The Pfizer COVID-19 vaccine is here and the first wave of vaccinations is set to start across Canada next week.

The vaccine’s launch has been marred by hesitancies and concerns related to its adverse effects, however. While it’s been approved for use by Health Canada, recent allergic reaction incidents in the United Kingdom — which started vaccinating people this week — have caused concern. As have reports of clinical trial participants in the United States developing Bell’s palsy.

According to a recent poll, one in five Canadians are on the fence about getting vaccinated, and many would prefer to wait and see its effects before getting it themselves. 

But how worried should you actually be about allergic reactions or other adverse effects of the Pfizer vaccine? Here’s what you need to know.

What happened in the U.K.?

Earlier this week, the U.K. began vaccinating health-care workers with the recently approved Pfizer vaccine. However, two workers with a history of severe allergic reactions experienced anaphylaxis after receiving their shot. Anaphylaxis is a sudden onset of restricted breathing or drop in blood pressure that happens after exposure to a substance someone is allergic to.

Both workers were treated and have since recovered, according to British officials. 

Why do vaccines sometimes produce allergic reactions?

Dr. Supriya Sharma, a medical adviser with Health Canada, says researchers are currently looking into what part of the vaccine caused the allergic reaction. 

Vaccines are usually made of two major components — the mRNA, the active ingredient and a series of non-medicinal ingredients. While it’s technically possible to be allergic to the virus proteins, it’s usually the non-medical ingredients that lead to adverse reactions.

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“More often than not with vaccines, it may be an allergy to the non-medicinal ingredient,” Sharma told reporters earlier this week. “If it is due to the vaccine, then we see if we can find out a component, because then as an alternative, somebody that has an identified allergy could have an alternative vaccine. Technically, you can be allergic to almost any molecule or substance.”

So that’s where researchers in the U.K. are at right now — trying to determine exactly what part caused the reactions, so people with a history of severe allergic reactions could possibly receive an alternative vaccine without that component.

People with a history of anaphylaxis were left out of Pfizer’s trials, according to the company. Among those who participated in the Pfizer trials, a very small number of people had allergic reactions. 

According to the U.S. Food And Drug Administration (FDA), 0.63 percent of participants who received the vaccine reported potential allergic reactions, compared to 0.51 percent of people who received a placebo.

Should I be worried if I have allergies?

U.K. health officials are currently warning people with a history of severe anaphylactic allergic reactions to avoid the vaccine for now and consult with their doctors. Canadian officials said earlier this week they’re closely monitoring the situation in order to determine the best course of action here.

The reaction experienced by the workers is “very rare,” according to British officials. 

If you have a basic pollen or cat allergy, there’s no need to worry. But if you do have a history of severe reactions, some officials are warning you to be aware of the risks.

“If I were a person that had an underlying allergic tendency, I might want to be prepared that I might get a reaction, and therefore be ready to treat it,” the U.S.’s Dr. Anthony Fauci said earlier this week. 

The Canadian government is developing a program to assist anyone who has an adverse reaction to a vaccine, including the COVID-19 one.

“In the very unlikely event of an adverse reaction though, we want to make sure that Canadians have fair access to support,” Prime Minister Justin Trudeau said earlier this week. 

“Vaccines are safe and effective.”

He said it is “very unlikely” people will experience an adverse reaction.

What about getting Bell’s palsy from the vaccine? 

Concerns have also been brought up around several U.S. clinical trial participants who developed Bell’s palsy after receiving the vaccine. 

Bell’s palsy is a type of facial paralysis, usually temporary, that involves the sudden weakness or freezing of one side of the face. 

This week, Pfizer Canada president Cole Pinnow said the four occurrences of Bell’s palsy among close to 22,000 subjects represented a frequency not above what is expected in the general population.

“I can appreciate the concern, but I’m going to defer to scientific experts that look at the data in totality,” said Pinnow. “From what I understand, they say that that incidence is on par with the normal population and therefore isn’t considered to be statistically significant.”

According to the U.S. FDA, there is “no clear basis upon which to conclude a causal relationship at this time.”