BOSTON (BLOOMBERG) - Some people who get Moderna Inc's vaccine experience delayed rashes that can be four inches (10cm) wide or more and take as long as six days to resolve, according to a report in the New England Journal of Medicine.
Researchers reported details on 12 such cases of reactions in people who had received their first dose.
They were typically treated with ice and antihistamines although some patients needed steroid treatments.
About half also got skin reactions after the second dose, though they were less severe.
While there have been previous reports of the reactions that occur mainly with the Moderna shot, their appearance can be surprising.
The details should help educate doctors and reassure people who develop them, said Dr Kimberly Blumenthal, the lead author of the paper and co-director of the clinical epidemiology programme at Massachusetts General Hospital in Boston.
"Our aim was to show how dramatic they can be while at the same time no one had it more severe with dose 2," she said in an emailed response to questions.
"So many of the patients I cared for with this were concerned about them. Is this an infection? (No!) Does this mean I cannot have dose 2? (No!) Will it happen with dose 2? (Not necessarily!)."
"This is a nuisance, but it is not dangerous," she said in an interview.
Evidence of the reactions appeared in earlier studies.
Moderna's final-stage trial of the vaccine had also noted delayed reactions at the site of the first injection in about 0.8 per cent of the 30,420 participants, and among 0.2 per cent at their second dose, the researchers noted. Those reactions typically resolved within about five days.
Reports of rashes after vaccines, colloquially called "Covid arm," started appearing in a pharmacists’ Facebook group earlier this year.
The pharmacists shared their experiences in the "Pharmacy Staff for Covid-19 Support" group, posting pictures of large red circles on their arms.
People asked what members of the 44,000-person community knew about the phenomenon and whether others saw similar effects.
The delayed reactions to the Moderna vaccine are different from the immediate injection-site reactions that commonly occur with both the Pfizer Inc. and Moderna messenger RNA vaccines, Dr Blumenthal said.
"This is a Moderna-specific reaction that pops up after the first week and can last a number of days," she said. "I have not been told about any from Pfizer," she said.
Exactly what is causing them is unknown, but it may be some sort of immune reaction to inactive components of the vaccine, she said.
While most of the outbreaks developed on the shoulder and upper arm near the injection site, others appeared on the elbow and the fingers and palm of the hand.
In some cases, the tissue below the skin surface hardened and lost its smooth appearance in response to the inflammation.