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How do influenza viruses change?
Influenza viruses have one characteristic that enables them to cause annual epidemics and even pandemics: the ability to change. Type A viruses often change their surface antigens or proteins. These changes can be minor – known as antigenic drift – or major – known as antigenic shift.

Antigenic drift: “ordinary” influenza
Antigenic drift occurs constantly among influenza A viruses, which is why we see new strains every year. Some annual influenza epidemics are worse than others. This happens when the new strains are very different from earlier strains. The more a strain differs from previous ones, the less immunity you will have to it.

Antigenic shift: pandemic influenza
Sometimes, major changes occur in the surface antigens (proteins) of influenza A viruses. Such changes can create a virus that is different from recently circulating strains, leading to a pandemic. People have very little or no immunity to it since they will not have been infected with it or vaccinated against it before. This lack of immunity allows the virus to spread more rapidly and more widely than an ordinary influenza virus.

How does antigenic shift occur?
Antigenic shift usually occurs in two ways: either as a sudden “adaptive” change when a normal virus reproduces, or from an exchange of genes between a human strain of influenza A virus and an animal strain. This genetic exchange or “re-assortment” produces a new virus capable of causing a pandemic in humans.

Genetic exchange occurs when an animal becomes infected with a human and an animal influenza virus at the same time – called a “co-infection.” The animal in which this genetic exchange takes place is often described as a “mixing vessel.” The domestic pig is a likely mixing vessel because it is open to both human and avian (bird) influenza. However, more recently experts fear that people may also serve as mixing vessels (see Figure 1).

Previous influenza pandemics
Influenza A viruses have undergone antigenic shift three times in the last century, resulting in pandemics with large numbers of both disease and death.

Pandemics during the last century

PANDEMIC	SPANISH FLU	ASIAN FLU	HONG KONG FLU
STRAIN	A(H1N1)	A(H2N2)	A(H3N2)
YEAR	1918-1919	1957-1958	1968-1969
Likely origin	Not known (first cases identified in Europe and USA)	China	China
Estimated deaths:
Global Canada	20 - 40 million 30,000 - 50,000	1 million 12,000 or more	1-4 million 12,000 or more
Age group most affected	Healthy young adults (20 - 50 years)	Very young and very old	Very old and those with underlying medical conditions

Can we use past pandemics to estimate the impact of future pandemics?
Most estimates of the impact of a future pandemic are based on information from previous pandemics. However, it is important to remember that important details of these events are still disputed, especially the true number of deaths.

Predictions based on previous pandemics need to take into account that the modern world is very different from 1918 with huge improvements in nutrition, healthcare and opportunities for interventions. It is important to understand that all impact predictions are estimates and that the actual impact of the next pandemic may turn out to be very different.

Where will the next pandemic influenza virus start?
While a new pandemic strain of the influenza virus could first emerge anywhere, including Canada, it is most likely to emerge in China and the Far East, as most previous pandemics appear to have done.

In this part of the world, dense human populations, domestic pigs and wild and domestic birds live close together, making it easy for the mingling of human and animal viruses through co-infecting, and the resulting genetic exchange that could give rise to a pandemic strain.

Key questions about pandemic influenza

Understanding pandemic influenza

Causes of pandemic influenza

	How do influenza viruses change?
	Antigenic drift: “ordinary influenza”
	Antigenic shift: pandemic influenza
	How does antigenic shift occur?
	Can we use past pandemics to estimate the impact of future pandemics?
	Where will the next pandemic influenza start?

The Nature of Avian Influenza

Controlling pandemic influenza

Vaccines

Antiviral drugs

Interventions

How will pandemic influenza affect Canada?

Canada, Alberta and the Calgary Health Region’s response to pandemic influenza

How does the plan work?

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