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About Influenza Pandemics by Centers for Disease
Control
Oct. 17, 2005 - The following information about flu
pandemics was prepared by the U.S. Centers for Disease Control.
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.
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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.
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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/.
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