Influenza Pandemics: A Worldwide Outbreak of
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.
Appearance (Emergence) of Pandemic
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).
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
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.
Influenza Pandemics during the 20th
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
"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.
"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.
"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
influenza virus and an avian influenza virus. The 1918-19 pandemic virus appears
to have an avian origin.
Stages of a Pandemic
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:
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
Pandemic alert period
Human infection(s) with a new subtype, but no human-to-human spread, or at most
rare instances of spread to a close contact.
Small cluster(s) with limited human-to-human transmission but spread is highly
localized, suggesting that the virus is not well adapted to humans.
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: increased and sustained transmission in general population.
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
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
Vaccines to Protect Against Pandemic
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
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
Antiviral Medications to Prevent and
Treat Pandemic Influenza
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
and rimantadine. Monitoring of avian viruses for resistance to influenza
antiviral medications continues.
Preparing for the Next Pandemic
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.
pandemics are different from many of the threats for which public health and
health-care systems are currently planning:
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
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.
in many locations could be limited, depending on the severity and spread of an
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.
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