What's In The Flu Shot, Anyway? (2024)

Flu season is just around the corner, and that means it’s time to get you flu shot. But despite how easy it is to go into a local pharmacy and get vaccinated against influenza, questions about what goes on behind the scenes abound. What are the flu vaccine ingredients in 2023? Are there egg-free options? What strains of the flu does the vaccine protect against? And how important will it be to get your flu vaccine in 2023? This is what you need to know.

Why You Need a Flu Vaccine in 2023

We could be facing a tough 2023-2024 flu season, experts say. The flu season in the Southern Hemisphere, which lasts from about April to October, usually predicts what we in the Northern Hemisphere can expect come fall and winter. And during the ongoing Southern Hemisphere flu season this year, Australia is seeing a relatively high number of cases. However, according to several measures, the circulating strains of the flu do not appear to be more severe than those in other years, The Guardian reports.

Of note, children are being hit hard, with about 80% of those being hospitalized for the flu at sentinel surveillance sites in Australia being children, according to The Guardian. That’s not because the virus targets kids. Rather, vaccination rates for kids are low this year in the country — emphasizing how important it is to get your children their flu shot this season.

Additionally, we may see flu season spike earlier than normal this year, with a peak around November rather than January or February, according to Johns Hopkins. However, recommendations for the timing of getting your flu shot hasn’t changed; most people should get it in September or October. You should definitely be up-to-date by the end of October, though later is better than never. Unless you’re in the throes of the flu, it’s never too late to get the shot.

Although the flu has been around for some time and doesn’t invoke the same fear as COVID, it’s crucial to protect your family from it. “Even a normal flu year is still something to worry about,” Michael A. Ben-Aderet, M.D., an infectious disease physician at Cedars Sinai, told AARP. One-hundred ninety-nine kids were reported to the Centers for Disease Control and Prevention to have died from flu during the 2018-2019 flu season alone, and statistical modelling suggests that 434 children may have actually died from the flu that season. The flu kills thousands of adults aged 18 to 49 each year (an estimated 2,450 during the 2018-2019 flu season) and sends tens of thousands to the hospital (66,869 in the 2018-2019 season).

The flu shot isn’t perfect, true, but it does greatly reduce your risk of getting the flu — and, if you do get the flu, of getting seriously sick with it. Flu vaccines usually have an effectiveness between about 40% and 60%, which is a lot more than nothing.

What’s in the Flu Shot?

Every year, scientists around the world do their best to get one step ahead of the flu by developing that year’s iteration of the flu shot. As a reminder, vaccines work by giving your body a chance to fight off an altered version of a virus or bacteria, so that if and when it encounters the live virus in the wild, it already knows how to react, and you never get sick. But what’s in a flu shot is a little more complicated.

The recipe starts with the four most common influenza strains from around the world, injected into fertilized chicken eggs or mammalian cells, deactivated so it doesn’t give you the actual flu, mixed with a grab-bag of preservatives, antibiotics, and sugars. This combination is then formulated for a shot or spray to make it in time for the 2023-2024 flu season. For those science-is-f*cking-awesome types out there, this is indeed awesome.

It’s also complex as hell — something that keeps virologists on their toes every year. Influenza strains constantly mutate, but scientists get one shot at the annual vaccine, making their best guess some 30 weeks in advance to get the flu shot out to the public.

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Flu Vaccine Ingredients 2023: The Strains

Every year, vaccine developers take virus samples from labs across the world and mix and match them. This year’s vaccine relies on four viruses — this is called a “quadrivalent” vaccine. The four viruses in the vaccine are somewhat different for the three different types of flu vaccines, which are egg-based (the usual vaccine type), recombinant, and egg-free.

According to the FDA, those viruses are…


First, let’s break down the terminology: “A” refers to the type of influenza that infects birds, humans, pigs, horses, seals, and dogs; H#N# refers to the different proteins found in the outer shell of the virus (hemagglutinin and neuraminidase); “pdm” is short for “pandemic” (remember, these are grown to mimic once-live viruses that did some harm); and “09” is the year of said pandemic (the 2009 virus accounted for some 203,000 deaths with a higher-than-normal population of children dying).

This year’s A(H1N1)pdm09 component is different for flu vaccines that are egg-based compared to those that are cell-based and recombinant-based.

For egg-based vaccines, the component changed compared to last year; it now contains the A/Victoria/4897/2022 (H1N1)pdm09-like virus, compared to last year’s A/Victoria/2570/2019(H1N1)pdm09-like virus. The new strains refers to a flu strain akin to the one seen in the 2009 pandemic that was created in 2022 in a lab in Victoria.

For cell-based and recombinant vaccines, the component is also different from last year. It is now A/Wisconsin/67/2022 (H1N1)pdm09-like virus, compared to last year’s A/Wisconsin/588/2019 (H1N1)pdm09-like virus.


The second component is a variant of the 2009 H1N1 swine flu. H3N2 was first found in pigs in 2010, then in humans in 2011. The biggest human outbreak was in 2012 with some 309 reported cases.

This year’s H3N2 vaccine component for the Northern Hemisphere is the same as last year’s: an A/Darwin/9/2021 (H3N2)-like virus in the egg-based vaccine and an A/Darwin/6/2021 (H3N2)-like virus in the cell-based and recombinant vaccines.

Influenza B (x 2)

Influenza B viruses tend to be the non-pandemic variety because they spread primarily among humans alone. They’re slower to mutate than Influenza A viruses, but are just as infectious among humans.

Both of the Influenza B viruses in the 2023-2024 flu vaccine, used in all three vaccine types, are the same as those used during the previous flu season in the Northern Hemisphere: B/phu*ket/3073/2013 (B Yamagata lineage)-like virus and B/Austria/1359417/2021-like virus (B/Victoria lineage).

Evergreen Flu Vaccine Ingredients: The Preservatives and Additives

Beyond the three to four viral components, a number of additives and preservatives are required to make vaccines effective — and to keep them from going bad. These ingredients, sometimes covered as trade secrets by drug companies in less public drugs, have led to many a conspiracy theory that anti-vaxxers would have you latch onto. It’s really much more boring than that.

Here are some of the ingredients you will find in the 2023-2024 flu vaccine — and why they’re there.

The Ingredient: Aluminum Salts

In: Most vaccines

Use: Boosts body’s response to the vaccine

The Ingredient: Sugar or gelatin

In: Most vaccines

Use: Preservative

The Ingredient: Formaldehyde

In: Most vaccines

Use: Kills viruses or inactivates toxins

The CDC says: “Formaldehyde is diluted during the vaccine manufacturing process, but residual quantities of formaldehyde may be found in some current vaccines. The amount of formaldehyde present in some vaccines is so small compared to the concentration that occurs naturally in the body that it does not pose a safety concern.”

The Ingredient: Antibiotics

In: Most vaccines

Use: Prevents bacterial contamination

The Ingredient: Thimerosal

In: Few flu vaccines; only multi-dose vials

Use: Preservative

The CDC says: “Thimerosal has a different form of mercury (ethylmercury) than the kind that causes mercury poisoning (methylmercury). It’s safe to use ethylmercury in vaccines because it’s processed differently in the body and it’s less likely to build up in the body — and because it’s used in tiny amounts. Even so, most vaccines do not have any thimerosal in them.”

The Ingredient: Egg proteins

In: Some vaccines

Use: Growing the vaccine

The CDC says: “Because influenza and yellow fever vaccines are both made in eggs, egg proteins are present in the final products. However, there are two new flu vaccines now available for people with egg allergies.”

Egg-Free Flu Vaccines in 2023

Most flu shots are created by injecting the above combination of ingredients into an egg, deactivating it, adding the other components, and shipping it. This creates what’s called an egg-based flu vaccine. The process has been around for decades. More recently, scientists have created more advanced methods, designed for people with egg allergies. These are the options:

Cell-Based. Unlike the regular egg-based flu shot, cell cultures are made without eggs. Instead, they’re made from mammalian cells. Previously, eggs would still be involved in the process because the four viruses used in the vaccine were created originally with the usual egg-injection process. Last year, however, the quadrivalent cell-based vaccine became cell-based from start to finish, so it’s completely egg-free.

Advantages: Flucelvax, the one cell-based flu vaccine approved for use in the U.S., is safe for people with egg allergies. Cell-based vaccines are fast to manufacture from soup to nuts (good for Spanish flu of 1912-type situations), and some independent studies have found that they’re 10% to 30% more effective than egg-based flu vaccines. It’s available for people 6 months and older.

Disadvantages: Cell-based flu vaccines have only been around since 2012, so they’re still the new kids on the block and aren’t available everywhere.

Recombinant. These vaccines don’t contain flu viruses themselves. Instead, they use proteins derived from the viruses. The proteins are combined with a different, non-flu virus that grows well in insect cells. The virus replicates, producing more of the protein that is then isolated and added to the vaccine.

Advantages: Flubok Quadrivalent, the one recombinant vaccine that is FDA-approved for the 2022-2023 season, is another completely egg-free vaccine for those with egg allergies. The recombinant flu vaccine manufacturing process, like with cell-based vaccines, is faster.

Disadvantages: These vaccines have a slightly shorter shelf life than most current flu vaccines, with expiration dates 9 months after the production date. They are only approved for patients 18 years or older.

New for the 2023-2024 flu season, the CDC says that people who have egg allergies may receive any flu vaccine — including the egg-based flu shot. In their own words: “People with egg-allergy may receive any flu vaccine (egg-based or non-egg based) that is otherwise appropriate for their age and health status. Additional safety measures are no longer recommended for flu vaccination beyond those recommended for receipt of any vaccine.”

Delivery of the Flu Shot in 2023

Not all flu shots are the same. Some aren’t even shots. (Let’s hear it for the nasal spray!) Here are your 2022 flu vaccine options:

Nasal Spray: The nasal spray vaccine is the only kind to include a live attenuated influenza vaccine. Although it can’t give you the flu, it does have a higher likelihood of inducing flu-like symptoms. This vaccine does not contain thimerosal or other preservatives. It’s available only for patients aged 2 to 49.

For: People who can’t stand the needle or jet spray or who want to avoid preservatives.

By Needle or Jet: Inactivated shots are usually given with a needle, but Afluria Quadrivalent can be given to adults with a jet injector, which is basically a high-powered spray that penetrates the skin.

For: Needles can be used for everyone aged 6 months and older. The jet spray is approved for adults aged 18 to 64.

2023 Flu Shot Dosages

Dosages are something you should put in the hands of your pediatrician. Of course, it doesn’t hurt to stay informed. Here are three takeaways.

  • Children 6 months through 35 months of age are recommended to receive 0.25 milliliters or 0.5 milliliters of inactivated vaccine per dose, depending on the type of vaccine.
  • Everyone aged 3 years and up should receive 0.5 ml, with the exception of adults aged 65 and up taking Fluzone High-Dose Quadrivalent, which is a relatively new vaccine with a dosage of 0.7 ml.
  • Children 6 months through 8 years of age need two doses if they have never received a flu vaccination before. These children should receive their first dose as early as possible in the flu season so that there’s time to get the second dose (which must be administered at least four weeks later) before the end of October.
  • People aged 65 and older are now recommended to take a high-dose flu vaccine for greater protection. These vaccines include Fluzone High-Dose Quadrivalent vaccine, Flublok Quadrivalent recombinant flu vaccine, and Fluad Quadrivalent adjuvanted flu vaccine.

Antivirals: What to Do When You Already Have the Flu

When you come down with the flu, antiviral medicine can shorten its duration. The FDA has approved six influenza antiviral drugs in the U.S. But they’re all in somewhat short supply, so you might not be given them if you’re not a child or elderly. This is why we get our flu shots, people!

The antiviral you probably already know goes by the brand name of Tamiflu, which you can get over the counter with relative ease even if you’re not young or elderly. This drug, along with two others that work in the same manner (brand names Rapivab and Relenza), block an enzyme the virus needs to replicate. They can shave up to a day off your illness, but they need multiple doses to keep the drug working. Tamiflu, for instance, requires patients to take it twice a day for five days.

The newest antiviral, Baloxavir marboxil (aka Xofluza), is a single-dose antiviral drug approved in 2018 by the FDA. Baloxavir is for people with basic flu who are 5 years or older and have had symptoms for less than 48 hours. In a phase 2 trial published by The New England Journal of Medicine, it shaved off upwards of 28 hours of flu symptoms (from 80.2 hours to 53.7 on average). This antiviral stands out in that it’s the only one that gets to the root of replication, messing with the virus’ RNA to stop it from reproducing. Also, it’s one of the only ones to come in a single dose, so you can pop it once and forget about it.

What's In The Flu Shot, Anyway? (2024)
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