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Home » CDC proposes possible cause of COVID-19 vaccine allergic reactions

CDC proposes possible cause of COVID-19 vaccine allergic reactions

Last updated on September 9th, 2021 at 11:36 am

After the recent launch of the Pfizer and Moderna vaccines, some severe COVID-19 vaccine allergic reactions were observed in patients who received them. Allergic reactions are observed with all vaccines, with a rate of around 1 in 1 million. As of 19 December 2020, there have been six reported severe allergic reactions in 272,000 people who have received one of the vaccines.

In the Pfizer clinical trial data, the researchers observed that around 0.63% of the vaccinated group in phase II/III trials experienced a significant allergic reaction.

So does this mean that COVID-19 vaccine allergic reactions are a major concern about the safety of these vaccines? And do we know why we’re seeing them, even if very rarely? I think we can answer both questions in this article.

Are the COVID-19 vaccine allergic reactions serious?

Yes, allergic reactions to anything is serious, but they are generally treated quickly and are rarely life-threatening.

The CDC is recommending:

If you have ever had a severe allergic reaction to any ingredient in a COVID-19 vaccine, CDC recommends that you should not get that specific vaccine. If you have had a severe allergic reaction to other vaccines or injectable therapies, you should ask your doctor if you should get a COVID-19 vaccine. Your doctor will help you decide if it is safe for you to get vaccinated.

CDC recommends that people with a history of severe allergic reactions not related to vaccines or injectable medications—such as allergies to food, pet, venom, environmental, or latex—may still get vaccinated. People with a history of allergies to oral medications or a family history of severe allergic reactions, or who might have an milder allergy to vaccines (no anaphylaxis)—may also still get vaccinated.

If you have a severe allergic reaction after getting the first shot, you should not get the second shot. Your doctor may refer you to a specialist in allergies and immunology to provide more care or advice.

The CDC has also recommended certain safeguards to assure that anyone who has received the vaccine and may experience an allergic reaction:

  • All people who get a COVID-19 vaccine should be monitored on-site.
  • People with a history of severe allergic reactions should be monitored for 30 minutes after getting the vaccine. All other people should be monitored for 15 minutes after getting the vaccine.
  • Vaccination providers should have appropriate medications and equipment—such as epinephrine, antihistamines, stethoscopes, blood pressure cuffs, and timing devices to check your pulse—at all COVID-19 vaccination sites.
  • If you experience a severe allergic reaction after getting a COVID-19 vaccine, vaccination providers should provide rapid care and call for emergency medical services. You should continue to be monitored in a medical facility for at least several hours.

These safeguards will help provide assurance that healthcare workers can quickly treat the 1 out of 50-100 thousand people who might have a serious allergic reaction to the vaccine.

This does not mean that the vaccine is dangerous. Our medical system is adept at dealing with allergic reactions and people who are at risk of serious allergic reactions are well aware of their condition, and they are often prepared themselves.

It is not my intent to dismiss the seriousness of these adverse events, but allergic reactions occur frequently without this vaccine. Back to the Pfizer clinical trial, about 0.5% of those who received the placebo also had a serious allergic reaction to something. Over 99.99% of people who receive the vaccine are not going to have an allergic reaction.

So what might be causing this?

Well, we don’t actually know. Some scientists are speculating that it might be polyethylene glycol (PEG) which is a component of mRNA vaccines from Pfizer and Moderna. Polyethylene glycol is a small organic molecule that has numerous uses in medicine including as a laxative (if you’ve ever had a colonoscopy, you will be familiar with it) and as an excipient in many drugs to protect proteins from being cleared quickly from the bloodstream. It is also used in soaps and shampoos.

In the mRNA vaccines, the polyethylene glycol stabilizes the lipid nanoparticles that “surround” the mRNA fragments, keeping them from being destroyed by the body before they can enter the cell and induce the manufacturing of the S-protein antigen of the SARS-CoV-2 virus.

According to a recent report in Science:

Severe allergy-like reactions in at least eight people who received the COVID-19 vaccine produced by Pfizer and BioNTech over the past 2 weeks may be due to a compound in the packaging of the messenger RNA (mRNA) that forms the vaccine’s main ingredient, scientists say. A similar mRNA vaccine developed by Moderna, which was authorized for emergency use in the United States on Friday, also contains the compound, polyethylene glycol (PEG).

PEG has never been used before in an approved vaccine, but it is found in many drugs that have occasionally triggered anaphylaxis—a potentially life-threatening reaction that can cause rashes, a plummeting blood pressure, shortness of breath, and a fast heartbeat. Some allergists and immunologists believe a small number of people previously exposed to PEG may have high levels of antibodies against PEG, putting them at risk of an anaphylactic reaction to the vaccine.

PEG was considered to be biologically inert, but a recent study showed that as many as 72% of people have some antibodies against PEGs, although that does not mean that all of those individuals would be predisposed to being allergic to PEG. And of those individuals, about 7% may have a level that could predispose individuals to anaphylactic reactions.

Despite this, there are a lot of products with PEG on the market, and we’re just not observing that 7% of the population is experiencing an anaphylactic reaction after using it. It is ubiquitous in many different modern healthcare and cosmetic products.

But there’s more to this. The amount of PEG in the Moderna and Pfizer vaccines is several orders of magnitude lower than in almost any other PEG-containing healthcare product. PEG laxatives, like those used in over-the-counter stool softeners, have doses in the level of around 20 g. The Pfizer and Moderna vaccines contain around 0.00005g of PEG per dose.

For those of you who seem to think if “a lot can hurt so can a little,” that betrays knowledge of how the immune system works and the basics of toxicology. It’s the dose that matters, and the amount of PEG that is in a single dose is in microgram levels. Unless someone is extremely sensitive to PEG, which they might have noticed after using a shampoo, or if they’re constipated, they’re probably not going to react to the PEG in these vaccines.


Let’s be clear about a few points about the COVID-19 vaccine allergic reactions:

  1. The risk of a severe allergic reaction after receiving the COVID-19 mRNA vaccines is extremely rare, in the range of 1 out 50-100 thousand doses. The risk of dying of COVID-19 is around 2 out of 100 cases, making the benefits of the vaccine far exceed this minor risk.
  2. We don’t know what causes the allergic reaction, although polyethylene glycol is heading the list of suspects. However, it’s hard to believe that the extremely small amount of PEG in these vaccines, compared to other common uses, would cause anything more than the handful of cases we’ve seen.
  3. This does not mean that the allergic reactions were overlooked in the clinical trials. They were observed, and the FDA has watching carefully.

This isn’t quite a myth about these COVID-19 vaccine allergic reactions since we are observing these extremely rare events, but it has not risen to even close the level to dispute the safety of them.



Michael Simpson

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