
Preparedness Plan.
Source:
www.cdc.gov |
