Updated: December 10, 2020 4:31 AM EST | Originally published: December 9, 2020 6:35 AM EST

Medical authorities in the U.K. are urging the public not to be alarmed after issuing a warning for the newly approved COVID-19 vaccine.

Three people developed “rare” allergic reactions to the Pfizer/BioNTech vaccine on Tuesday and Wednesday. As a result, the U.K.’s Medicines and Healthcare Products Regulatory Agency (MHRA) said that any person with a history of anaphylactic reactions to vaccines, medicine or food should not receive the shot. It also said that vaccinations should only be carried out in places where resuscitation facilities are available.

Regulators said anaphylaxis “is a known, although very rare, side effect with any vaccine.” All three patients—including two healthcare workers—are “recovering well.”

“Most people will not get anaphylaxis and the benefits in protecting people against COVID-19 outweigh the risks,” the MHRA said.

The advice came just a day after hospitals began administering the vaccine to patients and medics on Tuesday, making the U.K. the first country to roll out a fully-tested vaccine. Canadian regulators approved the vaccine on Wednesday. The U.S. Food and Drug Administration is expected to approve the Pfizer vaccine in the coming days.

Read more: FDA Finds Pfizer/BioNTech’s COVID-19 Vaccine Safe and Effective

Two people who received the vaccine in the U.K. on Tuesday, both medical staff with a history of anaphylactic reactions, developed allergic responses the same day. On Wednesday evening, British regulators said a third person had suffered a “possible allergic reaction” after receiving the vaccine.

The new guidance says: “Any person with a history of anaphylaxis to a vaccine, medicine or food should not receive the Pfizer BioNTech vaccine,” according to a statement issued by the MHRA on Wednesday evening. “A second dose should not be given to anyone who has experienced anaphylaxis following administration of the first dose of this vaccine.”

The MHRA, which became the first medical regulator in the world to approve the Pfizer vaccine on Dec. 2, said the news of the reactions did not impact its assessment of the vaccine’s overall safety. “You can be completely confident that this vaccine has met the MHRA’s robust standards of safety, quality and effectiveness. The safety data has also been critically assessed by the government’s independent advisory body, the Commission on Human Medicines. No vaccine would be approved unless it meets these stringent standards – on that you can be sure.”

The statement went on: “We have in place a robust and proactive safety monitoring strategy for COVID-19 vaccines which allows for rapid, real-time safety monitoring at population level. The fact that these incidents were picked up and reviewed shows that to be the case.”

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Write to Billy Perrigo at billy.perrigo@time.com.

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