Influenza Pandemics
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 are already in existence among people, whereas
pandemic outbreaks are caused by new subtypes or by subtypes that have never circulated
among people or 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.
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 appear (or "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. This change results in a new influenza A virus subtype. The appearance
of a new influenza A virus subtype is the first step toward a pandemic, but the
new virus subtype also must spread easily from person to person to cause a pandemic.
Once a new pandemic influenza virus emerges and spreads, it normally 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 and the World
Health Organization have large surveillance programs to monitor and "detect" influenza
activity around the world, including the emergence of possible pandemic strains
of influenza virus.
Influenza
Pandemics During the 20th Century
During
the 20th century, the emergence of new influenza A virus subtypes caused three
pandemics, all of which spread around the world within 1 year of being detected.
- 1918-19, "Spanish
flu," [A (H1N1)], caused the highest number of known influenza deaths:
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 complications later. 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 the 1970s.
- 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 origin
of the 1918-19 pandemic virus is not clear.
Stages
of a Pandemic
The
World Health Organization (WHO) has developed a global
influenza preparedness plan, which defines
the stages of a pandemic, outlines WHO's role 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 phase 1 and phase
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/or other scientific parameters.
The
distinction between phase 3, phase 4 and
phase 5 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/or other scientific parameters.
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, and then manufacturers 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, it is expected that the U.S. government will work with many partner groups
to make recommendations to guide the early use of vaccine.
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 for the treatment
and/or prevention of influenza. All four work against influenza A viruses. However,
sometimes influenza virus strains can become resistant to one or more of these
drugs, and thus the drugs may not always work. For example, the influenza A (H5N1)
viruses identified in human patients in Asia in 2004 and 2005 have been resistant
to amantadine and rimantadine. Monitoring of avian viruses for resistance to influenza
antiviral medications is continuing.
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 its effect in the United States could be severe. 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,
between 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.
Recent
examples of avian influenza outbreaks and infections in Hong Kong in 1997, 1998,
and 2002 and the ongoing widespread outbreaks of avian influenza among poultry
in Asia, show the importance of preparing for a pandemic. It has been 36 years
since the last pandemic.
Influenza
pandemics are different from many of the threats for which public health and the
health-care system are currently planning:
- The
pandemic will last much longer than most other emergency events 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 because of how widespread an influenza pandemic would be.
Because
of these differences and the expected size of an influenza pandemic, it is important
to have completed planning and preparedness activities to be able to respond promptly
and adequately. For this reason, the U.S. Department of Health and Human Services
(HHS) supports pandemic influenza activities in the areas of surveillance ("detection"),
vaccine development and production, antiviral stockpiling, research, and public
health preparedness. In addition, a draft National Pandemic Influenza Preparedness
Plan was issued by HHS for public comment in August 2004. To view the draft plan
or to obtain more information about pandemic influenza, visit the HHS Web site
at http://www.dhhs.gov/nvpo/pandemics/
Centers
for Diseases Control and Prevention
http://www.cdc.gov/
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