A health-care worker in Alaska had a serious allergic reaction after getting the new coronavirus vaccine developed by Pfizer and BioNTech — the first such case reported in the United States since shots began going into arms earlier this week, state officials revealed Wednesday.

The Alaska case echoes two similar cases in Britain in which health-care workers had serious but nonfatal allergic reactions to the vaccine. But the British workers had histories of severe allergic reactions, whereas the Alaska woman had none, state health officials said. She is now stable and was discharged from a hospital where she was kept overnight.

In randomized trials, Pfizer reported no such allergic responses, known as anaphylaxis, but people with a history of such severe reactions were excluded from participating.

Federal health officials praised Alaska hospital officials for following monitoring guidelines and catching the woman’s reaction and treating her promptly. Those guidelines advise that people with no history of allergic responses be observed for 15 minutes after receiving a shot; those with severe allergic histories should be observed for 30 minutes.

The Alaska health-care worker, described as middle-aged but otherwise not identified, began flushing and experiencing other signs of an allergic response about 10 minutes after she received the shot Tuesday afternoon at Bartlett Regional Hospital in Juneau.

“When she arrived at the emergency room she was feeling short of breath. She was not wheezy. Her heart rate was elevated,” said Lindy Jones, the attending physician who treated her with epinephrine, commonly used to treat severe allergic responses. The symptoms came and went, however, and the worker stayed overnight in the intensive care unit.

The vaccine shots were administered on Dec. 14 across the country, a sign of hope amid a pandemic that has killed more than 300,000 people nationwide. (The Washington Post)

By Wednesday morning she was in stable condition, no longer symptomatic and not on any medication. But because of her severe allergic reaction, she would not receive a second vaccine dose in three weeks as is the normal protocol with this vaccine.

“She kept a very positive attitude,” said Nobel Anderson, a doctor who treated her overnight. “She was excited that she got the first dose and was disappointed that she will not be getting the second dose. And she encouraged all of us to press on.”

Anne Zink, chief medical officer for Alaska, said the incident will not alter plans for distributing the vaccine.

“This is a new pandemic; this is a new virus. We’re learning a lot together,” Zink said.

Scientists do not know precisely why the new vaccine is triggering, in rare instances, a severe reaction.

“What we need to find out is what component of the vaccine is doing this,” Paul Offit, a pediatrician and expert on vaccines at Children’s Hospital of Philadelphia, said Wednesday. “Severe hypersensitivity or severe allergic reactions can happen after a vaccine. It’s important to stick around for 15 minutes in case you have an allergic reaction so that you can be treated with epinephrine to ameliorate your symptoms.”

Although the reaction by the health-care worker is so far an isolated case, it drew involvement from top officials at the Centers for Disease Control and Prevention, including Jay Butler, the agency’s deputy director for infectious diseases, who participated in the news conference with Alaska doctors Wednesday afternoon.

“This is an instance in which the systems worked very well,” Butler said, saying that experts from around the country were consulted when the CDC was alerted Tuesday about the Alaska incident.

The CDC has put out guidance saying that people with a history of severe reactions to any other vaccine or injectable therapy “may still receive vaccination, but they should be counseled about the unknown risks of developing a severe allergic reaction.”

A spokeswoman for Pfizer said the company doesn’t have all the details about what happened in Alaska but is “actively working with local health authorities to assess.”

“The prescribing information has a clear warning/precaution that appropriate medical treatment and supervision should always be readily available in case of a rare anaphylactic event following the administration of the vaccine,” said spokeswoman Jerica Pitts.

Pitts said that prospective participants in the late-stage clinical trial were excluded if they had a history of severe allergic reactions associated with a vaccine or to any component of this vaccine.

People who have mild allergies to food, pets, environment or latex can still get the Pfizer-BioNTech vaccine. Some side effects to the vaccine — such as fever, fatigue, headache or pain at the injection site — are common and go away in a day or two, health officials say. Experts say the new coronavirus vaccine has similar side effects to the widely distributed shingles vaccine.

Public health officials said the adverse event was reported to the long-standing early-warning system run by the CDC and the Food and Drug Administration. That three-decade-old safety system, known as the Vaccine Adverse Events Reporting System (VAERS), collects information about possible side effects or health problems after vaccination. The system looks for unusual or unexpected patterns that require a closer look. Anyone can report a reaction or injury, including health-care providers, patients and patients’ representatives, such as caregivers or attorneys.

Carolyn Y. Johnson contributed to this report.