UK health authorities have warned the public not to take the recently-approved COVID-19 vaccine developed by Pfizer-BioNTech for those who have had a “significant history of allergic reactions” due to adverse effects observed after 24 hours among two health care workers who got their first shot.

The two “responded adversely” after receiving the vaccine during UK’s first day of mass vaccinations that saw thousands of UK residents receive the first jab, according reports from the National Health Service (NHS) England.

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Reports reveal that the two developed symptoms of anaphylactoid reaction, citing that one common denominator between the two affected individuals was that they both carried an adrenaline auto injector and had a history of allergic reations.

“As is common with new vaccines the MHRA [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,” said Stephen Powis, the national medical director for NHS England, in a statement.

The two individuals are currently “recovering well” according to authorities.

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In response to the incident, the MHRA issued an updated advice for health care professionals who intend to receive the vaccine, stating that persons who have had a significant allergic reaction to a vaccine, medicine or food — such as previous history of anaphylactoid reaction, or those who have been advised to carry an adrenaline autoinjector — should not receive the Pfizer/BioNtech vaccine.

The advice also states that vaccines “should only be carried out in facilities where resuscitation measures are available.”