Causes of Pandemic Flu (Influenza)

Learn about the difference between seasonal and pandemic flu and the causes and impacts of pandemic flu.

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What is the difference between seasonal flu and pandemic flu?

Seasonal flu:
  • outbreaks happen every year in Canada, usually between November and April
  • is caused by one or more human flu viruses that are circulating in the community
  • some people may have immunity from past exposures and flu vaccination
Pandemic flu:
  • is rare (3 to 4 times per century) and can spread in multiple disease "waves" that are usually separated by several months.
  • is caused by a new ('novel') flu virus
  • humans have little or no immunity to the new virus because they have not been exposed to it before
  • can easily infect and spread between people, leading to a worldwide outbreak

What causes pandemic flu

There are 4 types of influenza viruses (A, B, C and D). Only influenza A viruses have caused influenza pandemics. Influenza A viruses are divided into subtypes based on combinations of the two proteins found on the virus that help it enter and exit human cells:

  1. Hemagglutinin (H)
    • 18 different known subtypes of H
  2. Neuraminidase (N)
    • 11 different known subtypes of N

Each influenza A strain is named according to its H and N subtypes (e.g. H1N1, H7N9, H5N1). It is possible to have any combination of H and N, but only certain ones have been known to infect humans.

Influenza A viruses are also found in animals including birds (avian flu) and pigs (swine flu). Wild birds, in particular, are natural carriers of influenza A viruses.

While most avian and swine flu viruses do not cause illness in humans, on rare occasions some avian and swine viruses have spread to people. These people had close contact with an infected bird or pig or its environment.

Influenza A viruses are constantly changing. Pandemic flu can happen when an animal influenza A virus (e.g. bird, pig) changes or mixes with a human flu virus to create a new flu virus strain that can easily infect humans and can be easily spread between humans.

Impacts of pandemic flu

The timing and impact of a flu pandemic is unpredictable. History has shown that flu pandemics generally occur three to four times per century. The pandemics of the last 100 years were in:

  • 1918-1919 (H1N1 "Spanish Flu")
  • 1957-1958 (H2N2 "Asian Flu")
  • 1968-1969 (H3N2 "Hong Kong Flu")
  • 2009-2010 (H1N1 commonly known as "Swine Flu")

Pandemic flu can cause severe illness (e.g. pneumonia) and death in people who were otherwise healthy.

It is estimated that 20 to 50 million people died worldwide during the Spanish Flu, and an estimated 1-4 million deaths occurred during the Asian Flu. During the 2009 H1N1 pandemic, it is estimated that:

  • between 100,000 and 400,000 people died worldwide, and
  • 70% of the deaths in Canada were in people younger than age 65.

Before a pandemic, it is impossible to know how many people will:

  • become ill
  • require hospitalization
  • die

The impact of a pandemic flu depends on several factors such as:

  • how easily the virus spreads
  • how sick it makes people
  • which groups of people (e.g. age, health, geographical location) are most affected

Not all novel influenza viruses turn into flu pandemics. Two avian influenza A viruses, H5N1 and H7N9, have sometimes made people sick in other parts of the world. The spread of these viruses between people has been very rare and does not happen easily.

There have been rare cases of novel avian influenza A virus infection in humans in Canada. Each of these cases involved a returning traveller who became infected with a novel virus while travelling in an affected part of the world. In all of these instances, there was no spread to other people in Canada.

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