The Truth About What's Really In The Flu Shot

The flu shot has been around for decades, but as ubiquitous as it is, what's actually in it isn't well known, so you might wonder what's actually in that flu shot that you've been getting every year, after all.

Since there are many different flu viruses, scientists take an educated guess at which ones are most likely to be circulating in the upcoming year, and include the four top contenders in the flu shot for that year. Since hindsight is 20/20 but foresight is not, the chances of choosing the correct strains of virus are a bit of a gamble, but during seasons when targeted strains match those that end up circulating, flu vaccines are around 40-60% effective at preventing illness and even more effective at preventing hospitalization and death (via CDC).

Infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security, told MSN Health, "Strains are chosen based on what is circulating in terms of flu viruses. Because the northern and southern hemispheres have opposite flu seasons, one may be used to influence the vaccine components of another."

Once virus strains are chosen, they are either killed and included in an "inactivated" vaccine, or weakened and used in a "live virus" vaccine nasal spray, according to Healthline. The flu shot contains "dead" influenza viruses, so it cannot cause the flu (via Medical News Today). But even when inactive, the viruses trigger the body's immune system to create antibodies to fight the illness, so it provides effective protection against the chosen strains. The CDC recommends that, with few exceptions, everyone six months and older receive the flu shot, especially those at risk of developing complications.

The vaccine nasal spray uses live viruses that are weakened, or attenuated (via CDC). These live forms of the influenza virus can't actually cause the flu, though. They are cold-adapted, meaning that they can only multiply in the nose where the temperature is cooler than the rest of the body. The viruses aren't able to multiply in warmer parts of the body, like the lungs. This form of the vaccine is approved for healthy, non-pregnant people who are between the ages of 2 and 49 years old. It's not recommended for some people, such as those younger than 2 or older than 50, those with weakened immune systems, those who have cochlear implants, and children ages 2 through 4 years old who have asthma.

Before the viruses go into the vaccine, they're grown in fertilized chicken eggs (via Medical News Today). The egg proteins help the viruses to replicate. Later, the viruses are separated from the egg and put into the vaccine. This means that the vaccine may contain small amounts of egg proteins.

However, it's still safe to get a flu shot if you're allergic to eggs, according to the Mayo Clinic. If you just have a mild allergy, there's generally no cause for concern, as the amount of egg proteins that may be present in the vaccine is so small. If you have a severe allergy to eggs, doctors may recommend that you get the shot in a medical setting where you can be monitored for allergic reactions for 30 minutes following the vaccination. There are currently two flu vaccines available that don't contain egg proteins, approved for use in those 18 years and older. Talk to your doctor if you're unsure which vaccination you can receive.

In order to inactivate the influenza viruses that are used in the vaccine injection, formaldehyde is used in the vaccine's production (via Healthline). Formaldehyde is a water-soluble gas that's found in many household products, from glue to pressed-wood furniture. And it's also produced right in our own bodies, during a process called the 'one carbon cycle' (via Science Daily). Formaldehyde is a by-product of this chemical reaction that's necessary for creating DNA and amino acids. Our bodies use an enzyme to then break it down into a less dangerous chemical called formate, and any DNA damage caused by the formaldehyde can be repaired with another enzyme.

While regular exposure to large amounts of formaldehyde is dangerous, the amount in the flu shot isn't anything to worry about, according to Healthline. Most is removed from the solution before the vaccine is packaged, so the amount that's left behind is even lower than the amount that naturally occurs in the body. The FDA has stated that there's no safety concern posed by formaldehyde in the flu shot and there's no evidence that it's been linked to cancer.

In order to prevent bacteria from contaminating the vaccine, preservatives and antibiotics are added to some vaccines in small doses (per Healthline). When multiple doses of a vaccine are stored in a single vial, a preservative called thimerosal is added to kill any bacteria or fungi in the vial.

This is a mercury-based preservative, so many experts agree that it should be reduced or eliminated in vaccines, according to the CDC. However, it contains a type of mercury called ethylmercury which is different from the type found in fish and is cleared from the body more quickly. Aside from relatively minor side effects like swelling and redness at the injection site, there is no evidence that the amount of thimerosal contained in vaccines is dangerous.

Small amounts of antibiotics are also necessary to prevent the spread of bacteria while a vaccine is being produced and stored (via Medical News Today). While some antibiotics like penicillin can cause severe reactions, the flu shot does not contain these types and instead contains other forms like gentamicin or neomycin, which are commonly found in topical medications and lotions.

Since vaccines can be exposed to heat and light during storage and transportation, they need stabilizers to prevent them from losing their potency, points out Healthline. The influenza vaccine uses sucrose, sorbitol, and monosodium glutamate (MSG) to keep it stable. Sucrose is simply table sugar, like the kind you'd add to your coffee, and sorbitol is an artificial sweetener present in chewing gum. While some people are sensitive to MSG, a flavor enhancer often found in processed foods, only a very small amount is present in the flu vaccine.

The vaccine also requires an emulsifier called polysorbate 80. This is the same substance used in sauces and salad dressings to keep them from separating. It's used in the flu shot to keep all the ingredients evenly distributed. In large doses, polysorbate 80 can cause reactions, but the amount found in the flu vaccine is very small.

Per Healthline, you should consult with your doctor if you have any concerns about whether the flu vaccine is right for you, especially if you have a weakened immune system, an allergy to any of the ingredients in the shot, or a history of Guillain-Barré syndrome.