Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, told reporters late Friday that the reactions had been reported in more than one state besides Alaska and that the FDA is probing five reactions.

“We are working hand in hand with the Centers for Disease Control and Prevention (CDC), and we’ve actually been working closely with our United Kingdom colleagues, who of course reported the allergic reaction. I think we’ll be looking at all the data we can from each of these reactions to sort out exactly what happened, and we’ll also be looking to try to understand which component of the vaccine might be helping to produce them,” Marks said.

“I think we have at this point the right ... mitigation strategy with the availability of treatment for a severe allergic reaction being at the ready, and we’ll continue to monitor it very closely,” he added.

Marks said the FDA was not certain what caused the reactions but indicated a chemical called polyethylene glycol, which is present in the vaccines produced by Pfizer and BioNTech as well as by Moderna “could be the culprit.” He added that the reaction some people have experienced could be more common than once thought.

“We’ll obviously be monitoring very closely what’s going on. We’re working very closely with the CDC on these, and there have been meetings between the CDC and FDA pretty much every day this week making sure we’re keeping very close track of what’s going on,” he said.

The reports of allergic reactions in Alaska follow two similar cases reported last week in Britain, the first nation to approve Pfizer’s vaccine.

The FDA’s current guidance says that most Americans with allergies should be cleared to take the vaccine but that people who’ve had severe reactions to other vaccines should not get vaccinated. It also said Friday that people with a history of severe allergic reactions to any components of the Moderna shot should avoid getting that vaccine.

“FDA takes very seriously the safety of the medical products that we authorize and approve, and certainly in a vaccine setting, it’s one of the reasons Dr. Marks and his team, in collaboration with the CDC, has set up a very robust surveillance system for assessing safety. One of the things that FDA does very well and uniquely is really getting to the bottom of events like allergic reactions so we can completely understand the circumstances and better inform the public and also our regulatory decisions,” said FDA Commissioner Stephen Hahn.