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What is influenza?
What is avian influenza?
What is pandemic influenza?
What is seasonal influenza and how does it differ from pandemic influenza?
Why might pandemic influenza be a problem?

What is influenza?

Influenza (flu) is a contagious respiratory illness caused by an influenza virus. Human influenza viruses are spread from person-to-person when small, virus-laden respiratory secretions are released into the air during coughing, sneezing, or talking. Flu symptoms include:

fever (usually high)
headache
extreme tiredness
dry cough
sore throat
runny or stuffy nose
muscle aches
gastrointestinal symptoms such as nausea, vomiting, and diarrhea, but these symptoms are more common in children than in adults

Symptoms can range from mild to severe, and in some cases complications brought about by the flu will exacerbate other health problems and cause death.

What is avian influenza?

Avian (bird) influenza (caused by influenza A virus H5N1) is a virus that occurs naturally among wild birds but can be deadly to domestic fowl (domestic fowl have less immunity to this strain of the influenza A virus). In certain limited situations, it has been passed directly from infected birds to humans, but there have been no documented cases of subsequent person-to-person transmission. Avian influenza, specifically highly pathogenic avian influenza (HPAI), has the potential to become pandemic because it is especially virulent; it can be transmitted from birds to humans, and, if the virus evolves (or mutates), it could gain the capability to spread easily from one person to another. Humans have no immunity to this virus and no vaccine against it is available.

What is pandemic influenza?

Pandemic influenza occurs when a virulent human influenza virus causes worldwide flu outbreaks, often resulting in a massive number of casualties. At the time of this writing, there is no pandemic flu underway in the world. In contrast, seasonal influenza occurs every year and results from the circulation of one or two influenza A or B virus strains among susceptible human populations. In the northern hemisphere, seasonal influenza tends to occur between November and February.

Certain characteristics of modern society tend to increase the probability of pandemic influenza occurrences, including the rapid travel of large numbers of people, increasing urbanization, and environmental destruction or deterioration. Conversely, substantial advances in the biosciences and medicine have generated important knowledge about what viruses are and how they function, and this knowledge has been applied to develop better therapeutics and preventives. Health deliver systems therefore are in a better position than ever before to protect humanity from viral diseases with efficacious and safe vaccines, treat illnesses with a wide variety of therapeutics, and apply good hygienic practices.

What is seasonal influenza and how does it differ from pandemic influenza?

Seasonal influenza is caused by strains of influenza viruses that have been circulating among human populations for some time. These viruses may undergo genetic drift from year to year, which means that their genetic makeup is subject to minor modifications. Persons who have developed immunity to these viruses by either having been vaccinated or infected tend to retain much of that immunity, though it diminishes over time. However, two World Health Organization working groups (one each for the northern and southern hemispheres) decide each year how the annual vaccine should be composed in order to achieve maximum efficiency. The annual vaccine is of the trivalent type; i.e., it is composed of three components, which are designed to protect against two influenza A strains and one influenza B strain.

An influenza pandemic may at times be distinguishable from seasonal flu outbreaks (i.e., non-pandemic flu seasons) by the age distribution of the people who died from influenza-related illnesses and the sheer number of victims it generates. For example, during a non-pandemic flu season the highest number of casualties occur in persons over 65 years of age and very young children. However, during the 1968-1969 Asian influenza pandemic, persons under 65 years of age accounted for about half the number of influenza-related deaths in the U.S. Younger people usually have a much higher risk (sometimes 20-fold) for influenza-related mortality during a pandemic, while the elderly face similar risk during both pandemic and inter-pandemic years if the virus is of the same subtype. As for casualties, seasonal influenza usually causes between 30,000 and 40,000 excess deaths in the U.S. every year, but pandemic influenza may cause five to twenty times that number of deaths.

Why might pandemic influenza be a problem?

An influenza pandemic would be a problem for many reasons. A virus with pandemic capabilities is:

Highly virulent. People have little to no natural, or innate, immunity to a new virus.
Easily transmitted from person-to-person. Its spread will be aided by the modern habit of frequent and rapid travel.
Able to evolve resistance mechanisms to human defenses such as vaccines and antiviral drugs.

Pandemics are also unpredictable and have no periodicity or pattern. There is no way to know when or if a pandemic will occur, nor what age group it will affect the most.

Increased mortality during an influenza pandemic is due to more that just the symptoms of influenza and pneumonia (a common, and often serious, influenza-related illness). Cardiopulmonary and other chronic diseases may be exacerbated by the flu, which weakens the body's immune system. The numbers of casualties from influenza each year are likely underestimated because diagnoses may not take into account the effect influenza has on these other diseases.

The occurrence of a pandemic also results in significant economic losses and social disruption. Sick people do not work, so schools and businesses close, and basic services such as public transportation and food delivery are interrupted. Family members may decide to stay home and take care of their sick relatives, thus exposing themselves to the virus and becoming sick in turn. With high infection rates, health care facilities may be overwhelmed and have shortages in hospital beds, medicines, supplies, and staff. To address surge capacities (i.e., a health care system's ability to meet an increased demand for medical care in the event of a large-scale public health emergency or disaster, alternative sites will have to be converted to health care facilities to cope with the enlarged demand.

Furthermore, difficult decisions as to distribution will have to be made if vaccines and medications are scarce, which is possible especially at the beginning of a pandemic. Governments and health officials will have to make decisions on who are to be given priority when scarce vaccines and antiviral medications are disbursed.

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