Britain's medicine regulator says anyone with a history of anaphylaxis to a medicine or food should not get the Pfizer-BioNTech COVID-19 vaccine, giving fuller guidance on an earlier warning.
Starting with the elderly and frontline workers, Britain began mass vaccinating its population on Tuesday, part of a global drive that poses one of the biggest logistical challenges in peacetime history.
The Medicines and Healthcare Products Regulatory Agency said on Wednesday there had been two reports of anaphylaxis and one of a possible allergic reaction since the rollout began.
"Any person with a history of anaphylaxis to a vaccine, medicine or food should not receive the Pfizer-BioNTech vaccine," MHRA Chief Executive June Raine said.
"Most people will not get anaphylaxis and the benefits in protecting people against COVID-19 outweigh the risks.
"You can be completely confident this vaccine has met the MHRA's robust standards of safety, quality and effectiveness."
Anaphylaxis is an overreaction of the body's immune system, which the National Health Service describes as severe and sometimes life-threatening.
The fuller guidance, clarifying that the main risk was from anaphylaxis specifically, was issued after consulting experts on allergies.
The MHRA had initially advised anyone with a history of a "significant allergic reaction" not to take the shot.
Pfizer and BioNTech said they were supporting the MHRA's investigation.
Last week, Britain's MHRA was the first in the world to approve the vaccine, developed by BioNTech and Pfizer, while the US Food and Drug Administration and European Medicines Agency continue to assess the data.
A top US official said on Wednesday Americans with known severe allergic reactions may not be candidates for Pfizer's vaccine until more was understood about what had happened.
Canada's health ministry said it would look at the reported adverse reactions but said adverse events were to be expected and would not necessarily change the risk or benefit of the shot.
MHRA chief Raine told MPs such allergic reactions had not been a feature of the Pfizer's clinical trials.
Pfizer has said people with a history of severe adverse allergic reactions to vaccines or the candidate's ingredients were excluded from their late stage trials.
However, the allergic reactions may have been caused by a component of Pfizer's vaccine called polyethylene glycol, or PEG, which helps stabilise the shot and is not in other types of vaccines.
The EMA said that all quality, safety and efficacy data would be taken into account in assessing the vaccine, including data generated outside the EU.
