It wasn't even two days before authorities in the UK issued a warning: People with significant allergies should avoid getting vaccinated against COVID-19 for now.
The UK was the first country in the world to give a COVID vaccine emergency approval (EUA). The one that's been rolled out so far was developed by the German pharmaceutical company BioNTech and its US partner, Pfizer.
Almost the same day that it was approved, many hundreds of people got vaccinated — and two of them showed allergic reactions.
Warning for people with allergies
British authorities have described the warning as a mere safety precaution for allergy sufferers.
They say it is possible that people with allergies may experience reactions to the Pfizer-BioNTech vaccine — as was the case for two employees of the UK's National Health Service (NHS).
Both of those NHS staff members had existing allergy conditions, and both recovered soon after receiving the vaccine, said Stephen Powis, National Medical Director for NHS England, at a UK parliamentary committee hearing.
People with existing allergies are being advised to be careful, however, especially if they experienced an anaphylactic shock in the past.
That's what scientists call a significant allergic reaction to food, medicines or vaccines.
Anaphylactic shock is a serious bodily reaction that can be life-threatening. It is the strongest known form of an allergic reaction and considered an absolute medical emergency.
In most cases, at least two organ systems react in the body: First, airways and the skin. Sometimes the gastro-intestinal-system reacts, and in many cases the cardiovascular system reacts.
Those sorts of reactions can come on very suddenly and a patient's condition can quickly deteriorate.
In the UK, people with allergies will not get the COVID vaccine for now.
Tell them about your allergies
There are many forms of allergic reactions, aside from the extreme, life-threatening ones. Patients can develop a skin rash, or it can become difficult for them to breathe, or the face and tongue can swell.
Authorities in the UK advise people with existing and known allergies to make that clear before they even go to get vaccinated.
Irrespective of an individual's allergy history, people with allergies will not get the vaccine for now.
That's especially the case for people who carry an adrenalin "autoinjector" pen, as was the case with the two NHS staff, who experienced a reaction.
Pfizer has told the German news agency, dpa, that the vaccine was generally well-tolerated by test participants in Phase 3 clinical trials, and that no major safety concerns were reported after an independent review of the test data.
About 44,000 people took part in the trials, and 42,000 had already received a second dose, they said.
Margaret Keenan, who was the very first person to receive the vaccine in the UK, has yet to receive her second dose.
Keenan called on everyone in the UK to take part in what has been described as the largest ever vaccination campaign — even those who are not in any of the known risk groups, such as Keenan herself, who is 90 years old.