British regulators say people with severe allergies should not receive the Pfizer/BioNTech vaccine after two NHS staff members suffered an allergic reaction after being inoculated.
The UK was the first country in the world to approve Pfizer's vaccine, with the first shots administered to elderly patients and health workers on Tuesday.
Patients will now be asked if they suffer from any allergies before they are given the shot.
The Medicines and Healthcare products Regulatory Agency (MHRA) advised NHS trusts that anyone who had a history of significant allergic reactions to medicines, food or vaccines should not receive the shot at this time.
The two NHS workers, who suffer severe allergies and carry adrenalin pens with them, had a reaction shortly after they were vaccinated.
It is understood they had an anaphylactoid reaction, which tends to involve a skin rash, breathlessness and sometimes a drop in blood pressure.
This is not the same as anaphylaxis, which can be fatal.
Both workers are fine now.
Prof Stephen Powis, national medical director for the NHS in England, said: "As is common with new vaccines, the MHRA have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination after two people with a history of significant allergic reactions responded adversely yesterday. Both are recovering well."
MHRA chief Dr June Raine said regulators were keeping a close eye on the allergic reactions.
"We know from the very extensive clinical trials this wasn’t a feature," she told a parliamentary committee on Wednesday.
"But if we need to strengthen our advice now that we’ve had this experience in the vulnerable populations – the groups that have been selected as a priority – we get that advice to the field immediately.”