Regulators say people with a history of significant allergic reactions should not have the Pfizer/BioNTech Covid jab.
It came after two NHS workers had allergic reactions on Tuesday.
The Medicines and Healthcare products Regulatory Agency said the advice applies to those who have had reactions to medicines, food or vaccines.
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 (ana-fi-lak-toid) reaction, which tends to involve a skin rash, breathlessness and sometimes a drop in blood pressure. This is not the same as anaphylaxis (ana-fi-laxis) which can be fatal.
Both NHS workers have a history of serious allergies and carry adrenaline pens around with them.