LONDON • Britain's medicine regulator has said that anyone with a history of anaphylaxis or severe reaction to a medicine or food should not get the Pfizer-BioNTech Covid-19 vaccine, giving fuller guidance on an earlier allergy warning about the shot.
Starting with the elderly and front-line 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 country's Medicines and Healthcare Products Regulatory Agency on Wednesday said that there had been two reports of anaphylaxis and one of a possible allergic reaction since the vaccine rollout began.
"Any person with a history of anaphylaxis to a vaccine, medicine or food should not receive the Pfizer BioNTech vaccine," the agency's 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 that this vaccine has met the (agency's) robust standards of safety, quality and effectiveness."
Anaphylaxis is an overreaction of the body's immune system, which the National Health Service in Britain describes as severe and sometimes life-threatening.
The fuller guidance clarifies that the main risk is from anaphylaxis specifically. The agency had initially advised anyone with a history of a "significant allergic reaction" not to take the shot.
Pfizer and BioNTech said they are supporting the agency's investigation.
Last week, the British agency became the first in the world to approve the vaccine, developed by Pfizer and its German partner BioNTech, while the US Food and Drug Administration (FDA) and the European Medicines Agency continue to assess the data.
Dr Moncef Slaoui, a top US health official, said on Wednesday that people with known severe reactions will be asked to not take the vaccine, until the FDA understands exactly what happened.
It will probably also ask providers to monitor whether people develop a rare but temporary and not very serious form of facial paralysis called Bell's palsy, after data showed that four people out of about 19,000 in the vaccine arm of the trial got the condition.
Canada's health ministry has said it would look at the reported adverse reactions in Britain, but added that adverse events were to be expected and would not necessarily change the risk or benefit of the shot, after the country approved the vaccine.
Dr Raine told lawmakers such allergic reactions had not been a feature of 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, which is reflected in the British agency's emergency approval protocol.
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.
Imperial College London's expert in allergy and immunology Paul Turner, who has been advising the British agency on its revised guidance, said: "As we've had more information... the initial concern that maybe it affects everyone with allergies is not true.
"The ingredients like PEG which we think might be responsible for the reactions are not related to things which can cause food allergy. Likewise, people with a known allergy to just one medicine should not be at risk."
In the United States, documents released by the FDA in preparation for an advisory committee meeting said 0.63 per cent of people in the vaccine group and 0.51 per cent in the placebo group had reported possible allergic reactions in trials. Professor of experimental medicine Peter Openshaw, from Imperial College London, said those numbers were very small.
"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," he said.
AGENCE FRANCE-PRESSE, REUTERS