Anaphylaxis can be life-threatening, causing breathing difficulties, a fast heartbeat and confusion.
Symptoms usually occur within minutes or even seconds after exposure to a food or medicine, and so people who receive their jab are kept for around 15 minutes to ensure they are well.
Health officials have reassured events like these were not unexpected because the vaccine was new.
And Pfizer said the vaccine was well tolerated during the trials with no serious safety concerns .
It has been trialled extensively on thousands of people worldwide and been declared safe and effective for use.
But regulators in the UK will be studying people given the jab over time to make sure other side effects don t appear.
Alaska healthcare worker with no history of drug allergies suffered serious anaphylactic-like reaction after getting Pfizer s coronavirus vaccine
A healthcare worker in Juneau, Alaska, suffered a severe allergic reaction within 10 minutes of receiving Pfizer Inc s coronavirus vaccine on Tuesday
She remains stable in the ICU and is expected to be discharged later on Wednesday
The woman has no known history of allergies and is said to be disappointed that she can t receive the vaccine s second dose
The anaphylactic-like reaction is believed to be the same one suffered by two British healthcare workers last week, both of whom have since recovered
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