WASHINGTON — A health care worker in Alaska had a serious allergic reaction after getting Pfizer’s coronavirus vaccine Tuesday and remained hospitalized Wednesday morning under observation.

The middle-aged worker had no history of allergies but had an anaphylactic reaction that began 10 minutes after receiving the vaccine at Bartlett Regional Hospital in Juneau, Alaska, a hospital official said. The reaction included flushing and shortness of breath.

Dr. Lindy Jones, emergency department medical director at the hospital, said the reaction subsided soon after the worker was treated with epinephrine. He said the worker remained enthusiastic that she had received the vaccine and was set to be discharged later Wednesday.

“She is healthy, and she is doing well,” Jones said.

Government officials were scrambling Wednesday to learn more about the case, according to three people familiar with their response.

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With millions of Americans expected to be vaccinated by the end of the year, the incident is likely to prompt federal officials to be even more watchful for any sign of serious side effects. The Alaska woman’s reaction was believed to be similar to the anaphylactic reactions two health workers in Britain experienced after receiving the Pfizer-BioNTech vaccine last week. Both of them recovered.

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The reaction is likely to come up Thursday, when Food and Drug Administration scientists are scheduled to meet with an outside panel of experts to evaluate Moderna’s coronavirus vaccine, which uses the same kind of technology as Pfizer’s. Although the Moderna and Pfizer-BioNTech vaccines are similar in their ingredients, it is not clear whether an allergic reaction to one would occur with the other. Both consist of genetic material called mRNA encased in a bubble made from a mixture of fats. The two companies use different fats.

Pfizer’s trial in the United States involving more than 40,000 people did not find any serious adverse events caused by the vaccine, although many participants did experience aches, fevers and other side effects. Severe allergic reactions to vaccines are typically linked to the vaccine because of their timing.

A Pfizer spokesperson, Jerica Pitts, said that the company does not yet have all the details of the case but is working with local health authorities. The vaccine comes with information warning that medical treatment should be available in case of a rare anaphylactic event, she said. “We will closely monitor all reports suggestive of serious allergic reactions following vaccination and update labeling language if needed,” Pitts said.

After the workers in Britain fell ill, authorities there initially warned against giving the vaccines to anyone with a history of severe allergic reactions. They later clarified their concerns, changing the wording from “severe allergic reactions” to specify that the vaccine should not be given to anyone who has ever had an anaphylactic reaction to a food, medicine or vaccine. That type of reaction to a vaccine is “very rare,” they said.

Pfizer officials have said the two British people who had the reaction had a history of severe allergies. One, a 49-year-old woman, had a history of egg allergies. The other, a 40-year-old woman, had a history of allergies to several different medications. Both carried EpiPen-like devices to inject themselves with epinephrine in case of such a reaction.

Pfizer has said that its vaccine does not contain egg ingredients.

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The British update also said that a third patient had a “possible allergic reaction” but did not describe it.

In the United States, federal regulators issued a broad authorization for the vaccine Friday to adults 16 years and older. Health care providers were warned not to give the vaccine to anyone with a “known history of a severe allergic reaction” to any component of the vaccine, which they said was a standard warning for vaccines.

But because of the British cases, FDA officials have said they would require Pfizer to increase its monitoring for anaphylaxis and submit data on it once the vaccine comes into use. Pfizer also said that the vaccine is recommended to be administered in settings that have access to equipment to manage anaphylaxis. Last weekend, the Centers for Disease Control and Prevention said that people with serious allergies can be safely vaccinated with close monitoring for 30 minutes after receiving the shot.

Anaphylaxis can be life-threatening, with impaired breathing and drops in blood pressure that usually occur within minutes or even seconds after exposure to a food, medicine or even a substance like latex to which the person is allergic.