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The warning by Britain's regulators over the Pfizer-BioNTech COVID-19 vaccine is only directed at people with a history of certain significant allergic reactions.

And it comes after UK officials reported a small number of adverse reactions in people who received the jab on their first day of its rollout.

Two cases of anaphylaxis and one possible allergic reaction were recorded, prompting a revision of guidelines.

Put simply, Anaphylaxis is a condition can cause throat swelling, breathing difficulty and problems swallowing. The UK’s National Health Service even describes it as severe and sometimes life-threatening.

Late on Wednesday, the UK regulator said anyone with a history of anaphylaxis to a vaccine, medicine or food should not get the vaccine.

An expert from the London School of Hygiene & Tropical Medicine advised Reuters that anyone who’s ever needed to use an EpiPen should delay having this vaccine, at least until a reason for the adverse reaction has been clarified.

Severe allergies are very rare, however. That's according to London's Queen Mary University, which also told us that in 2012 there were only seven hospital admissions per 100,000 people for severe allergies, they said.

Some experts Reuters spoke with praised UK regulators' caution, while others -- such as a representative of the Mayo Clinic in the US -- said these added restrictions in Britain were"overdoing it".

The vaccines maker Pfizer had excluded people with a history of significant adverse reaction from late-stage trials of it’s COVID-19 vaccine.