The science behind the flu shot

Dr. Stanley Martin, MD

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

Have you ever wondered 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. 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.

But 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. And this year, with COVID-19 to worry about, it’s more important than ever to get one. You certainly don’t want to catch both viruses at once. Protect yourself where you can.

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, 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 cannot 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, 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 are administered through a shot in the arm or through a nasal spray, are made using an egg-based manufacturing 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 over 4,000 patients 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. 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. To learn more about the flu shot or where to get one, visit Geisinger.org/flunews. 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.

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Dr. Stanley Martin is the director of infectious diseases division at Geisinger.


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