It's not the news anyone wanted.
The UK regulator has since warned anyone with a history of "significant" allergic reactions not to currently receive the Pfizer/BioNTech Covid-19 vaccine.
The precise details of the cases are yet to emerge while our regulator the MHRA investigates.
But the expert consensus seems to be that it won't turn out to be a huge setback - apart for the two individuals involved - and may in fact be good news for the rest of us.
First, allergic reactions to vaccines are a known problem.
"Allergic reaction occurs with quite a number of vaccines, and perhaps even more frequently with drugs, so it is not unexpected," said Professor Stephen Evans of the London School of Hygiene and Topical Medicine.
Data from more than 18,000 volunteers in the main clinical trial for the vaccine showed a very low incidence of allergic reactions and hardly any more than in the placebo part of the trial.
However, the trial protocol excluded those with any history of severe allergic reaction to any of the vaccine ingredients.
This means it's possible anyone with a history of severe allergic reaction to many things were excluded which is why we might have seen an allergic response in the first day of general use, that wasn't seen in the trial.
The instructions to healthcare professionals giving the vaccine (here in case you're interested) make it clear that the vaccine shouldn't be given to people with any allergy to the ingredients.
But as you can see from the list, they're not easily recognisable and don't include any of the common allergens sometimes found in vaccines like egg or gelatin.
This fact may have made it possible with people who were unwittingly sensitive to one or more of the ingredients was included.
And it's quite possible that people who are prone to allergic reaction to one thing, can have responses to new allergens: "For the general population this does not mean that they would need to be anxious about receiving the vaccination," Prof Evans said.
"One has to remember that even things like marmite can cause unexpected severe allergic reactions."
For now the MHRA is advising that no-one who has experienced allergic reactions severe enough for them to be prescribed an EpiPen should have the vaccine.
And in that sense it's fortunate that these two cases occurred so early in the very safe confines of a hospital.
They triggered the necessary "yellow card" response that alerts anyone administering the vaccine.
"The prompt reporting of these events using the yellow card scheme and the rapid issuing of additional information to guide practice shows that the safety monitoring system is working well," said Dr Penny Ward, from Kings College London.