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.

National Health Service medical director Stephen Powis said the advice had been changed as a precaution after two NHS workers reported anaphylactoid reactions from the vaccine.

“Two people with a history of significant allergic reactions responded adversely yesterday,” Powis said. “Both are recovering well.”

The Medicines and Healthcare Products Regulatory Agency (MHRA) initially advised anyone with “a history of a significant allergic reaction to a vaccine, medicine or food” to avoid taking the vaccine.

However, by the end of Wednesday that guidance was set to be refined after discussions with experts on the nature of the reactions.

“We’re tweaking advice to make it very clear that if you’ve got a food allergy, you’re not more at risk,” Imperial College London’s Paul Turner, an expert in allergy and immunology who has been advising the MHRA on their revised guidance, told Reuters.

Pfizer and BioNTech said they were supporting the MHRA’s investigation.

Last week Britain’s MHRA became the first in the world to approve the vaccine, developed by Germany’s BioNTech and Pfizer, while the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) continue to assess the data.

A top U.S. official said on Wednesday that Americans with known severe allergic reactions may not be candidates for Pfizer’s COVID-19 vaccine until more was understood about what had happened.

Canada’s health ministry said it would look at the reported adverse reactions in Britain, but said adverse events were to be expected and would not necessarily change the risk/benefit of the shot, after the country approved the vaccine.

ALLERGIC REACTION

MHRA Chief Executive June Raine told lawmakers 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, which is reflected in the MHRA’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 Turner said: “As we’ve had more information through, 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,” Turner told Reuters.

The EMA said in an email that all quality, safety and efficacy data would be taken into account in assessing the vaccine, including data generated outside the EU.

In the United States, the FDA released documents on Tuesday in preparation for an advisory committee meeting on Thursday, saying the Pfizer vaccine’s efficacy and safety data met its expectations for authorization.

The briefing documents said 0.63% of people in the vaccine group and 0.51% in the placebo group reported possible allergic reactions in trials, which Peter Openshaw, professor of experimental medicine at Imperial College London, said was a very small number.

“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.

However, Gregory Poland, a virologist and vaccine researcher with the Mayo Clinic in Rochester, Minnesota, said that the MHRA and NHS had overreacted initially.

“I would not have broadened to the degree they did,” he said.

“It’s reasonable to let the world know about this, and to be aware of it in terms of people who have had reactions like this to vaccines. I think to say medicines, foods or any other allergies is past the boundary of science.”