The first rollout of the first clinically approved COVID-19 vaccine just hit its first speedbump. British health regulators issued a warning today after two health-care workers developed a serious allergic reaction to the Pfizer vaccine. They are now checking histories on patients before administering the vaccine, and for the moment will not vaccinate those with a “significant history” of allergic reactions.

What kind of allergic reactions? That’s still a bit ambiguous, but Sky News reporter Thomas Moore hints that the concern is about allergic reactions to both other vaccines or food:

U.K. regulators said Wednesday that people who have a “significant history” of allergic reactions shouldn’t receive the new Pfizer-BioNTech vaccine while they investigate two adverse reactions that occurred on the first day of the country’s mass vaccination program.

Professor Stephen Powis, national medical director for the National Health Service in England, said health authorities were acting on a recommendation from the Medical and Healthcare Products Regulatory Agency.

“As is common with new vaccines the MHRA 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,” Powis said in a statement. “Both are recovering well.”

NBC’s foreign correspondent Richard Engel reports from the UK that regulators are serious about this new restriction:

As Moore points out, that doesn’t mean that people have to choose between a vaccine or nothing. Moderna’s vaccine should be available soon, although perhaps less plentiful in the UK as the US has bought most of its production line through Operation Warp Speed. The UK also has the AstraZeneca/Oxford vaccine near the end of its Phase 3 testing, which is a different vaccine type and one that doesn’t use the novel messenger-RNA technology on which Pfizer and Moderna rely. Either of those might end up being suitable for patients with histories of allergic reactions.

Still, it’s a new concern.  As the Associated Press reports, Pfizer didn’t see this issue in its Phase 3 trials, which either suggests that they didn’t include people with histories of allergies or this reaction can be expected to be a very narrow problem. The FDA plans to meet tomorrow to make a final decision on Pfizer’s emergency-use application, and the US health care system is primed to start delivering it — to nursing homes and health-care workers first, followed by the most at-risk patients. If the problem remains limited, perhaps the FDA will follow through tomorrow, but they may decide to follow the UK’s new guidance and tell people with significant allergies to wait a bit for another vaccine.