Influenza Pandemic: An Introduction
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.
Influenza Pandemic Versus Seasonal Outbreak
An influenza pandemic is different from a seasonal outbreak or "epidemic" of influenza. Seasonal outbreaks are caused by subtypes of influenza viruses that already circulate among people, whereas influenza pandemic outbreaks are caused by:
- New subtypes
- Subtypes that have never circulated among people
- 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.
How Influenza Pandemic Viruses Emerge in a Population
There are many different subtypes of influenza or "flu" viruses. The subtypes differ based upon certain proteins on the surface of the virus, which include the hemagglutinin (HA) protein and the neuraminidase (NA) protein.
Pandemic influenza 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. Such changes result in a new influenza A virus subtype. The appearance of a new influenza A virus subtype is the first step toward a pandemic
(see Pandemic Stages). However, to cause an influenza pandemic, the new virus subtype must also have the capacity to spread easily from person to person.
Once a new
influenza virus emerges and spreads, it usually becomes established among people and moves around, or "circulates," for many years as seasonal epidemics of influenza.
Influenza Pandemics in Recent History
During the 20th century, the emergence of several new
influenza A virus subtypes caused three influenza
pandemics, all of which spread around the world within a year of being detected.
1918-1919: The "Spanish Flu" Influenza Pandemic
The 1918-1919 "
Spanish flu" [A (H1N1)] caused the highest number of known influenza deaths. However, the actual
influenza virus subtype was not detected in the 1918 influenza 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.
1957-1958: The "Asian Flu" Influenza Pandemic
The 1957-1958 "Asian flu" [A (H2N2)] caused approximately 70,000 deaths in the United States. The Asian flu was first identified in China in late February 1957, and then spread to the United States by June 1957.
1968-1969: The "Hong Kong Flu" Influenza Pandemic
The 1968-1969 "Hong Kong flu" [A (H3N2)] caused approximately 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.
Viruses containing a combination of genes from a human influenza virus and an
avian influenza virus caused both the 1957-1958 and 1968-1969 pandemics. The 1918-1919 pandemic virus appears to have an avian origin as well.
Vaccines for an Influenza Pandemic
A vaccine probably would not be available in the early stages of an
influenza 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 an influenza pandemic occurs, the U.S. government will work with many partner groups to make recommendations guiding the early use of available vaccine.
Medications for an Influenza Pandemic
There are four different antiviral medications (
amantadine,
rimantadine,
oseltamivir, and
zanamivir) that are approved by the U.S. Food and Drug Administration (FDA) for the treatment and/or prevention of influenza. All four drugs 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 humans in Asia in 2004 and 2005 have been resistant to amantadine and rimantadine. Researchers are continuing to monitor avian viruses for resistance to other antiviral medications.
Possible Impact of an Influenza 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 an influenza 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 to 734,000 hospitalizations
- 18 to 42 million outpatient visits
- 20 to 47 million cases of illness.
Between 15 percent and 35 percent 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.
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