Flu Shot Facts:
How It’s Made

Flu Fact
Research shows that most flu strains begin in Asia and spread around the world on roughly the same course and timeframe each year.

Although the flu shot is usually given in the fall, the vaccine’s production begins the January before. The timeline is based on when the vaccine needs to reach the public. Because strains of the influenza virus mutate annually, a new vaccine must be produced each year.

A typical timeline

The Centers for Disease Control in Atlanta and the World Health Organization continually monitor the different strains of flu virus.

January – May

The Food and Drug Administration analyzes the data and decides which strains should be included in the new vaccine.
The CDC sends a small amount of frozen virus (called seed virus) to the FDA. The FDA tests it and then sends it to the vaccine’s manufacturers.
The virus is injected into fertilized chicken eggs and the eggs are incubated to allow the virus to grow. The virus grows in the egg white. The flu shot usually includes a few different strains of virus, and each strain is grown separately.
After several days, the eggs are opened and the egg whites are removed. The virus is then purified from the egg white and treated with a chemical that inactivates it, which is why it is almost impossible to get the flu from the vaccine.

June

The FDA tests the strains to make sure they are purified and that there will be enough virus in each dose to make the vaccine effective.

July

The different strains of virus are mixed into one vaccine and then the vaccine is packaged in cold storage for distribution.

August

The FDA licenses the final product and the vaccine is shipped.

October & November

People get their flu shots.

More about the flu shot: Should you get one?

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