What Ingredients Are in the Flu Shot?
If you read the ingredient list of your average flu vaccine, you may notice words like formaldehyde, polysorbate 80, and thimerosal.
Some of these ingredients, such as thimerosal, have made news in recent years because of concerns that they may pose health risks.
Here's a rundown of the typical ingredients you'll find in a flu vaccine, and the real story behind those possible risks.
What's in a flu shot?
When you get a flu vaccine, you have two options:
Inactivated influenza vaccines that contain flu viruses that have been killed, so they can't cause the flu.
Live influenza vaccine (LAIV or FluMist) nasal sprays that contain a live but weakened form of the virus.
Here are some ingredients you'll find in the flu shot:
Egg protein
Many flu vaccines are made by growing the viruses inside fertilized chicken eggs. This means that they contain a small amount of egg protein.
A newer version of the vaccine, called Flucelvax, is grown in animal cells instead.
Preservatives
Vaccine manufacturers add the preservative thimerosal to multidose vaccine vials. Thimerosal helps prevent and kill dangerous bacteria and fungi in the vial.
Thimerosal contains mercury, which can be toxic in large doses. There isn't enough evidence to show the small amount contained in the flu vaccine is dangerous. But if you're concerned, thimerosal-free versions of the flu vaccine are available.
Stabilizers
Sucrose, sorbitol, and monosodium glutamate (MSG) are used to keep vaccines stable. They prevent vaccines from losing potency, even when exposed to heat and light.
Sucrose is the same table sugar you spoon into your coffee. Sorbitol is an artificial sweetener that's also found in chewing gum.
MSG is a flavor enhancer. Commonly thought of as an additive in Chinese food, it's used in many processed foods. Though certain people are sensitive to MSG, the amount found in the flu vaccine is very small.
Antibiotics
Neomycin, gentamicin, and other antibiotics are added to vaccines in very small amounts. They stop bacteria from contaminating the vaccine.
Polysorbate 80
This emulsifier prevents sauces and salad dressings from separating. In vaccines, polysorbate 80 keeps all the ingredients evenly distributed.
Though large doses can cause some people to have reactions, the amount in the flu vaccine is very small.
Formaldehyde
This natural compound is found in household products from glues and other adhesives to pressed-wood furniture. Formaldehyde is a gas that's soluble in water. It's used in the flu vaccine to inactivate influenza virus.
Routine exposure to large doses of formaldehyde is linked to eye and throat irritation, breathing trouble, and a higher risk for certain cancers.
However, according to the Centers for Disease Control and Prevention (CDC), most formaldehyde used in producing a vaccine undergoes removal from the vaccine solution before being packaged and sent to doctors and pharmacies.
Per the Food and Drug Administration (FDA), the level of formaldehyde that remains in a vaccine (such as the flu vaccine) is much lower than the amount that occurs naturally in the human body.
The residual amount of formaldehyde used in vaccines 'does not pose a safety concern,' and 'there is no evidence linking cancer to infrequent exposure to tiny amounts of formaldehyde via injection as occurs with vaccines.'
What are the side effects of the flu shot?
Most side effects from the flu vaccine are mild. People have reported symptoms like:
tenderness, redness, and swelling of the skin around the injection site
fever
fatigue
headache
Call your doctor or go to an emergency room right away if you have any of these more serious side effects, which may indicate an allergic reaction:
trouble breathing or wheezing
swelling of the eyes or lips
hives
weakness
fast heartbeat
dizziness
Benefits of the flu vaccine
A yearly flu vaccine is the single best way to prevent the flu and its complications.
Although vaccine effectiveness can vary from year to year, in general the vaccine can reduce doctor's visits and illness. During the 2023–2024 flu season, the flu shot prevented 9.8 million flu-related illnesses, 4.8 million medical visits, 120,000 hospitalizations, and 7,900 deaths.
The flu vaccine will lower your likelihood of getting sick. And if you do catch the flu, it's likely to be milder than if you weren't vaccinated.
The vaccine also prevents serious flu-related complications like:
pneumonia
bronchitis
asthma attacks
That's why it's especially important for young children, older adults, and anyone with chronic health conditions like chronic obstructive pulmonary disease (COPD), heart disease, and diabetes to get vaccinated.
Who should avoid the flu vaccine?
The flu vaccine is very effective, but it isn't right for everyone.
Don't get the vaccine if you've had a severe allergic reaction to any ingredient it contains, including egg protein.
You also should avoid the flu vaccine if you've had Guillain-Barré syndrome. In 1976, a swine flu vaccine was linked to an increased risk for Guillain-Barré, which causes the immune system to attack and damage the protective coating around nerve cells.
Guillain-Barré syndrome causes extreme weakness and tingling in the limbs, known as severe peripheral neuropathy. It can be life-threatening in rare cases.
There's no clear link between the current flu vaccine and Guillain-Barré. If any risk exists, it's very small, affecting about 1 out of every 1 million people vaccinated.
The vaccine also isn't recommended for babies under 6 months old because it hasn't been proven safe in infants.
Talk to your doctor if you have a weakened immune system, or if you take medicine to suppress your immune system. You may not respond as well to the vaccine.
If you're sick, you might want to put off the flu shot until you feel better.
Talking to your doctor
It's always a good idea to talk with your doctor, especially if you haven't gotten the flu vaccine before or if your health has changed. If you have an allergy or other condition that might make the vaccine risky for you, check with your doctor before getting vaccinated.
Here are a few questions to ask your doctor:
Is there any reason why I shouldn't get the flu vaccine?
What side effects might it cause?
What should I do if I have side effects?
Should I get the flu shot or nasal mist?
Optum Perks is owned by RVO Health. By clicking on this link, we may receive a commission. Learn more.
Outlook for flu vaccines
The flu vaccine is considered safe. You can't catch the flu from the vaccine, because the virus in the vaccine has been killed or weakened.
The live vaccine isn't recommended for people with weaker immune systems.
Preventing the flu
Getting the flu vaccine is one of the best ways to avoid the flu this season. Also try these other steps to protect yourself against the flu virus:
Wash your hands with warm water and soap, or use an alcohol-based hand sanitizer to kill germs throughout the day, especially before you eat.
Even if your hands are clean, keep them away from your eyes, nose, and mouth, which are entry routes for flu viruses and other germs.
Try to stay away from anyone who's sick.
If someone in your house catches the flu, disinfect any surfaces they touch, such as countertops and doorknobs.
Cover your nose and mouth whenever you sneeze. Cough and sneeze into your elbow to avoid contaminating your hands.
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