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We’ve all heard that we should get the flu vaccine each year, but what’s in it? And how does it work? Keep reading to find out.

Each year, it’s recommended that everyone age 6 months and older get a flu shot. Why? Because it’s the best way to protect yourself from the flu — helping to prevent serious illness, hospitalizations and even death.

Ever wonder how the flu shot works to provide this protection? Here’s the science behind the vaccine.

How does the flu shot work? 

“The vaccine works by introducing a small protein from the virus into your body. This causes your body to produce antibodies to fight the disease and creates some degree of immunity in case you’re faced with the real virus later,” explains Stanley Martin, MD, director of Geisinger’s Division of Infectious Diseases, adding that it takes about two weeks for the shot to be most effective, which is why it’s recommended that you get your vaccination in the fall, before flu season even begins. 

What if it’s later in the season and you still haven’t gotten your shot? Don’t worry — you’ve still got time. “Geisinger still offers flu shots through March,” says Dr. Martin. “And with COVID-19 continuing to spread, it’s once again important to be vaccinated against both viruses. You certainly don’t want to catch both at once. Protect yourself where you can.”

Flu Shot Science infographic

What’s in the flu vaccine?

Each year, researchers determine which strains of the flu virus are the most likely to occur during the upcoming season and the vaccines are designed accordingly. “Most flu vaccines protect against four types of viruses,” explains Dr. Martin. “An influenza A (H1N1) virus, an influenza A (H3N2) virus and two influenza B viruses.” 

The vaccine can be made from an inactivated virus — which means the particles have been grown in a culture and have lost their ability to cause disease. The vaccine can also be made from a recombinant virus — which means it’s been created synthetically and can't reproduce and cause disease. A third option is the live (but weakened) virus, which is used in the nasal spray vaccine. “All choices work equally well,” explains Dr. Martin, “but the live virus is not recommended for anyone with a weakened immune system, pregnant women or those with certain other medical conditions.” 

I’m allergic to eggs. Can I get a flu shot?

Most flu vaccines, whether they're administered through a shot in the arm or through a nasal spray, are made using an egg-based process. The virus is injected into fertilized chicken eggs, where it incubates and replicates for several days. This means a minute amount of egg protein may be in the vaccine. But the amount is so small it’s negligible to the vast majority of people with an egg allergy, and they can still tolerate the flu shot just fine. In fact, the flu shot has been given to many people with an egg allergy without any serious adverse reaction. Only in cases where someone has a history of severe anaphylaxis — a type of allergic reaction requiring hospitalization — in response to eggs should there be cause for concern. If you’re one of these few people, not to worry: There is a limited supply of egg-free flu vaccines for those with a demonstrated history of anaphylaxis.

Other ways of preventing the flu

“The flu vaccine is your best defense against catching the flu,” says Dr. Martin. “Other precautions to take are the same ones we’ve been following to protect against COVID-19. Avoid close contact with others, stay home when you’re sick, wear a face mask when you’re out in public and wash your hands often.”

Get your flu shot at Geisinger

Every year, we make it as easy as possible for you to get vaccinated against the flu. You can make an appointment, walk into a clinic or provider’s office or visit a drive-through location. The flu shot is your “best shot” against catching the flu. That’s why we do everything we can to keep you, and the rest of the community, protected. See all the ways to get your flu shot.

Next steps:

5 myths about this year’s flu season
Can you get the flu and COVID-19 at the same time? Find out.
4 reasons to get your flu shot

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