Breakdown of What’s in the Flu Shot

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The annual influenza vaccine, often called the “flu shot,” protects against the influenza viruses you’re most likely to be exposed to every flu season. With few exceptions, it’s safe and effective for most people ages 6 months and older.

The ingredients of the flu vaccine change each year, depending on the flu virus strains that are expected to be circulating and getting people sick. Reading about flu shot ingredients can be confusing.

This simple guide discusses each ingredient so you feel more at ease before flu season.

An illustration with what to know about the flu shot

Verywell / Theresa Chiechi

Flu Shot Ingredients

The flu shot is a quadrivalent vaccine. This means it protects against four different flu viruses: two influenza A viruses and two influenza B viruses.

Flu shots vary in terms of ingredients, manufacturing method, brand, and delivery. Let’s go over the types of flu shots and some of the typical flu shot ingredients.

Types of Flu Shots

There are two main versions of the flu shot, both of which use your body’s natural defenses to trigger an immune response. The two types are:

  • Inactivated influenza vaccines (IIV): Contain a dead, or inactivated, version of the influenza virus
  • Live attenuated influenza vaccines (LAIV): Contain a weakened, or attenuated, version of the virus

There are three ways the flu shot can be delivered:

  • Injection via needle, usually into the arm
  • Nasal spray
  • A jet injector that can penetrate the skin without a needle

All flu vaccines on the market meet the safety and efficacy requirements set by the Food and Drug Administration (FDA).

Egg Proteins

Egg-based flu vaccines are produced by growing inactivated versions of the flu virus in fertilized chicken eggs, where they replicate before being extracted and put into vaccines. This is the most common flu shot manufacturing process, and it’s been used safely for over 70 years.

Examples of standard-dose quadrivalent flu vaccines made with egg-based manufacturing methods include:

  • Afluria Quadrivalent
  • Fluarix Quadrivalent
  • FluLaval Quadrivalent
  • Fluzone Quadrivalent
  • FluMist Quadrivalent (nasal spray)

While most flu vaccines contain egg proteins, there are now two ways of manufacturing flu vaccines that don’t require any exposure to eggs:

  • Cell-based flu vaccines: Grown in cultured cells from mammals
  • Recombinant flu vaccines: Don’t contain egg proteins or the flu virus

There are currently two egg-free flu vaccines available in the United States:

  • Flublock Quadrivalent: A recombinant flu vaccine for adults ages 18 and older
  • Flucelvax Quadrivalent: A cell-based flu vaccine for people ages 6 months and older

Strains of the Flu Virus

Flu viruses change frequently, so the strains of the flu virus in the vaccine do as well.

According to the Centers for Disease Control and Prevention (CDC), flu shot composition is decided each year with the help of over 144 influenza centers in over 114 countries.

Laboratories at each center collect influenza surveillance data throughout the year and send virus samples to the World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza. The FDA makes the final determination on which strains to target in the U.S.

In the flu shot, the viruses are dead, so you can’t get the flu from the vaccine. Live attenuated vaccines contain a weakened version of the virus, so they’re also safe.


Some versions of the flu vaccine contain adjuvants, such as aluminum salts, which trigger the body to produce a stronger immune response. The amount of aluminum salts in the flu shot is very small. Aluminum salts are also commonly found in baby formula and drinking water.


Antibiotics, such as neomycin, are added to some flu vaccines to prevent bacterial contamination during the manufacturing and storage process. Antibiotics that commonly cause allergic reactions, such as penicillin, aren’t used by vaccine manufacturers.


Small amounts of preservatives such as thimerosal are added to multidose vials of the flu vaccine to prevent contamination by bacteria.

Because thimerosal is a mercury-based compound, some people worry it can cause mercury poisoning. But thimerosal contains only a tiny amount of ethylmercury—not methylmercury, the kind that typically causes damage. Still, if you’re concerned, most flu shots are mercury-free.


Stabilizers, such as gelatin or sugar (sucrose), help to protect vaccines from damage by heat or light after they’ve been manufactured.

Most of these ingredients occur in the body naturally.

Inactivating Ingredients

Inactivating ingredients, such as formaldehyde, are used to kill (“inactivate”) the influenza virus in the flu vaccine. Formaldehyde is removed from the vaccine after the manufacturing process, but trace amounts might still be present in the vaccine solution.

Excessive exposure to extreme amounts of formaldehyde over time may cause health problems, including certain cancers. However, formaldehyde is a natural compound that occurs in the body. The amount of formaldehyde found naturally in the body is much higher than the amount in any vaccine.

Formaldehyde in the Body

According to the FDA, the amount of formaldehyde in the average newborn’s body at just 6 to 8 pounds is already 50 to 70 times higher than the highest possible amount of formaldehyde in any vaccine.

What Does the Flu Shot Do?

By exposing you to a dead or weakened version of the flu virus, the flu shot causes your body to develop antibodies to the virus about two weeks after vaccination.

The antibodies can protect you from getting the flu. They can also protect you from developing severe influenza complications such as pneumonia, sinus infections, and ear infections if you do get sick.

The effectiveness of the flu shot can vary based on your age and health as well as how well the vaccine “matches” the flu viruses that are circulating in your area.

Still, the flu vaccine can reduce your risk of having to seek medical attention due to the flu by up to 40 percent to 60 percent.

Is the Flu Shot Safe?

The CDC recommends that all people ages 6 months and older get the flu shot every year, with few exceptions.

Generally, the flu shot is safe and effective.

Possible Side Effects

The most common flu shot side effects include:

  • Redness, swelling, or tenderness at the site of injection
  • Headache
  • Fever
  • Muscle soreness
  • Nausea

More rarely, the flu shot (like other vaccines) can cause dizziness or fainting. Some studies have shown extremely rare cases of Guillain-Barré syndrome (GBS) after the flu shot, while others have shown no association. The nasal spray has not been associated with any cases of GBS.

Additional side effects from the nasal spray flu vaccine can include:

Allergic Reactions

After getting any vaccine, you should watch for signs of a severe allergic reaction. These could include:

  • Difficulty breathing
  • Increased heart rate
  • Swelling, especially in the eyes or lips
  • Hives
  • Dizziness
  • Weakness

Severe allergic reactions to vaccines are rare. Seek immediate medical attention if you see any of these signs or symptoms a few minutes or hours after getting the flu shot, as they could be fatal.

Allergic Reactions After a Flu Shot

If you have previously had a severe, life-threatening allergic reaction to the flu shot or to any ingredients in the flu vaccine, you shouldn't get a flu shot.

Egg Allergies

If you have an egg allergy, it’s most likely still safe for you to get a flu shot. These are the CDC recommendations for people with a history of egg allergy:

  • If you’ve had only hives or other mild symptoms after exposure to eggs, you can still get any version of the flu vaccine.
  • If you’ve had more serious egg allergy symptoms, such as lightheadedness, vomiting, swelling, or respiratory distress, you should get the flu shot at an inpatient or outpatient medical setting under the supervision of a healthcare provider who can treat you for an allergic reaction.
  • If you’ve ever had a severe allergic reaction to the flu vaccine, you shouldn’t get the flu shot.

In Children

Most children ages 6 months old and up should get an annual flu vaccine. Side effects in children are typically as mild as those in adults.

The nasal spray flu vaccine is approved for most children and adults between the ages of 2 and 49. However, some children shouldn't get the nasal spray flu vaccine, including:

  • Children ages 2 to 17 who are taking medications containing aspirin or salicylate
  • Children ages 2 to 4 who have asthma or a recent history of wheezing
  • Children who are immunocompromised or immunosuppressed

In Adults 65 and Up

Adults ages 65 and up should get an annual flu shot. They shouldn't get the nasal spray flu vaccine.

The regular flu shot is safe and effective for older adults. However, vaccines don’t always prompt a strong immune response among people in this age group. There are two vaccines designed specifically for those ages 65 and older:

  • The high-dose flu vaccine (Fluzone High-Dose): Contains four times the amount of inactivated virus as a typical flu shot
  • The adjuvanted flu vaccine (Fluad Quadrivalent): Made with MF59 (an adjuvant that uses squalene) to trigger a stronger immune response

Flu Prevention

The flu shot is one of the best ways to protect yourself from the seasonal flu, but there are other steps you can take to stay as healthy as possible. Learn when to get the flu shot and other flu prevention tips.

When to Get Vaccinated

It’s a good idea to get the flu shot at the beginning of the flu season, usually September or October in the U.S.

If you aren’t able to get vaccinated by the end of October, don’t skip it; flu season typically peaks in February.

The CDC recommends early vaccination (usually in July or August) for children as well as for people in the third trimester of pregnancy. Adults, especially those aged 65 and older, should avoid early vaccination because protection from the vaccine wanes over time.

Hygiene Tips

In addition to the seasonal flu vaccine, these preventive healthy habits can help to protect you and your loved ones from the flu.

  • Stay home if you’re sick and avoid close contact with others who may be sick.
  • Wash your hands frequently with soap and water or alcohol-based hand sanitizer.
  • Disinfect surfaces that are frequently touched.
  • Cover your nose and mouth with a mask or tissue as often as possible, especially when you sneeze or cough.
  • Get plenty of sleep and practice other healthy habits such as getting regular exercise and eating a nutritious diet.


The flu vaccine is safe and effective for most people in protecting against four strains of the flu virus. It’s recommended that all people ages 6 months and older get a flu shot every year, ideally by October.

Some ingredients in the seasonal flu vaccine vary from year to year based on the strains of flu virus that are circulating the most in the community. Others, such as preservatives, stabilizers, adjuvants, antibiotics, and inactivating ingredients, are common in vaccines and have been extensively tested for safety and effectiveness.

A Word From Verywell

The flu shot has been used safely in the U.S. since the 1940s.

It can protect you from catching, spreading, or getting severe complications from the seasonal influenza virus. Talk with your healthcare provider if you have questions about flu shot ingredients or which version of the flu shot you should get.

Frequently Asked Questions

  • Where can you learn more about flu shot ingredients?

    You can learn more about flu shot ingredients from the CDC. The CDC releases detailed information about the composition of the seasonal flu vaccine each year. The Food and Drug Administration (FDA) also releases information about flu shot ingredients each year.

  • Does one flu shot protect against multiple types of influenza?

    Flu shots in the U.S. are quadrivalent vaccines. This means they protect against four flu viruses. These include two types of influenza A viruses and two influenza B viruses.

  • How long does the vaccine protect you from the flu?

    After you get a flu shot, it will take your body about two weeks to produce antibodies against the virus. After that, protection from the flu shot will last about six months. Because protection wanes over time, most older adults should wait until September or October to get their seasonal flu shot.

  • Which viruses are in the 2021-2022 flu shot?

    The 2021-2022 flu shot is a quadrivalent flu vaccine that protects against four different flu viruses. In the U.S., these include two influenza A viruses and two influenza B viruses.

    According to the FDA, the specific viruses are an A/Victoria/2570/2019 (H1N1) pdm09-like virus, an A/Cambodia/e0826360/2020 (H3N2)-like virus, a B/Washington/02/2019-like virus (B/Victoria lineage), and a B/Phuket/3073/2013-like virus (B/Yamagata lineage).

20 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Laura Dorwart
Laura Dorwart is a health journalist with particular interests in mental health, pregnancy-related conditions, and disability rights. She has published work in VICE, SELF, The New York Times, The Guardian, The Week, HuffPost, BuzzFeed Reader, Catapult, Pacific Standard,, Insider,, TalkPoverty, and many other outlets.