It goes away on its own in about a week and generally doesn't happen with the second dose.

PITTSBURGH (KDKA) – About a week after the Moderna vaccine, some people develop a reaction.

“We’ve had a few cases. Folks have reached out because it’s a little unusual,” says Asti Drug pharmacist Chris Antypas.

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Genese Hendrickson says her reaction looked like someone had ripped an adhesive bandage off.

“It was a big rectangle, two and a half to three inches long, maybe an inch and a half to two inches wide,” says Hendrickson, adding, “It left that imprint where it would be bubbled and red. It did itch.”

An itchy, red rash pops up on the arm that was injected. This can happen up to a week later.

“It is some sort of immune reaction,” says Dr. James DeAngelo of Allergy and Clinical Immunology Associates. “It’s technically not an allergy. It’s a hypersensitivity. We just don’t know the mechanism yet.”

It can be oval, hard and warm. It’s more common in men younger than 60.

“It was raised and hot,” Genese describes.

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“Other than being unsightly, it’s not cause for great concern. It’s not happening so frequently that we expect it to happen,” says Antypas, “Whenever there’s an adverse reaction, we like to report it.”

The good news — it goes away on its own in about a week.

“It was maybe around day three or day four and it lasted for about four days before it went away,” says Genese.

And generally, it does not happen with the second dose.

“I did have my second dose and I did not have any red arm or anything this time,” Genese says. “I’m glad I got it.”

The allergist is taking it cautiously, though. For the few patients he has seen with this, he’s having them get their second dose closer to six weeks later, while he sorts through the available information.

“I’m putting it in a little bit of a hold,” says Dr. DeAngelo.

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Because this reaction has only been with the Moderna vaccine, its nickname is Moderna Arm. It has not happened with the Pfizer vaccine.