LONDON — U.K. regulators say people who have a “significant history’’ of allergic reactions shouldn’t receive the new Pfizer-BioNTech vaccine while they investigate two adverse reactions that occurred on the first day of the country’s mass vaccination program.

Stephen Powis, the national medical director for the U.K.'s National Health Service in England said Wednesday that health authorities were acting on a recommendation from the Medical and Healthcare Products Regulatory Agency.

He says the agency has advised, on a precautionary basis, that "people with a significant history of allergic reactions do not receive this vaccination."

Powis added that both people are recovering well.

Dr. June Raine, head of the U.K.'s Medical and Healthcare Products Regulatory Agency, told Parliament on Wednesday that the reactions were not seen in clinical trials for the drug.

“We know from the very extensive clinical trials that this wasn’t a feature,” Raine said, according to the Associated Press. “But If we need to strengthen our advice, now that we have had this experience with the vulnerable populations, the groups who have been selected as a priority, we get that advice to the field immediately.

The FDA is currently weighing whether to approve the Pfizer-BioNTech vaccine for emergency use in the United States.