JSYK: Flu vaccines are actually grown in eggs. 

By Korin Miller
October 15, 2019

It's officially flu season, and you know what that means: It's also flu shot myth season.

While every single person should be getting a flu shot every single year (unless you're medically unable to, because of age, allergies, or other medical exemptions), flu myths still scare lots of people off from getting the jab. (Quick FYI: You cannot get the flu from the flu shot, and you absolutely do need to get it each year).

But there's one myth out there that seems understandable, at least in theory: That people with egg allergies can't get the flu shot. While, again, that's also untrue, you might be wondering where that specific myth came from in the first place. Here's what to know about the link between egg allergies and flu shots. 

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So, what exactly do eggs have to do with the flu shot?

It's all about how the flu vaccine is made. The most common way is through an egg-based manufacturing process, which has been in use for more than 70 years, per the Centers for Disease Control and Prevention (CDC). To make the vaccines, candidate vaccine viruses (CVVs), or flu viruses that are prepared by the CDC or other public health organization, are grown in eggs, according to the Centers for Disease Control and Prevention (CDC). Those CVVs are then injected into fertilized hen’s eggs and incubated for several days to allow the viruses to replicate.

The fluid (which contains the virus) is harvested from the eggs. For flu shots, the flu viruses are then killed, broken up, and purified, the CDC explains. For the nasal spray vaccine, the starting CVVs are weakened viruses and go through a different production process.

Basically, "you can think of the egg as a test tube," says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. "Viruses have to grow in something living—you can’t just grow them on a Petri dish the way bacteria do.” And because those flu viruses are grown in eggs, "little traces of egg protein, although they are miniscule, are at least theoretically associated with the final vaccine product," he says.

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So, can you still get the flu shot if you’re allergic to eggs?

Yes, absolutely. “An egg allergy isn’t a problem for [the flu vaccine],” says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. “The amount of egg protein that’s in a vaccine isn’t enough to trigger a severe reaction.”

It’s been known for a long time that people who get any vaccine (not just the flu vaccine) can have an allergic reaction to some part of the vaccine, Dr. Schaffner says. “It was long thought that it might be due to the eggs, but in the last 10 years, there have been a number of studies by allergists that have shown that the traces of egg protein in flu vaccine are not the cause of these allergic reactions,” he says.

All people with egg allergies used to have to be observed for an allergic reaction for 30 minutes after they got the flu vaccine, the CDC says, but now the organization just recommends that people with a history of a severe reaction to egg (that is, any symptom other than hives) get vaccinated in an inpatient or outpatient medical setting, like a hospital, clinic, health department, or doctor’s office, under the supervision of a health care provider who is able to spot and manage severe allergic conditions.

There are egg-free flu vaccines available, like cell-based flu vaccines and recombinant flu vaccines, but “those are being made for different reasons, not because of people who are allergic to eggs,” Dr. Adalja says.

Overall, though, if you’re allergic to eggs, definitely still get your flu shot. “We need to put that whole [myth] to rest,” Dr. Schaffner says.

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