WASHINGTON — The Food and Drug Administration reiterated Thursday that the newly authorized Pfizer-BioNTech coronavirus vaccine should continue to be used with no new restrictions despite several reports of health-care workers who had a severe allergic reaction after receiving the injection.

Two of those incidents happened in the United Kingdom last week, and a third in Alaska on Tuesday. Another Alaska hospital employee had a brief but much less serious reaction on Wednesday.

The FDA said it is closely monitoring these situations and is teaming with the Centers for Disease Control and Prevention to investigate what incited these responses. While that is being investigated, the FDA is working with Pfizer to update fact sheets and prescribing information to reflect the evolving information. The FDA said that would underscore an existing requirement – that facilities administering the vaccine must be capable of immediately treating any severe allergic reaction.

The vaccine developed by the pharmaceutical giant Pfizer and the German company BioNTech passed strict safety reviews during months-long randomized clinical trials involving tens of thousands of people. But three incidents of anaphylaxis – a sudden allergic response that can be reversed quickly with medication – are a complication for officials hoping to gain public acceptance of the vaccine.

They’re also a biochemical mystery. No one knows what component of the vaccine incited the anaphylactic reactions.

“While the totality of data at this time continue to support vaccinations under the Pfizer [emergency use authorization] without new restrictions, these cases underscore the need to remain vigilant during the early phase of the vaccination campaign,” FDA vaccine expert Doran Fink said at the start of an all-day meeting of the FDA advisory committee examining a similar vaccine developed by Moderna, which is poised for emergency authorization in the coming days.

“We learned of these cases through established safety surveillance systems that worked exactly as designed, and FDA is coordinating with the CDC to further investigate the cases in the U.S., and to communicate our findings in a timely manner,” Fink said.

Fink said the totality of data does not suggest new restrictions on use of the Pfizer-BioNTech vaccine are required. Unlike traditional vaccines, these do not include preservatives and do not include ingredients grown in chicken eggs.

Two cases of anaphylaxis occurred last week in the United Kingdom, involving health care workers with a history of severe allergic reactions. They fully recovered after treatment.

A third incident happened Tuesday at Bartlett Regional Hospital in Juneau, Alaska. A health-care worker with no prior history of severe allergic reactions suffered shortness of breath, rapid heartbeat and flushed skin. The first symptoms began 10 minutes after receiving the vaccine injection.

The worker was treated overnight. By the next morning she was stable and not on medication. She then stayed a second night in the hospital “under observation,” according to a hospital statement. “She is still encouraging her colleagues to get the vaccine,” the statement added.

That was the first and, as of Thursday afternoon, only reported case of anaphylaxis since the vaccines began to be administered Monday nationwide.

Another employee at Bartlett Regional Hospital who received the vaccine Wednesday “experienced eye puffiness, lightheadedness, and scratchy throat ten minutes after being injected with the vaccine,” according to a statement from the city of Juneau. He was given a standard treatment of epinephrine, Pepcid and Benadryl and “felt completely back to normal within an hour and was released,” the statement said.

The hospital said that case was not considered anaphylaxis.

Vaccine expert and pediatrician Paul Offit of the Children’s Hospital of Philadelphia said Wednesday that, among all vaccines, about one in a million vaccinations triggers a severe allergic reaction. The guidance for this new vaccine requires people to be monitored for 15 minutes after the injection to see if they have a reaction, or 30 minutes if they have a history of severe allergic responses.

Milder side effects, such as fever, headache, fatigue and pain at the injection site, have also been reported by volunteers in randomized trials, particularly, after the second dose and among younger people who tend to have more robust and reactive immune systems. Those side effects go away in a day or two and are not considered a cause for concern.

Tal Zaks, chief medical officer of Moderna, said at Thursday’s hearing that there are three components of the vaccine that theoretically could cause rare reactions. He said that the National Institutes of Health convened an expert panel Wednesday to discuss the possibilities.

He pointed out that several components of the Moderna vaccine – which has not been linked to any anaphylactic reactions in the randomized trials – are proprietary, and differ from components in the Pfizer-BioNTech vaccine.

“While we all say there’s a [lipid nanoparticle] here delivering mRNA therefore they must be the same . . . I actually think as far as the component that is likely to be the culprit here, I would not necessarily assume that,” Zaks said. “We will be looking very carefully as has been noted, and continue to collaborate with colleagues to understand the mechanism here.”

Moderna executive Jacqueline Miller said Thursday that, after scouring a database from previous use of the same vaccine technology in trials for eight other vaccines with 1,700 people, there was a single case of an anaphylactic event reported, and it was more than two months after the shot was given. The lengthy time delay makes it unlikely that it was directly linked to the shot.

The randomized trials of both the Pfizer-BioNTech and Moderna coronavirus vaccines have not produced cases of anaphylaxis, but those trials did not permit participation by anyone with a known allergy to any component of the vaccine.

States and jurisdictions are continuing to receive deliveries of the first week’s 2.9 million Pfizer dose allocation and provide vaccinations every day. States routinely provide data to the CDC on vaccine coverage, and the CDC is awaiting the first data reports from states about vaccine administration that began Monday.

More than 1.5 million doses had been distributed, but not necessarily administered, by Thursday morning, according to the Department of Health and Human Services.

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