Pandemic Flu

What Is a Pandemic Flu?

A pandemic flu is a global outbreak of disease that occurs when a new influenza A virus subtype 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 (the flu). Influenza viruses that already circulate among people cause seasonal outbreaks, while an outbreak of pandemic flu is 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 Does a Pandemic Flu Emerge?

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 flu 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 flu 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 flu (see Pandemic Stages). However, to cause a pandemic flu, the new virus subtype must also have the capacity to spread easily from person to person. Once a new pandemic flu virus emerges and spreads, it usually becomes established among people and moves around or "circulates" for many years as seasonal epidemics of influenza.

 

Pandemic Flu in Recent History

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. The pandemics included:

 

• 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 1918-1919 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 (see Spanish Flu).

 

• 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 spread to the United States by June 1957.

 

• 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 that contained 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 pandemic flu virus now appears to have been avian in origin.

 

 

Vaccines

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 is 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.

 

Influenza Medications

There are four different antiviral drugs (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 flu medicines may not always work, because strains of the influenza virus can become resistant to one or more of these medications. For example, the influenza A (H5N1) viruses that were identified in humans in Asia in 2004 and 2005 have been resistant to amantadine and rimantadine. However, researchers are still monitoring avian viruses for resistance to other antiviral medications.

 

The Impact of a Pandemic Flu

Many scientists believe it is only a matter of time until the next pandemic flu occurs. The severity of the next pandemic cannot be predicted, but modeling studies suggest that the impact of another pandemic flu 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 flu 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, 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 healthcare systems are currently planning.

 

A pandemic flu will last much longer than most public health emergencies, and may include "waves" of influenza activity separated by months of inactivity. In the 20th century pandemics, a second wave of influenza activity occurred 3 to 12 months after the first wave.

 

If another pandemic flu were to occur:

 

• The number of healthcare workers and first responders available to work can be expected to be reduced

 

• Healthcare workers and first responders will be at high risk of illness through exposure in the community and in healthcare 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 the flu.

 

Organizations Preparing for a Pandemic Flu

Because of these differences and the expected size of a pandemic flu, 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 following areas:

 

 

In May 2005, the secretary of Health and Human Services created a multi-agency National Influenza Pandemic Preparedness and Response Task Group. This unified initiative involves Centers for Disease Control and Prevention (CDC) and many other agencies (international, national, state, local, and private) in planning for a potential pandemic flu. Its responsibility includes revision of a U.S. National Pandemic Influenza Response and Preparedness Plan.