What is a pandemic - pandemic influenza?

Three Types Of Influenza:

1. Seasonal

Seasonal Influenza is a contagious respiratory illness caused by a virus. There are three types of influenza viruses: A, B and C. Influenza A (having different subtypes and strains) and B viruses (having different strains) cause seasonal epidemics of disease (when).  Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics. 

An epidemic occurs when the number of people who become infected rises well beyond what is expected within a country or a part of a country.

Seasonal influenza occurs each year, usually during the winter months, and affects from 5% to 20% of the population. The best protection against this illness is to get annual vaccine inoculation, and to practice cough and sneeze etiquette and hand washing.

2. From a new/novel virus (Antigenic Shift)

Influenza A viruses are found in many different animals, including ducks, chickens, geese, pigs, whales, horses and seals. 

Avian Influenza is found mostly in birds, both domestic and wild. Infection of humans is very rare; however, avian influenza viruses do sometimes change and develop an ability to infect humans and spread from person to person. 

Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptoms similar to humans, such as cough, fever and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus. 

This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. This new virus can cause illness in people and can be transmitted easily from person to person.

3. Pandemic

A pandemic is defined as an outbreak of global proportions – affecting a larger geographic area and infecting more people that an epidemic. It happens when a novel virus emerges among humans - it causes serious illness and is easily human transmissible (spreads easily from person-to-person).

Pandemic Influenza occurs when a new or novel strain of influenza A virus caused by antigenic shift infects humans and spreads readily from person to person, causing influenza pandemics in people. Humans possess little or no natural immunity to such a new strain. The symptoms – fever, cough, sore throat, body aches, headache, chills, fatigue and muscle pain – are similar to those seen with seasonal flu, but, in a severe pandemic, 30% or more of the population gets sick and more people suffer from complications (e.g. pneumonia, respiratory failure), or even die. A severe pandemic influenza virus would likely spread around the world in a matter of months.

From CDC and Medical News Today sources

For further information see:

World Health Organization - six phases

The World Health Organization (WHO) has identified six phases of a pandemic influenza, to enable agencies world-wide to identify and plan and prepare for phases leading up to the declaration of a pandemic, using common language. These phases are Inter-pandemic Period (two phases; Pandemic Alert Period (three phases); and Pandemic Period (one phase); followed by a Post-Pandemic Period (see chart below).

A pandemic is expected to come in waves, with as many as three waves over an 18-month period. Preparations need to consider the effects that several pandemic events may have on the community and congregation and ministry.

WHO Six Phases of Pandemic Influenza
Period     Phase Description
Inter-pandemic Period


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 is considered low.

2No 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 Period3Human infection(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.
4Small cluster(s) with limited human-to-human contact transmission but spread is highly localized, suggesting that the virus is not well adapted to humans.
5Larger 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 Period6Increased and sustained transmission in general population.
Post Pandemic PeriodReturn to Inter-Pandemic Period.