Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said at a press conference that the allergic reactions had been reported in more than one state, including in Alaska.

Marks also said that a chemical called polyethylene glycol (PEG) that is an ingredient in the Pfizer vaccine – as well as the Moderna Inc vaccine authorized on Friday – “could be the culprit” causing the reactions.

Marks said that allergic reactions to PEG could be somewhat more common than previously understood.