At least 4 people have had allergic reactions after Pfizer's vaccine, among hundreds of thousands who've been vaccinated. Here's what we know.

• Hundreds of thousands of healthcare workers across the US and UK are beginning to get Pfizer's COVID-19 vaccine.
• So far, there have been at least 4 reports of non life-threatening allergic reactions, shortly after people got their first shots.
• 2 allergic reactions happened in the UK last week, and 2 more were recorded at an Alaska hospital this week.
• It's still not clear what's causing the reactions.
• The FDA says people should only refrain from vaccination if they've had a severe allergic reaction to ingredients in this specific vaccine, or reacted to a previous dose of Pfizer's coronavirus vaccine.

At least four people have had allergic reactions shortly after receiving Pfizer's new COVID-19 vaccine in the US and the UK.

But, that doesn't necessarily mean people with food allergies or who carry epi pens should worry when getting vaccinated against the coronavirus.

Data so far suggests that this vaccine is safe, for the overwhelming majority of allergic people who get it. It's still not completely clear why these four known reactions happened, or whether they were related to the vaccine administration.

Four allergic reactions, among hundreds of thousands of vaccinations

Last week, two National Health Service workers in the UK developed non life-threatening allergic reactions after receiving their first Pfizer shots. Both recovered after receiving a shot of epinephrine.

This week, two more people, this time in Alaska, had allergic reactions shortly after recieving Pfizer's vaccine. They both received their shot at Bartlett Regional Hospital in Juneau.

One of those people had no history of allergic reactions, and had to be admitted for an overnight stay. Benadryl and epinephrine didn't immediately suppress her reaction, which included an "increased heartbeat, shortness of breath, and skin rash and redness," according to the city of Juneau.

Both of the US healthcare workers who had reactions after recieving the vaccine in Alaska have said they don't want news of their cases to dissuade any of their colleagues from getting the vaccine, as the pandemic continues pummeling the US, with more than 3,000 COVID-19 deaths a day. Pfizer's shot has already demonstrated 95% efficacy at preventing symptomatic COVID-19 infections, in trials of tens of thousands of adults of various ages and colors around the world.

For the time being, the US Food and Drug Administration is recommending that everyone who is eligible to receive the vaccine in the US (for the most part, that's just healthcare workers and nursing home residents for now) should be allowed to take it, with two exceptions:

• people who've "had a severe allergic reaction after a previous dose of this vaccine"

and

• people who've "had a severe allergic reaction to any ingredient of this vaccine."

UK regulators have taken a more conservative approach, suggesting that "people with a significant history of allergic reactions do not receive this vaccination."

But the vaccine has already been injected into more than 137,800 people across the UK, and in the US, more than 2.9 million doses were shipped out to states this week.

New York City Mayor Bill de Blasio announced Thursday that 5,200 healthcare workers in his city have gotten shots, since distribution started there on Monday.

Experts are still puzzled about what might be going on with the four reported allergic reactions, among hundreds of thousands of vaccine administrations, as there isn't really much in terms ingredients in the vaccine that might prompt an allergic reaction, and there were no allergic reactions reported in Pfizer's vaccine trials. People with food allergies were allowed to participate.

Dr. Sanjeev Jain, a board-certified allergist, immunologist, and CEO of Columbia Allergy previously told Insider it's possible that these people are having extremely rare allergic reactions to the polyethylene glycol component in the vaccine, but there could be something else going on too.

During an FDA advisory committee meeting on Thursday, Dr. Stanley Perlman, a University of Iowa immunologist, asked regulators about any possible explanations they've come up with for the allergic reactions.

"We and CDC are continuing to investigate cases and consider data," FDA vaccine expert Doran Fink said during the meeting. "At this point, we don't have enough information to make definitive recommendations one way or another, and as we continue to investigate and evaluate the data we will consider whether additional recommendations need to be made."

Moderna's COVID-19 vaccine, which is also expected to receive an FDA greenlight later this week, did not have any anaphylaxis or allergic reactions after shots were administered in its trial either. That vaccine is expected to become the US's second emergency authorized vaccine to prevent symptomatic COVID-19 infections, and it could be OK'd as early as Friday.