People with a history of significant allergic reactions should not have the Pfizer/BioNTech Covid jab, regulators say.
It came after two NHS workers had allergic reactions on Tuesday.
The advice applies to those who have had reactions to medicines, food or vaccines, the Medicines and Healthcare products Regulatory Agency said.
The two people had a reaction shortly after having the new jab, had treatment and are both fine now.
They are understood to have 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 NHS workers have a history of serious allergies and carry adrenaline pens around with them.
Professor Stephen Powis, medical director for the NHS in England, said both individuals were recovering well.
He said this was "common with new vaccines", describing it as a precautionary measure.
Dr June Raine, head of the MHRA, said it was only right to take this step now that "we've had this experience".
Reactions like this are uncommon, but do happen with other vaccines, including the annual flu jab.
Several thousand people were vaccinated on Tuesday in hospital clinics on the first day of the UK rollout of the new Covid jab.
Prof Peter Openshaw, an expert in immunology at Imperial College London, said: "The fact that we know so soon about these two allergic reactions and that the regulator has acted on this to issue precautionary advice shows that this monitoring system is working well."(BBC)