Flu Vaccine, What's in It?

 

 Flu shots contain tiny amounts of inactive, or dead, flu virus. They also contain other ingredients, such as stabilizers, preservatives, and substances that encourage an immune response.

The flu vaccine contains many ingredients, all of which are safe and necessary to keep flu vaccines shelf-stable and effective. The viruses that cause the flu, known as influenza viruses, are constantly changing. To ensure the flu vaccine remains effective, researchers and manufacturers work together to update the vaccine every year.

Different flu vaccines have slightly different ingredients. For instance, the vaccine may be:

• An injection: In this case, it usually contains tiny amounts of deactivated, and therefore not harmful, flu viruses.
• A nasal spray: In this case, it contains live viruses that have been weakened, and are therefore not harmful.  

What is in a flu shot and why?

Image credit: lechatnoir/Getty Images

Many vaccines for the flu and other viral infections contain similar ingredients. The purpose of each ingredientTrusted Source is either to make the vaccine effective or ensure that it is safe.

Many studies over the years have shown that flu vaccines are safe and effectiveTrusted Source, reducing flu cases and related hospitalizations.

Below, learn about seven ingredients in flu shots and the function of each:

Influenza viruses

Flu vaccines contain tiny amounts of the viruses that the vaccine protects against. 

In the shot, these viruses are inactivated, or dead, so they cannot cause the flu. The nasal spray contains live viruses, but they are weakened, or attenuated, so that they, too, cannot cause the flu.

The presence of these inactive viruses triggers the body’s natural defense mechanism — the immune system — which produces antibodies to fight these viruses. 

The body remembers, or stores, their appearance, so that it can quickly recognize any live versions of these viruses and create antibodies to fight them as well.

Traditional flu shots are trivalent, or three-component, vaccines. This means that they protect against three viruses: two influenza A viruses, H1N1 and H3N2, and one influenza B virus.

The specific viruses in an annual shot depend on which are likely to circulate during that year’s flu season. Researchers make this prediction.

The influenza viruses contained in the egg-based trivalent 2024 to 2025Trusted Source flu vaccine are:

• the influenza A virus H1N1, also known as the Victoria strain
• the influenza A virus H3N2, also known as the Thailand strain
• an influenza B virus known as the Austria strain

The viruses in the cell- or recombinant-based trivalent vaccines are:

• the influenza A virus H1N1, also known as the Wisconsin strain
• the influenza A virus H3N2, also known as the Massachusetts strain
• an influenza B virus known as the Austria strain

Flu resources

For more information and resources to help keep you and your loved ones healthy this flu season, visit our dedicated hub.

Formaldehyde

Formaldehyde, a chemical typically present in the human body, is a product of healthy digestive function.

In high doses, formaldehyde is toxic and potentially lethal. However, the tiny amounts present in flu vaccines are harmless.

Formaldehyde’s role in a flu shot is to inactivateTrusted Source toxins from viruses and bacteria that may contaminate the vaccine during production.

Thimerosal

Thimerosal is a preservative, and it keeps vaccines from becoming contaminated.

This ingredient is only presentTrusted Source in multi-dose vials, which contain more than one dose. Without it, the growth of bacteria and fungi are common in these vials.

Single-dose vials, prefilled syringes, and nasal sprays do not need a preservative, because the risk of contamination is so low.

Thimerosal has been safely included in vaccines since the 1930s. It comes from an organic form of mercury called ethylmercury, a safe compound that — unlike other forms of mercury — does not remain in the body.

Ethylmercury is different from the standard form of mercury that can cause illness in large doses, and it is also different from the mercury found in seafood, called methylmercury, which can stay in the body for years.

However, many health experts no longer recommend flu vaccines containing thimerosal. 

Chicken egg proteins

These proteins help the viruses grow before they go into the vaccine.

The inactivated influenza viruses present in vaccines are usually grown inside fertilized chicken eggs, where the virus replicates. Then, the manufacturers separate the virus from the egg and include it in the vaccine. 

As a result, the finished vaccine may contain small amounts of egg proteins.

The CDCTrusted Source advise that people with egg allergies can still receive egg based vaccines if they are appropriate for their age and health status. 

Gelatin

Some vaccines may include gelatine as a stabilizer. It helps keep the vaccine effective from the point of production to the moment of use. Stabilizers also help protect the vaccine from the damaging effects of heat or freeze-drying.

Currently, the only flu vaccine that contains gelatin is FluMistTrusted Source. As the vaccine uses such a low quantity of gelatin, it generally does not present any safety issues. However, it may not be suitable for people have gelatin-related allergies. 

Antibiotics

Antibiotics in flu vaccines keep bacteria from growing during the production and storage of the products.

Vaccines do not contain antibiotics that can cause severe reactions, such as penicillin. Instead, they contain other forms, such asTrusted Source kanamycin or neomycin, which is also an ingredient in many topical medications, such as lotions, ointments, and eye drops.

»MORE:Get an online treatment plan for your cold, flu, or sinus infection in as little as 15 minutes with Optum Now Online Care.

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Benefits of flu vaccines

Receiving a flu vaccine has several benefitsTrusted Source, including:

• Preventing the person and those around them from developing the flu.
• Reducing the risk of hospitalization, particularly among children and older adults.
• Protecting vulnerable groups, including babies, older people, and people with chronic diseases.
• Protecting people during and after pregnancy by reducing both the risk of flu-associated acute respiratory infections and the likelihood of the infant getting the flu.
• Preventing complications in people with chronic diseases.

• Who should avoid flu shots?

  The CDC recommend that everyone 6 months and older receive the flu vaccine every year, though they also provide guidelinesTrusted Source about who should either avoid the vaccine or take extra precautions.

  Age, current and past health status, and allergies to any ingredients in the flu vaccine are factors to consider.

  The following groups should not receive the flu vaccine or may require additional precautions:

  • infants under 6 months of age
  • people with severe allergies to any of the ingredients, such as gelatin or eggs
  • anyone who has had a severe allergic reaction to a previous flu shot
  • people who have had Guillain-Barré syndrome
  • people who are not feeling completely healthy

  Side effects of flu shots

  The flu vaccine cannot cause the fluTrusted Source because it contains either inactivated or weakened viruses that are no longer infectious or synthetic, lab-made variants.

  A flu shot may cause slight flu-like symptoms, however. These usually appear soon after the shot and may last a few days. They can include:

  • a headache
  • a fever
  • nausea
  • muscle aches

  The most commonTrusted Source side effect is a slight soreness or redness in the arm, at the site of the injection.

  In rare circumstances, the flu vaccine can cause serious side effects, such as allergic reactions. These usually occur within a few minutes to hoursTrusted Source after vaccination, and they are treatable.

  Many myths about vaccinations circulate — including that they weaken the immune system, cause autism, or contain unsafe toxins. These claims are not based on scientific evidence.

  »LEARN MORE:Debunking the anti-vaccination myths

  Summary

  Flu shots contain various ingredients that work together to ensure that the vaccine is safe and effective. The specific ingredients vary slightly among vaccines.

  Ingredients often include deactivated influenza viruses, chemicals that boost the body’s response to the vaccine, preservatives to prevent contamination, and stabilizers. 

  It is generally advisable to get a flu shot in September or October, but getting one any time during flu season will help.

  Medically reviewed by Angelica Balingit, MD — Written by Peter Morales-Brown — Updated on July 7, 2025