What's in the COVID-19 Vaccines?

Doctor filling syringe with vaccine dose.

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Key Takeaways

  • The Pfizer-BioNTech and Moderna vaccines are similar in that they both use mRNA technology to deliver a small amount of genetic information to trigger the body's immune response.
  • The ingredients for both vaccines are very similar.
  • Reports of allergic reactions to Pfizer’s COVID-19 vaccine have been made. Polyethylene glycol is the chemical suspected to have induced an allergic reaction. 

As the first doses of COVID-19 vaccines are administered across the country, a few reports of allergic reactions to Pfizer-BioNTech’s vaccine have raised some concerns for those who are prone to allergic reactions.

Despite these concerns, experts say the risk of experiencing an allergic reaction in response to the authorized vaccines is still low when taking into account the ingredients present in both. Here's what we know about the Pfizer and Moderna vaccine ingredients and their role in allergic reactions.

What This Means For You

If you have any questions about reactions and sensitivity to COVID-19 vaccines, contact your healthcare provider to discuss your allergy profile. Keeping a running list of all known allergies to have handy at your doctor’s office. 

What's in the Vaccine?

Pfizer-BioNTech and Moderna’s COVID-19 vaccines are both mRNA vaccines, which use a copy of a natural chemical called mRNA to provoke the body’s immune response. When the immune response is activated, it protects the body from acquiring an infection.

“The RNA is packaged in a similar manner in both vaccines, which requires the use of polyethylene glycol, the chemical suspected to induce allergic reactions in a few patients who had an allergic reaction to the Pfizer vaccine,” Sanjeev Jain, MD, PhD, board-certified allergist and immunologist at Columbia Allergy based on the West Coast, tells Verywell. 

While research determined polyethylene glycol (PEG)—a polymer or substance containing very large molecules—is safe for use, sensitivity is possible and can cause reactions.

People who are allergic to PEG or polysorbate (which is not in the vaccines but is related to PEG) should not get an mRNA vaccine, according to the Centers for Disease Control and Prevention (CDC).

Pfizer-BioNTech’s vaccine contains:

  • A nucleoside-modified messenger RNA (modRNA) encoding the viral spike glycoprotein of SARS-CoV-2
  • Lipids, or 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
  • Monobasic potassium phosphate
  • Sodium chloride (salt)
  • Dibasic sodium phosphate dihydrate 
  • Sucrose (sugar)

The Moderna vaccine contains similar ingredients such as:

  • Messenger ribonucleic acid (mRNA) encoding the spike glycoprotein of SARS-CoV-2
  • Lipids, or fatty substances, including: SM(sphyngomyelin)-102, Polyethylene glycol [PEG] 2000 dimyristoyl glycerol [DMG], 1,2-distearoyl-sn-glycero-3-phosphocholine [DSPC], and cholesterol
  • Tromethamine
  • Tromethamine hydrochloride
  • Acetic acid
  • Sodium acetate
  • Sucrose (sugar)

“The vaccine primarily contains salts and stabilizers in the forms of sugars and lipids, which don’t cause allergic reactions,” Jain says.  

Both vaccines are similar when it comes to ingredients. “The primary difference between the two is that the packaging of the RNA in the Moderna vaccine allows for storage in a regular freezer, compared to ultra-cold freezers required for the storage of the Pfizer vaccine,” Jain says.

Although allergic reactions to the vaccines are possible, Jain says the risk of a reaction to the current COVID-19 vaccines is fairly low. “Most allergic reactions can be attributed to the preservatives or the vial stoppers that are made with latex," he says. "The vaccine does not contain any of these ingredients."

Allergic Reactions

There is some concern that vaccines can cause anaphylaxis, a severe and potentially life-threatening allergic reaction. According to Jain and Lakiea Wright, MD, board-certified allergist and immunologist at Brigham and Women’s Hospital in Massachusetts, the COVID-19 vaccines are unlikely to trigger anaphylaxis.

The Vaccine Adverse Event Reporting System (VAERS) reported that there were 10 cases of anaphylaxis after about 4.04 million first doses of the Moderna vaccine (2.5 cases per million doses) and 21 cases of anaphylaxis after about 1.89 million first doses of the Pfizer-BioNTech COVID-19 (11.1 cases per million doses). All people with follow-up information available had recovered or been sent home from the hospital at the time of the reports, and no deaths were reported.

Although it is rare, other allergic reactions can still occur which can be mild to life-threatening. According to the CDC, people should receive the vaccination in a facility where anaphylaxis can be treated.

While the risk of allergic reactions to the COVID-19 vaccine is low, there is a possibility that a person could react differently. “Any time you take a medication, your body can perceive the medication as foreign, as a threat, and your immune system can mount a response which triggers your allergy cells to fire off,” Wright says.

Wright stresses that reports of allergic reactions to the Pfizer and Moderna COVID-19 vaccines do not mean that people with all types of allergies shouldn’t get the vaccine. A person’s eligibility for the vaccine depends on their specific allergy history. 

Managing Allergic Reactions

If you decide to get the COVID-19 vaccine, observe for any symptoms or allergic reactions.

A number of symptoms can all indicate a systemic reaction to the vaccine, including:

  • A tickle or clearing of the throat or a sensation of the throat closing
  • A high-pitched sound while breathing
  • Postnasal drainage
  • Sneezing
  • Coughing
  • Wheezing
  • Tight chest or shortness of breath
  • Abdominal pain or cramping
  • Hives
  • Itching
  • Swelling of the lips, face, or throat
  • Difficulty swallowing
  • Dizziness
  • Fainting
  • Fast heart rate
  • Nausea
  • Vomiting
  • Diarrhea

According to Wright, there are ways to treat allergic reactions, like antihistamines. "In some cases, it may be appropriate to treat with steroids," he adds. "For a severe allergic reaction, for example, anaphylaxis, the primary treatment is epinephrine."

According to the Food and Drug Administration (FDA), people who have had a severe allergic reaction after a previous dose of the vaccine or allergic reaction to any ingredient of the vaccine should not get the Pfizer and BioNTech vaccine at this time.

In addition, people with a history of multiple allergies, in general, may require additional monitoring, according to Wright. Monitoring may include a 30-minute observation post-vaccination. (The cases of anaphylaxis after the mRNA vaccines typically occurred within 15 minutes of vaccination.)

Plan Accordingly

If you are planning to take the COVID-19 vaccine, Wright recommends discussing your allergies with your healthcare provider. “If you have any concerns about past allergic reactions and risk for vaccination, make sure you discuss this in detail with your healthcare provider," Wright says.

If you experience symptoms away from the vaccination shot site, Jain states that you might be experiencing a systemic reaction. “It is a good idea to do a body inventory prior to your injection; make a mental note of any active allergy symptoms for the day,” Jain says. “This will best help the medical staff in determining any change in condition after the shot.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kayla Hui, MPH
Kayla Hui, MPH is the health and wellness ecommerce writer at Verywell Health.She earned her master's degree in public health from the Boston University School of Public Health and BA from the University of Wisconsin-Madison.