British health officials recommended that people with severe allergy reactions not be given the vaccine. Such reactions to vaccines are rare, even in people who have allergies to food or bee stings.
The decision by British drug regulators to recommend against use of the coronavirus vaccine made by Pfizer and BioNTech in people who have a history of severe allergic reactions has raised a number of concerns.

The regulators issued the warning after two health care workers, both with such a history, had a serious reaction, anaphylaxis, after receiving the vaccine on the first day it became available in Britain. 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 or medicine, or even a substance like latex to which the person is allergic.

Both workers were treated and have recovered, the regulators said.British authorities have since 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.

Their update also said that a third patient had a “possible allergic reaction,” but did not describe it.Officials of the U.S. Food and Drug Administration said on Thursday that because of the British cases they would require Pfizer to increase its monitoring for anaphylaxis and submit data on it once the vaccine comes into use. A panel of expert advisers to the F.D.A. voted on Thursday to recommend authorizing the vaccine for emergency use, but also expressed concerns about the need to track anaphylaxis. The agency usually accepts the experts’ recommendations, and the authorization is expected within days. Vaccinations could start next week in the United States.