The UK's Medicines and Healthcare products Regulatory Agency announced on Wednesday that those with a history of "significant" allergic reaction to drugs, food or vaccines should not take the Pfizer-BioNTech coronavirus vaccine.
The decision came after two staff members of the National Health Service who were given the vaccine experienced an "anaphylactoid reaction," referring to allergic symptoms. Both had histories of allergies and are now reportedly recovering.
The UK became the first country in the world to deploy the coronavirus vaccine after the Pfizer-BioNTech jab was approved by the MHRA last week.
Dr. June Raine, the head of the MHRA, told a parliamentary hearing: "We know from the extensive clinical trials that this was not a feature, but if we need to strengthen our advice now that we have had this experience in vulnerable populations, the groups selected as a priority, we get that advice out immediately."
A spokeswoman for Pfizer said they had been advised by the MHRA of two reports "that may be associated with allergic reaction" due to being administered their vaccine.
"As a precautionary measure, the MHRA has issued temporary guidance to the NHS while it conducts an investigation in order to fully understand each case and its causes. Pfizer and BioNTech are supporting the MHRA in the investigation," she added.
"In the pivotal phase three clinical trial, this vaccine was generally well tolerated with no serious safety concerns reported by the independent Data Monitoring Committee. The trial has enrolled over 44,000 participants to date, over 42,000 of whom have received a second vaccination."