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.

The national medical director for the National Health Service in England says health authorities were acting on a recommendation from the Medical and Healthcare Products Regulatory Agency.

Professor Stephen Powis says the agency has advised, on a precautionary basis, that “people with a significant history of allergic reactions do not receive this vaccination.” He says both people are recovering well.