Some people have delayed reactions to their first dose of a Covid vaccine, with their arms becoming red, sore, itchy, and swollen about a week after the shot.

The reactions seem uncomfortable but harmless. However, according to a letter published Wednesday in the New England Journal of Medicine, the angry-looking skin condition can be mistaken for an infection. Doctors said they wanted to share information about the cases to prevent unnecessary antibiotic use, alleviate patient worries, and reassure them they can safely get their second vaccine shot.

“We changed our patient handout when we saw this,” said Dr. Kimberly G. Blumenthal, author of the letter and an allergist at Massachusetts General Hospital, in an interview. “We said it was normal to get redness, itching and swelling when you got the vaccine. We have changed the wording so that it can start seven to ten days after receiving the vaccine. “

The letter describes the experiences of 12 people who had “delayed large local reactions” that started four to eleven days after the first vaccination with the Moderna vaccine within a median of eight days. The report is not a controlled study, but rather a series of cases that doctors became aware of because vaccine recipients were concerned and wanted to know if they should get the second shot.

Most were vaccinated at Massachusetts General Hospital, where both the Moderna and Pfizer-BioNTech vaccines were administered. But the delayed reactions only occurred in people who received the Moderna shot, said Dr. Blumenthal, adding, “I don’t understand why.”

Moderna reported delayed skin reactions in its large clinical study in 0.8 percent of recipients after the first dose and 0.2 percent after the second dose.

Updated

March 4, 2021, 1:24 p.m. ET

According to the letter from Dr. Blumenthal and 10 other doctors all 12 people reported typical symptoms such as a sore arm that often appeared shortly after inoculation, and these initial symptoms had disappeared.

Then met a delayed response. Five people developed large, raised skin lesions four or more centimeters in diameter near the injection site. Two had rashes elsewhere, one near the elbow and one on the palm. Some also had systemic symptoms such as tiredness and sore muscles at the same time.

Most treated the skin symptoms with ice and antihistamines. However, some required steroids in cream or pill form, and a doctor was prescribed an antibiotic that mistook the problem for infection.

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The median symptoms lasted six days and ranged from two to eleven days. All patients took the second shot. Half had no further delayed response, but three developed the same symptoms again and three showed milder responses than after the first shot.

Dr. Blumenthal said there were many unanswered questions about the reactions. Ten of the 12 patients were women, but it is not clear whether women are more prone to the problem or whether the imbalance occurred because more of the vaccinated health workers were female.

Some had allergies to drugs, wasp stings, or food, while others didn’t.

A skin biopsy in a patient indicated the condition was a drug reaction. But what exactly the patient’s immune system reacted to is not known.

“I hope the companies find out,” said Dr. Blumenthal.

She is currently aware of about 30 cases, mostly in women and all recipients of the Moderna vaccine. The hospital has created a register to keep track of them.