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The 2020-2021 Flu Shot: What’s in It and Why?

Everything you need to know about flu shot ingredients, varieties, and dosages.

The 2020-2021 flu season is unusual in many ways. That’s not to say it’s going to be particularly bad: Clues from the Southern Hemisphere, where the flu season is well underway, actu ally hint that it will be a mild year for influenza across the globe. But it will be exceedingly complicated. First of all, the mild flu season in the southern hemisphere is probably due in large part to masks, social distancing, and hand sanitizing. Second of all, coronavirus is still raging in the U.S. and if a flu season packs the hospital, the mortal dangers of both COVID-19 and the flu increase. Coming down with a cough and fever could also mean locking down in quarantine while you wait (probably way too long) for COVID-19 test results. It’s more crucial than ever to get your flu shot.  

This year, getting vaccinated for the flu will be a bit different too. For one, kids can get vaccinations and the flu shot at pharmacies, a new approval by the Trump administration. Many pharmacies can give adults the flu shot too. And it’s more important than ever to get vaccinated on time. August is too soon because protection might wear off by the heavy-hitting months later in the season. But September is ideal and you should definitely be up to date by the end of October, though later is better than never. There are also new and improved flu vaccine options for older adults, who are hit hard by both the flu and COVID-19. And the government may require some people to get flu shots, such as Massachusetts, which is mandating the vaccine for students.

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, 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 popular (slash effective) 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, and then formulated for a shot or spray to make it in time for the 2020 flu season. For those science-is-fucking-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. Fortunately, the CDC published its recommendations for the 2020-2021 flu shot on Friday, meaning it’s actually possible to understand what you’re having injected into yourself and your child (also, your parents if you can), wherever it is you get your flu shot. Another note: Unless you’re in the throes of the flu, it’s never too late to get the shot

2020-2021 Flu Shot Ingredients: The Strains

Every year, vaccines take virus samples from labs across the world and mix and match them. This year’s vaccine relies on four viruses (they call this a “quadrivalent” vaccine). Those viruses are… 


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, and the biggest human outbreak was in 2012 with some 309 reported cases. Going to a pig farm anytime soon? This one’s for you.

The CDC reports this year’s H3N2 vaccine component was updated from an A/Kansas/14/2017 (H3N2)-like virus to an A/Hong Kong/2671/2019 (H3N2)-like virus.

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, but still just as infectious among humans (and, apparently last season, harbor seals).

The CDC reports that the B/Yamagata virus component from the 2019-2020 flu vaccine remains the same for the 2020-2021 flu vaccine. This year’s B/Victoria vaccine component was updated from a B/Colorado/06/2017 (B/Victoria lineage)-like virus to a B/Washington/02/2019 (B/Victoria lineage)-like virus.


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). 

The CDC reports that this year’s A(H1N1)pdm09 component changed from A/Brisbane/02/2018 (H1N1)pdm09-like virus to A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus. That means a flu strain akin to the one seen in the 2009 pandemic that was created last year in a lab in the Maonan district of Guangdong is replacing the strain created in 2018 in Brisbane.

Evergreen Flu Shot 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 other, less-public drugs, have led to many a conspiracy theory that the likes of 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 2020-2021 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: Some vaccines; mostly 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.”

Preservative-Free, Egg-Free, and Other Flu Shot Formulations

Most flu shots are created by injecting the above combination of components into an egg, deactivating it, adding the other ingredients, and shipping it. This process has been around for decades, while more advanced methods were formulated very recently.

Cell-Based. Unlike the regular flu shot, cell cultures are made without eggs, using instead cells from mammals. This was not always an entirely egg-free vaccine because the four viruses used in it were created originally with the usual egg-injection process. This year, however, the quadrivalent cell-based vaccine is cell-based from start to finish, meaning it’s completely egg-free. 

Advantages: Flucelvax, the one cell-based flu vaccine approved for use in the U.S., is safe for anyone with egg allergies. Cell-based vaccines are faster to manufacture from soup to nuts (good for Spanish flu of 1912-type situations) and have been found in some independent studies to be 10 to 30% more effective.

Disadvantages: Flucelvax is for kids 4 and older, so toddlers will have to do with the regular shot. Furthermore, cell-based flu vaccines have been around since 2012, so they’re still the new kids on the block and can’t be found everywhere.

Recombinant. These vaccines don’t require the use of the flu virus. The recombinant flu vaccine has a slightly shorter shelf life than most other currently available injectable influenza vaccines. 

Advantages: Flubok Recombinant, the one recombinant vaccine that is FDA approved for the 2020-2021 season, is another completely egg-free vaccine for those with egg allergies. The recombinant flu vaccine manufacturing process, like cell-based vaccines, is faster to manufacture too.

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

The CDC says: “Persons with a history of egg allergy who have experienced only [hives] after exposure to egg should receive influenza vaccine. Any licensed, recommended influenza vaccine… that is otherwise appropriate for the recipient’s age and health status may be used. Persons who report having had reactions to egg involving symptoms other than [hives]… or who required epinephrine or another emergency medical intervention may similarly receive any licensed, recommended influenza vaccine… that is otherwise appropriate for their age and health status. If a vaccine other than [the cell-based or recombinant vaccine] is used, the selected vaccine should be administered in an inpatient or outpatient medical setting… Vaccine administration should be supervised by a health care provider who is able to recognize and manage severe allergic reactions.”

Flu Shot Delivery

Not all flu shots are the same. Some aren’t even shots (let’s hear it for the nasal spray!) Here’s your 2020-2021 flu options: .

Nasal Spray: The nasal-spray vaccine is the only kind to include a live attenuated influenza vaccine, meaning while 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 and is used only on patients aged 2 to 49.

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

By Needle or Jet: These inactivated shots are usually given with a needle, but Afluria Quadrivalent can be given to adults with a jet injector, basically, a high-powered spray that seeps through the skin.

For: Needles can be used for everyone 6 months and older, while the jet spray is approved this year for adults aged 18 to 64

Flu Shot Dosage & Ages

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

  • This year, 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 age 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 new vaccine introduced this year with a dosage of 0.7 ml.
  • Children 6 months through 8 years of age need 2 doses if they have never received a flu vaccination before. These children should receive their first dose as soon as possible so that there’s time to get the second dose (which must be administered at least 4 weeks later) before the end of October.

New Flu Shots for the 2020-2021 Season

There are two new flu shots available this year, and they are both designed for adults aged 65 and above. Older people generally have weaker immune responses to vaccines, so these special formulations help them get as much protection from the flu as possible. This is especially important in 2020 because older people face a heavy double burden from the flu and COVID-19.

Fluzone High-Dose Quadrivalent: This high-dose vaccine protects against four strains of the flu, replacing last year’s version which only protected against three. It is four times stronger than a normal vaccine to give older adults extra protection.

Fluad Quadrivalent: This new vaccine contains an extra ingredient called an adjuvant that promotes a stronger immune response to the flu vaccine. The ingredient, called adjuvant MF59, is a natural compound that can be made from vegetable oils or, more commonly, shark liver oil. An older version of this drug called Fluad protects against three flu strains, but this new drug vaccinates against four.

Antivirals: What to Do When You Already Have the Flu

When you get the flu, antivirals 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 individual. 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 OTC 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 and shave up to a day of your illness. These 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 last year by the FDA. Baloxavir is for people with basic flu who are 12 years and 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 is 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 that’s nice.