Thousands of Britons received the first clinically authorized, fully tested coronavirus vaccine on Tuesday, with people reporting minimal side effects.

But two health workers with a history of serious allergies had reactions after being given the vaccine, British drug regulators said on Wednesday. As they investigate what precisely caused the reactions, the regulators warned that people prone to severe allergic reactions should not receive the Pfizer-BioNTech vaccine for the time being.

Scientists said that for the vast majority of people, the new advice should not cause any concerns about receiving the coronavirus vaccine, and that it was difficult to protect against certain rare reactions with any new vaccine.

Dr. Anthony S. Fauci, the nation’s leading expert on infectious diseases, said on Wednesday that the allergic reactions were concerning but most likely rare, the kind of effects that show up when a vaccine moves out of testing and into broader distribution.