Image by Image via Pixnio/Victor Tangermann

Health authorities in the UK are advising that those with a history of strong allergic reactions should hold off on getting Pifzer and BioNTech's COVID-19 vaccine, Reuters reports.

The country became the first in the world to approve and start administering the vaccine to the elderly and frontline workers this week, but two people who had the shot have already developed adverse allergic reactions.

"As is common with new vaccines the [Medicines and Healthcare products Regulatory Agency] have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination, after two people with a history of significant allergic reactions responded adversely yesterday," National Health Service medical director Stephen Powis said in a statement.

Luckily, "both are recovering well," according to Powis.

While common, regulators were caught off guard. The allergic reactions, they said, didn't come up in the extensive tests for the vaccine — but that could be because people with a history of strong allergic reactions were excluded from the trials.

"We know from the very extensive clinical trials [of the vaccine] that this wasn’t a feature," MHRA chief executive June Raine told lawmakers, as quoted by Reuters.

The MHRA is now advising anybody with a previous history of "significant allergic reaction to a vaccine, medicine or food" to skip their Pfizer COVID-19 shot.