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The Pfizer-BioNTech COVID-19 vaccine is administered to a personal-support worker at the Ottawa Hospital, on Dec. 15, 2020, in Ottawa.Adrian Wyld/The Canadian Press

People with common allergies, such as to food, pollen or pets, are no more likely than the rest of the population to have a rare allergic reaction to the COVID-19 vaccine, according to experts.

Two U.S. health care workers had allergic reactions – one of them a serious reaction – after receiving the Pfizer-BioNTech COVID-19 vaccine this week, CBS News reported on Thursday. In Britain, two National Health Services workers had severe allergic reactions after receiving the vaccine earlier this month.

Health Canada has recommended that anyone who is allergic to any of the ingredients should not receive the vaccine. It also advised that anyone who has had a serious allergic reaction to another vaccine, drug or food to talk to their health professional beforehand.

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The likelihood of having an allergic reaction to any of the components of the vaccine, however, is extremely rare, said Tim Vander Leek, president of the Canadian Society of Allergy and Clinical Immunology.

“If anybody thinks that they may have a reason not to receive a vaccine like the Pfizer vaccine, I think it’s very important that they discuss that with their health care provider, as opposed to just avoiding it,” he said.

What caused the allergic reactions in the healthcare workers?

The answer is yet unknown. The allergen, or the substance that triggered the allergic reaction, in the two British cases is still under investigation, according to an e-mail from Britain’s Medicines and Healthcare products Regulatory Agency (MHRA).

“One theory is that it may have been the Polyethylene glycol (PEG), but this is not yet proven,” it said.

According to the BBC, both individuals have a history of serious allergies and both were reportedly recovering well the day after receiving the vaccine.

In the U.S. cases, both of which occurred at the same hospital in Juneau, Alaska, one male worker experienced eye puffiness, light-headedness and a scratchy throat minutes after receiving the vaccine on Wednesday, while a female worker showed signs of an anaphylactic reaction on Tuesday, including increased heart rate, shortness of breath, and a skin rash, CBS News reported. Both were treated, and their reactions were not life-threatening. The specific allergen was not reported.

What is polyethylene glycol?

Polyethylene glycol is a compound used in some medicines, cosmetics and foods, the MHRA said. This ingredient has been identified by Canada’s National Advisory Committee on Immunization as a potential allergen in the Pfizer-BioNTech vaccine.

It is unknown exactly how many people are allergic to PEG, since these allergies may not always be recognized, but research suggests it is extremely rare, Dr. Vander Leek said. For example, he noted in one review of all available medical literature between 1977 and 2016, only 37 cases of allergies to PEG were reported.

In a statement issued on Tuesday, the Canadian Society of Allergy and Clinical Immunology said PEG is found in multiple products that are tolerated safely on a daily basis by many individuals in Canada.

Like the Pfizer-BioNTech vaccine, the Moderna COVID-19 vaccine, which is expected to receive Health Canada approval soon, also contains PEG, so it is likely that the same recommendations – to not receive that vaccine if people have a known allergy to PEG or any other of the vaccine’s ingredients – will apply, Dr. Vander Leek said. However, he added, it is important to note that there are many different types of PEG molecules, and the molecules listed as ingredients in these two vaccines are different. Individuals with an allergy to one type of PEG may be able to tolerate another form of it, he said, noting this underscores the importance of consulting a health care provider.

What other ingredients are in the vaccine?

The medicinal ingredient in the Pfizer-BioNTech vaccine is mRNA. Besides PEG, other non-medical ingredients are as follows: ALC-0315 = ((4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate), 1,2-Distearoyl-sn-glycero-3-phosphocholine, cholesterol, dibasic sodium phosphate dihydrate, monobasic potassium phosphate, potassium chloride, sodium chloride, sucrose, and water for injection.

None of these other ingredients has been identified as a likely cause of allergy, Dr. Vander Leek said.

The Pfizer-BioNTech vaccine and some of the candidate COVID-19 vaccines have common reported side effects, including fevers and aches. How do you distinguish an allergic reaction from an expected side effect?

Common responses to vaccines, which can last a couple days to a week or two, are the result of your immune system getting activated, said Zainab Abdurrahman, a clinical immunologist and allergist and assistant clinical professor (adjunct) of pediatrics at McMaster University. A rise in body temperature, redness and swelling around the injection site, and a general sense of tiredness or feeling under the weather are all part of the immune system’s response to try to fight off and stop the spread of infection from what it perceives as a pathogen, Dr. Abdurrahman said.

“Those are all normal things that your body does when it is trying to respond to a threat. And we want it to think what you’re getting in the vaccine is a threat,” so that it can recognize and respond to the actual pathogen, she said.

An allergic reaction, by contrast, is not expected and occurs typically within the first few minutes to an hour of receiving a vaccine, Dr. Abdurrahman said.

This can involve rashes, breathing problems, swelling and a decrease in blood pressure.

Does having allergies to one substance make you more likely to be allergic to another?

People with asthma, for example, have a higher risk of environmental allergies, Dr. Abdurrahman said, but vaccine allergies are extremely rare. The chances of having anaphylaxis to a vaccine is about one in a million.

“It’s not so much that, [for example], having a peanut allergy is going to increase your risk of having a vaccine allergy; we don’t really see that,” she said.

While no COVID-19 vaccine is approved for children yet, there are trials underway to test them in adolescents. Are children more susceptible to allergies than adults?

There’s no reason to assume children will be more likely to have allergic reactions to the COVID-19 vaccine, Dr. Abdurrahman said.

Since children do typically experience more redness and fevers when they get other kinds of vaccines than adults, she says she expects it may be more common for them to have expected side effects, like localized pain, redness, swelling and high temperature. Still, there’s no knowing which vaccine children may eventually receive.

What are the options for people who have known allergies to the vaccine ingredients?

It may be possible for them to choose another vaccine that does not have the component to which they are allergic, when others become available, Dr. Vander Leek said. Alternatively, he said, there may be potential ways of desensitizing the body so that they can tolerate receiving it.

Though he noted it is yet unknown whether this could apply to the COVID-19 vaccine, desensitization, which involves gradually building up the dose from an extremely small amount, is used routinely when patients need to take life-saving medications to which they are allergic.

The initial COVID-19 vaccinations in Canada and around the world raise questions about how people react to the shot, how pregnant women should approach it and how far away herd immunity may be. Globe health reporter Kelly Grant and science reporter Ivan Semeniuk discuss the answers.

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