Those with strong allergic reactions should exercise caution.

A massive vaccination drive is currently underway in the United Kingdom. After officially approving the Pfizer coronavirus vaccine, the country administered its first shot to 90-year-old Margaret Keenan. 800,000 shots will be administered as part of the first wave of vaccinations, with at least 40 million to be administered in total. By the UK’s estimate, they’ll have enough vaccines to inoculate at least a third of their total population, with the first shots going to those in the health and care home professions, as well as people over the age of 80.

However, amid the excitement, the UK’s Medicines and Healthcare products Regulatory Agency has offered a word of warning. The vaccine is already known to cause some potentially intense side effects in those that receive it, such as aches and general weakness, but it could be especially dangerous to those who have a history of intense allergic reactions.

“Any person with a history of 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 regulatory agency said in their updated guidelines.

According to NHS national medical director Stephen Powis, this is a fairly common practice when new vaccines are deployed. While Pfizer’s vaccine has undergone rigorous clinical testing, it’s not possible to account for every possible permutation of the human body, which is why the MHRA is currently keeping a lookout for reports of intense allergic reactions, several of which have already occurred, so they can react quickly.

“We know from the extensive clinical trials that this was not a feature, but if we need to strengthen our advice now that we have had this experience in vulnerable populations, the groups selected as a priority, we get that advice to the field immediately,” said MHRA Head Dr. June Raine.