What is the difference between a pandemic and a normal flu season?

Influenza viruses are constantly changing. They can change in two different ways: 

"Antigenic Drift"

One way they change is called “antigenic drift.” These are small changes in the genes of influenza viruses that happen continually over time as the virus replicates. But these small genetic changes can accumulate over time and result in viruses that are look different to the body’s immune system. When this happens, the body’s immune system may not recognize those viruses. That is why the flu vaccine composition must be reviewed each year and updated as needed to keep up with changing viruses.

"Antigenic Shift"

The other type of change is called “antigenic shift.” Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in a completely different virus that infects humans. Such a shift occurred in the spring of 2009, when an H1N1 virus with a new combination of genes emerged to infect people and quickly spread, causing a pandemic. When shift happens, most people have little or no protection against the new virus.


While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.

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1. What types of flu vaccine are available?
2. When does the flu season start? When does it end?
3. Can a flu shot give me the flu?
4. Is this year's vaccine a good match?
5. Why do we count pediatric flu deaths and not adult flu deaths?
6. What is the difference between a pandemic and a normal flu season?
7. When should I get vaccinated?
8. Does the flu vaccine work right away?
9. Does the flu shot wear off if I get it too early?
10. What protection does the flu vaccine provide if I do get sick with flu?
11. Why do I need a new flu shot every year?
12. Where can I get a flu vaccine?