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Ask the Experts

What’s in a Flu Shot?

by Tim Gower  

The main ingredients in most influenza vaccines are parts of the virus (called antigens) that provoke the immune system to make antibodies that guard against the flu. Traditional flu vaccines contain three inactivated viruses (trivalent vaccines), one each from the H1N1, H3N2, and influenza B families. Each type has many different subclasses with different antigens. The subclasses change from year to year, and scientists track viruses throughout the world to prepare vaccinations for the upcoming flu season.

Injections with four inactivated viruses (quadrivalent vaccines) are also available. They contain another strain of influenza B and offer broader protection, but they may be more expensive.

Traditional flu vaccines contain a trace of egg in which the viruses are harvested. They also contain substances called adjuvants, such as aluminum salts, which boost the immune system’s response to the vaccine. Other ingredients include preservatives and stabilizers.

Another type of flu shot, the recombinant flu vaccine (Flublok) doesn’t use the influenza virus or chicken eggs during manufacturing. Instead, a certain protein is isolated from a naturally occurring vaccine virus. These proteins are then combined with portions of another virus that grows well in insect cells. This “recombinant” vaccine virus is then mixed with insect cells and allowed to replicate. The flu protein is then harvested from these cells and purified.

The flu nasal spray, FluMist—which is not approved for people 50 and older—contains live attenuated (weakened) viruses that won’t cause influenza in otherwise healthy people.

This article first appeared in the October 2018 issue of UC Berkeley Health After 50.

Also see Flu Shot: Does Time of Day Matter?