2 people had allergic reactions to Pfizer's COVID-19 shot. Here's what's in it.

• Two people with known, severe allergies in the UK had non life-threatening anaphylactic reactions shortly after receiving Pfizer's vaccine.
• They might've been reacting to one of the vaccine ingredients, or the reaction could have just been their body's immune system stimulation.
• "Allergic people should not be disheartened by this news, because there are other vaccines in the pipeline," one allergist said. "And over time we will know more about the profile of these individuals who had a reaction, so that we can take the necessary precautions."
• Pfizer said people with common food allergies were not excluded from this study, "because there is no trace of nuts, eggs, or any food in our vaccine."

Two UK National Health Service workers had non life-threatening anaphylactic reactions shortly after receiving Pfizer's new coronavirus vaccine - but allergists say there's no reason to panic, or assume that people with severe allergies won't be able to get vaccinated against the coronavirus down the line.

Both of the healthcare workers who reacted to the shot in the UK had known, severe allergies, which required them to carry adrenaline (epinephrine) pens, the NHS told Insider in an email.

They are both recovering fine now, but the UK's Medicines and Healthcare products Regulatory Agency (MHRA) is advising "that people with a significant history of allergic reactions" refrain from getting Pfizer's shot, for now, as the matter is under investigation.

People with severe allergies were not in the vaccine trials

Certain vulnerable groups, including pregnant women and people with severe allergies, were excluded from the vaccine trials, making those two individuals in the UK, in all likelihood, a few of the very first highly allergic people to test out the new Pfizer shot.

"Patients or subjects with severe allergic reaction history have been excluded from the clinical trials, which is classic," Moncef Slaoui, the chief scientist in charge of Operation Warp Speed for distributing vaccines in the US, said during a press briefing on Wednesday.

"The expectation would be that subjects with known severe reaction, allergic reaction, should not take the vaccine until we understand exactly what happened here."

Other vulnerable groups, however, including older adults, as well as people with autoimmune diseases, and common conditions including hypertension, asthma, diabetes, HIV, hepatitis B and hepatitis C, were all included in Pfizer's vaccine studies, as were people with more routine allergies.

"There is no criteria in our protocol that excludes participants with food allergies, because there is no trace of nuts, eggs, or any food in our vaccine," a Pfizer representative told Insider in an email.

Pfizer's special ingredient, mRNA, is packed together with sugar, salt, and fats

The Pfizer vaccine is based on a new kind of technology, designed to get the body to build up defenses against the novel coronavirus on its own. So far, it's looking like a very effective injection, which works extremely well to prevent future COVID-19 cases in individuals who get it.

The key ingredient in the Pfizer shot is what's called messenger RNA (mRNA), which is, essentially, a genetic punching bag for the body to learn how to fight against the proteins that help COVID-19 invade our cells.

In order to get the vaccine into a form that can be injected into people's arm muscles, this mRNA is packaged up with other ingredients, into a whitish, sterile, preservative-free solution, according to the US Food and Drug Administration.

"The mRNA is packaged in a particle, and the particle has a lipid component and others salts and sugars," Dr. Sanjeev Jain, a board-certified allergist, immunologist, and CEO of Columbia Allergy told Insider.

He said that "none of those ingredients appear to be highly allergenic," stressing that most of the chemicals in the new shot are quite benign.

Here is the full list of ingredients in Pfizer's shot:

• A nucleoside-modified messenger RNA (modRNA) encoding the viral spike glycoprotein of SARS-CoV-2 (this is what makes the shot work)
• Lipids (i.e. fatty substances) including:
  • (4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate),
  • 2-[(polyethylene glycol)-2000]-N, N-ditetradecylacetamide,
  • 1,2-distearoyl-snglycero-3-phosphocholine,
  • and cholesterol
• Potassium chloride (in other words, potassium and chlorine)
• Monobasic potassium phosphate
• Sodium chloride (a.k.a salt)
• Dibasic sodium phosphate dihydrate
• Sucrose (yes, that's sugar)

Three possible explanations for the allergic reactions

"Salt and stabilizers and all those things don't cause allergic reactions," Jain said.

But, he does have two suspicions about what might be behind the allergy issues.

"One of the ingredients does have a polyethylene glycol component, and polyethylene glycol is a polymer that some people can react to, very rarely," he said.

The other thing he thinks might've happened is that the two individuals had "non-specific" mast cell reactions to the vaccine in their bodies, which means the vaccine particle itself reacted with "the machinery that causes allergic reaction" in the body.

Finally, it could be the case that these highly-allergic people might've had some allergy treatment, or exposure to an allergen recently, and that the combination of their immune system being triggered by that event, at the same time as it was stimulated by a vaccine, was enough to send it into overdrive.

"The fact that their immune system got stimulated by the vaccine, that could have triggered a reaction," Jain said.

"That's why when we give people allergy shots or any kind of allergy treatment, we tell them, 'don't get your shot on the same day that you're getting your flu vaccine,' because you can have a bad reaction from your allergy shot if you get it on the same day as your flu vaccine."

Jain said it would be a big mistake for everyone who has allergies to assume they won't be able to get a vaccine, though.

"Allergic people should not be disheartened by this news, because there are other vaccines in the pipeline," he said. "And over time we will know more about the profile of these individuals who had a reaction, so that we can take the necessary precautions."