Flu Shot Ingredients: What is in the Flu Vaccine?

By Sarah Malka, MD
Medically reviewed
October 25, 2021

The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend everyone older than six months get an influenza shot each year to protect against the flu, a viral infection that attacks the respiratory system and causes fevers, body aches, nausea, vomiting, cough and congestion, and can cause serious illness in some people. 

Every year, researchers create flu vaccines based on predictions about which four strains will be most likely to circulate that season.

While immunization doesn’t guarantee someone won’t get infected, evidence shows getting a flu shot can prevent severe illness, hospitalization, and death related to influenza.

When the viruses in the flu shot are similar to circulating flu viruses, the vaccine has been shown to reduce the risk of having to see a doctor by 40-60%.

Getting a flu shot is much more effective at preventing serious illness or death than taking a flu medication (oseltamivir/tamiflu), which have not been shown to prevent hospitalization or death.  

Have questions about the vaccine?

Talk to your healthcare provider about your concerns and learn more about the type of flu vaccine that’s best for you.

In this article, I’ll go over some basics about flu shots, including what ingredients are in the vaccines, as well as their side effects and benefits.

Finally, I’ll discuss when it’s best to talk to your doctor or another healthcare provider.

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Flu Shot Basics

Influenza, commonly called the flu, is a contagious viral illness that spreads through droplets when an infected person talks, sneezes, or coughs.

Flu symptoms such as cough, sore throat, runny nose, congestion, body aches, and fever can be mild.

But for some people, such as those with compromised immune systems, influenza can cause severe illness or death.

To protect yourself and others against the influenza virus, the CDC recommends an annual flu vaccine.

While the flu shot does not guarantee that you will not get the flu, it makes symptoms milder and protects against severe illness, hospitalization, or even death.

The flu vaccine works by introducing inactive versions of several flu strains to the immune system, so it can develop antibodies to fight against the infection later on.

U.S. flu shots are quadrivalent vaccines, which means they protect against four types of flu viruses: two influenza A viruses (including H1N1) and two influenza B viruses.

The strains included in the shot change each year based on research about which strains will be most likely to circulate that year.

In some years, flu shots are more precise, while in other years, they are less effective. 

Along with inactive versions of the influenza virus, flu shots contain several ingredients that ensure the vaccine’s safety and effectiveness.

Ingredients in the Flu Vaccine

Every ingredient in the influenza vaccine is included for a specific purpose and either helps provide immunity against influenza viruses, keeps the vaccine safe and long-lasting, or helps produce the vaccine.

Here are the ingredients found in most flu vaccines:

Preservatives 

Preservatives help protect the flu vaccine from getting contaminated with bacteria or fungus.

But they’re not used in every vaccine dose.

Preservatives are only used in vials of vaccines that contain more than one dose to prevent germs from entering the container when it’s open.

Many flu shots are also available in single-dose vials, which means they don’t contain preservatives.

Egg protein

Because the antigens in the flu vaccines are created by growing the flu virus inside of fertilized chicken eggs, they contain trace amounts of egg protein.

If you’re allergic to eggs, you should tell your healthcare provider, who can help you identify a vaccine that’s safe for you to take, such as Flucelvax, which is grown in animal cells instead of eggs.

Many people with milder egg allergies are able to take the flu vaccine without adverse reactions, but you should discuss this with your provider.

Stabilizers

Stabilizers, such as sugar or gelatin, help keep the flu vaccine effective after it is manufactured.

They prevent the active ingredients in the vaccine from changing in temperature shifts or other conditions during storage.

Formaldehyde

Found in many household products, formaldehyde is a gaseous natural compound that is used during the production of the vaccine to weaken or kill viruses, toxins, and bacteria that could be in the product.

It also helps to inactivate the flu virus found in vaccines.

Most formaldehyde is removed from vaccines in the manufacturing process.

If and when formaldehyde is in vaccines, the final product contains tiny amounts.

In fact, there’s actually more formaldehyde already in the human body than in the vaccine.

Animal protein

While most flu vaccines are produced by growing flu viruses in eggs, some types of flu shot are grown in cultured cells (proteins) of mammals instead.

Currently, Flucelvax Quadrivalent is the only cell-based flu shot approved by the Food and Drug Administration in the United States.

Other ingredients

Flu vaccines also contain trace amounts of antibiotics such as neomycin and gentamicin, which help stop bacteria from entering and contaminating the vaccine.

Polysorbate 80, an emulsifier that’s often used to keep sauce or dressing from separating, helps to keep vaccine ingredients evenly distributed and soluble.

It’s found in many vaccines, including those for HPV and Hepatitis B.

The vaccine also contains aluminum, an ingredient that increases the vaccine’s effectiveness by helping your immune system respond to the vaccine more strongly.

Flu Vaccine Side Effects

Any medication or vaccine can come with the possibility of side effects.

The most common flu shot side effects include: 

Usually, flu vaccine side effects are mild and go away in a few days on their own.

Some studies have found a very small link between Guillain-Barré Syndrome (GBS) and the flu shot, but the risk is estimated to be lower than 1 or 2 cases for every million vaccinated people.

GBS is more common following an actual influenza infection than the vaccine.

You may have heard that getting a flu shot can give you the flu.

But because the vaccine contains a dead version of the virus, it can’t give you the flu and it cannot make you contagious to others.

Your body may develop flu-like symptoms as it develops an immune response from the vaccine. This is not the flu, and is likely to be much milder than the actual illness caused by influenza.

Benefits of Flu Vaccine

While you can still get sick with the flu after getting a flu shot, scientific evidence suggests influenza vaccines should make your symptoms milder, and are effective in protecting against severe illness and hospitalization from the flu.

During the 2019-2020 flu season, researchers estimate vaccination prevented 7.5 million flu illnesses, 3.7 million flu-related medical visits, more than 100,000 hospitalizations, and 6,300 deaths.

The flu vaccine can protect anyone six months and older.

It’s especially important for people who are more likely to become severely ill if they get the flu—people with weaker immune systems, older adults, children, and pregnant individuals. 

Along with protecting you from potential severe illness, getting a flu shot can also protect other people from getting sick or being hospitalized. 

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When to See a Doctor or Healthcare Provider

If you want to get a flu shot, visit your primary care provider or a pharmacy that offers them.

Many local pharmacies offer walk-in appointments, but you may need an appointment at your doctor or healthcare provider’s office.

Tell your provider if you have severe egg allergies so they can pick a safe vaccine option.

If you think you’re having a severe allergic reaction from a flu vaccine, call 9-1-1 right away and go to the hospital.

After receiving medical care, you can report the reaction to the Vaccine Adverse Event Reporting System (VAERS)

How K Health Can Help

Did you know you can get affordable primary care with the K Health app?

Download K to check your symptoms, explore conditions and treatments, and if needed, text with a doctor in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

Is mercury in the flu vaccine?
Thimerosal, a preservative in the flu vaccine, is a form of ethylmercury, which is different from the type of mercury that causes mercury poisoning (methylmercury). It’s very unlikely to build up in the body and cause mercury poisoning, and it’s used in extremely small amounts. And it’s not in most vaccines: Single-dose flu shots and most other common vaccines do not contain this ingredient at all.
Do flu shots contain pig blood?
No, flu shots do not contain pig blood. Some vaccines contain porcine gelatin (gelatin from pigs) as a preservative.
Can vegans get the flu vaccine?
Because most types of flu vaccine are created in hen’s eggs, they could contain trace amounts of egg. Still, the CDC recommends everyone receive the flu shot to prevent the spread of illness and protect you from severe illness, hospitalization, and death. If you’re vegan and concerned about the vaccine, talk to your healthcare provider about receiving a version of the flu shot that doesn’t contain egg, such as Flucelvax.
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Sarah Malka, MD

Dr. Sarah Malka is a board certified emergency medicine physician with K Health. She completed her residency at Harvard Medical School.