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

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.

The Centers for Disease Control and Prevention (CDC) recommend that everyone 6 months of age and older, with a few exceptions, have a flu vaccine every year.

The CDC confirm that getting the vaccine is the best way to avoid getting the flu and spreading it to other people.

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. Nasal spray vaccines are approved for people aged 2–49 only.

In light of the ongoing COVID-19 pandemic, reducing the spread of respiratory illnesses, including the flu, is more important than ever.

This article looks at the various ingredients that flu shots contain, their function, and the safety of the vaccines.

Many vaccines for the flu and other viral infections contain similar ingredients. The purpose of each ingredient 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 effective, 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 trivalent 2020–2021 flu vaccine are:

  • the influenza A virus H1N1, also known as the Guangdong-Maonan strain
  • the influenza A virus H3N2, also known as the Hong Kong strain
  • an influenza B virus known as the Washington strain

A person can also get a quadrivalent, or four-component, vaccine that protects against an additional influenza B virus. In 2020–2021, this is one known as the Phuket strain.

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 inactivate toxins from viruses and bacteria that may contaminate the vaccine during production.

Aluminum salts

Aluminum salts are adjuvants — they help the body develop a stronger immune response against the virus in the vaccine. This allows scientists to include smaller amounts of the inactivated influenza viruses in these vaccines.

As with formaldehyde and most ingredients in flu shots, the amount of aluminum present is extremely small.

Aluminum salts are also in drinking water and various health products, such as antacids and antiperspirants. They are not always present in flu vaccines, some of which are aluminum-free.

Thimerosal

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

This ingredient is only present 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.

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 CDC say that people with egg allergies can receive the standard flu vaccine, but that those severe allergies should do so in a supervised medical setting.

Egg-free flu shots are also available.

Gelatin

Gelatin is present in the flu shot as a stabilizer — it keeps 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.

Most flu vaccines use pork-based gelatin as a stabilizer.

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 as gentamicin or neomycin, which is also an ingredient in many topical medications, such as lotions, ointments, and eye drops.

Receiving a flu vaccine has several benefits, 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.

As an example of the last point: The vaccine decreases the rate of major heart problems in people with heart disease. It also reduces the rate of hospitalizations in people with chronic lung disease and diabetes.

The CDC recommend that everyone 6 months and older receive the flu vaccine every year, though they also provide guidelines 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

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

A flu shot may cause slight flu-like symptoms, however. These usually appear soon after the shot and last 1–2 days. They can include:

The most common 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 hours 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.

Read about anti-vaccination myths here.

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.

The CDC recommend getting a flu shot in September or October, but getting one any time during flu season will help.

How and where people receive their flu shots may vary due to the COVID-19 pandemic. The CDC provide more information about finding a shot here.