October 14, 2021

What Is in the Flu Shot?

Flu season is here, and many people are wondering what is actually in the flu shot and if the flu shot is safe.

Magdalena Chavez, PA-C, a physician assistant with Healthcare Associates of Texas explains, “some of my patients may be hesitant to get the flu vaccine because they aren’t sure if it is safe.  First of all, I would reassure them that the flu vaccine is safe, in fact safe enough that I make sure my own family gets it every year.  If they have any specific concerns, we can talk about those in detail.  I hope that my patients, especially those I’ve worked with for a long time, know that I have their best interests in mind, and I would never recommend anything that I thought could hurt them.”

Read on to learn about what ingredients are in the flu shot, the effects they have on humans, and why it matters.


What Ingredients Are in the Flu Shot?

The flu shot is effective and safe, but what’s in the flu vaccine? The flu shot contains many ingredients that don’t directly create an immune response. Instead, they help support, augment, and stabilize the vaccination process. Here is a breakdown of what is in the flu vaccine.

Dead Flu Viruses

When you elect to get a flu shot, you can usually choose between several different options. All forms of the injectable flu shot have inactivated flu viruses. Scientists call these viruses hemagglutinin glycoproteins. Hemagglutinins teach your body how to build antibodies to a particular virus, which is the goal of any flu vaccine. The viruses are inactivated (dead). This means they cannot cause influenza illness, and you cannot get the flu from the flu shot. Research shows that hemagglutinin-based vaccines are safe and well-tolerated.


Egg Protein (Ovalbumin)

doctor doing research

One way to make flu vaccines is by growing flu viruses inside fertilized chicken eggs. Flu viruses that grow in chicken eggs have small amounts of egg protein (ovalbumin). The amount is usually less than one microgram per dose. Egg protein generally does not affect a person when injected. Yet, some people wonder whether people with severe egg allergies should receive an egg-based flu vaccine. The U.S. Centers for Disease Control and Prevention (CDC) recommends that people with severe egg allergies receive egg-based vaccines while being supervised by a medical professional. People with egg allergies who are not comfortable with an egg-based shot can also receive an egg-free version.

Aluminum salts

Aluminum salts are sometimes added to vaccines to boost a vaccinated individual’s immune response. It may sound strange to learn that aluminum is in a shot. But, aluminum is also in the food we eat, the water we drink, infant formulas, and health products such as antacids. Experts report that we eat 7 to 9 milligrams of aluminum every day. Vaccine makers have used aluminum salts (like aluminum hydroxide and aluminum phosphate) for more than 60 years. Aluminum is not in the Alfuria vaccine, which is the brand name for the one of the top flu shots for 2021-2022. If a manufacturer does use an aluminum salt in their flu shot, it is less than a single milligram.


Preservatives (Thimerosal)

A preservative known as thimerosal is sometimes used in flu vaccines to prevent contamination. Thimerosal has been used in this capacity for 90 years, according to the CDC. Thimerosal is an ethylmercury derivative, and it does contribute a small amount of mercury when injected. But, this form of mercury does not linger in the body or cause illness. In fact, ethylmercury is different from the other kind of mercury you may hear about that is in fish. This type of mercury does linger in the body or cause illness. In the Afluria quadrivalent vaccine, there is up to 24.5 mcg of ethylmercury present. But, experts note that this is quickly metabolized by the body.



Formaldehyde is a compound that inactivates the viruses in flu vaccines. It is found naturally in the environment and is also produced by your own cells. It is also found in industrial products, like pressed wood and glue, and is used as a preservative. In large amounts, formaldehyde has been linked to a risk of developing cancer. However, the FDA notes that the amount of formaldehyde in a vaccine (between 0.005 to 0.1 milligrams) is generally 50 to 70 times less than the amount of formaldehyde that is naturally produced by a newborn baby. No evidence has connected the small amounts of formaldehyde in vaccines to sinister outcomes like cancer.

“Formaldehyde sound like a scary word, but like many preservatives, we need to keep in mind that we’re ingesting some amount of these chemicals, including formaldehyde every day in our food and water.  In fact, we make much larger amounts of aldehyde within the body any time we drink a cocktail or beer or wine.  Just like with alcohol, some of these compounds may be harmful if taken in large amounts but the amount of formaldehyde in the flu shot is miniscule compared to the amount in some foods we eat regularly,” Chavez adds.


woman getting a vaccine preparedWhen you read the ingredient list of a flu vaccine, you may see the name of many different chemicals. These chemicals, such as sodium chloride, help to stabilize a vaccine so that it does not become altered during the storage process. If you are wondering if an ingredient such as sodium chloride is safe for injection, you may be familiar with its other name, which is table salt. The amounts of sodium chloride and other stabilizers (like monobasic sodium phosphate, dibasic sodium phosphate, monobasic potassium phosphate, potassium chloride, sucrose, and calcium chloride) that are in a flu shot are very small. These small amounts are safe for humans. The Afluria shot has 4.1 milligrams of sodium chloride, compared to the daily salt intake recommendation of 2300 milligrams. The Afluria shot also contains less than 10 micrograms of sucrose (sugar), which is less than a single grain of sugar.


The flu shot has a small amount of antibiotic medications. These prevent bacterial growth and contamination. In the Afluria shot, an antibiotic known as Neomycin sulfate is present at about 82 nanograms. This is much smaller than the milligram- and gram-sized doses of Neomycin used in other medical settings. A second antibiotic, known as Polymyxin B, is present in the Afluria flu shot at about 14 nanograms. This antibiotic medication is safely used in a much larger milligram form, too. This means a nanogram dose is especially small and insignificant.

“The bottom line is that the flu virus itself is far more dangerous that any part of the flu vaccine.  You may experience some injection site reaction or some fatigue or aches after the shot, but it is so minimal compared to what the infection causes in the body,” Chavez emphasizes.


Is the Flu Shot FDA-Approved?

If you are wondering if the flu shot is FDA approved, the answer is yes. The U.S. Food and Drug Administration (FDA) meets every year to choose which strains or variants of the influenza virus to include in the flu shot. They base their decision on data about which strains appear to be circulating widely in that year and thus causing the most infections.

For the 2021-2022 flu season, the FDA chose four viruses that will go in each type of flu shot (egg-based and non-egg). The shots are approved by the FDA. Each shot has a specific safety profile and effectiveness level.

Let’s use the example of the Afluria quadrivalent (four-virus) shot, which was purchased in the biggest quantity this season. Afluria is 60% effective in protecting against lab-confirmed infection with influenza in people ages 18 to 64. The FDA reviewed trial data to approve the shot. The data shows that people who received the shot most commonly felt muscle aches (experienced by 25% of people ages 18 to 64, and 12% of people older than age 65). There were no deaths that occurred within 180 days of people receiving the vaccine that were caused by the vaccine.

“I always recommend the flu shot for anyone who is able to get it, which is basically anyone over 6 months of age that hasn’t had a history of severe allergic reaction to the shot.  It’s safe and effective.  And while there is the possibility of breakthrough infection as with any vaccine, patients who are vaccinated tend to have a much shorter and milder course of illness and less risk of severe complication such as pneumonia or respiratory distress,” Chavez explains.


Is the Flu Shot Safe?

After learning about what does the flu shot contain, you may still have a lingering question about is the flu shot safe.

Chavez says, “The flu shot is absolutely safe!  It has been FDA approved for decades now and has been very successful in reducing the numbers of sick patients and even death from the influenza virus.”

The CDC monitors the safety of all flu vaccines and notes that flu vaccines have established a good safety record over the past 50 years.

Yet, just because they have a high safety profile does not mean rare complications cannot occur from flu shots. Rarely, conditions such as Guillain-Barre syndrome, do occur. Guillain-Barre syndrome is a neurologic disorder that can lead to fatal paralysis of the muscles that help a person breathe. It has been reported in 1 to 2 people per million doses of flu vaccine administered. While it is a small and unwanted risk of the flu shot, the CDC notes that most people with Guillain-Barre syndrome fully recover after a few weeks.


The information featured in this site is general in nature. The site provides health information designed to complement your personal health management. It does not provide medical advice or health services and is not meant to replace professional advice or imply coverage of specific clinical services or products. The inclusion of links to other web sites does not imply any endorsement of the material on such websites.