An odd side effect is cropping up in people who get the Moderna coronavirus vaccine, according to a new study published in The New England Journal of Medicine.
They suffer something that has become known as “Moderna arm.”
It’s an angry, red rash. It almost always appears on the same arm where the vaccine was given. Sometimes it spreads to the lower part of the arm or hand.
It can be a very pronounced oval on the skin. It is hard, hot to the touch, and it can be itchy. It shows up an average of eight days after the first dose of the two-dose vaccine. It lasts for about a week.
Moderna Arm: A Nuisance But Not Dangerous
In about half the people who get it, the rash does not recur with the second dose of the vaccine. About 25% get a milder rash after the second shot. The remaining 25% get it at about the same severity.
The good news is that while the rash may be uncomfortable, it is not serious, according to the study.
“The big concern people might have had was that they were going to have a full-body, full-blown allergic reaction,” said Dr. Esther Freeman. She is one of the study’s authors. “On that point we can be reassuring.”
Co-author Dr. Kimberly Blumenthal added: “This is a nuisance, but it is not dangerous.”
Moderna’s final-stage trial of the vaccine noted the arm rash in only about 1 in 100 recipients. But the researchers believe the side effect may be far more prevalent.
The scientists do not know what causes it. “We believe it’s tied to the body’s immune system going to work,” Dr. Freeman said.
Strangely, the Pfizer vaccine, which has similar ingredients, does not cause the rash.
Some doctors are misdiagnosing Moderna arm as an infection and giving patients antibiotics for it. Antibiotics won’t help, but icing the rash can reduce the discomfort, Dr. Freeman said. Over-the-counter antihistamines and steroid creams may also help.
You can also try a natural solution such as butterbur extract. Butterbur is a marsh plant that has been used to fight allergies for centuries. It has not been tested specifically against Moderna arm, but it has proven effective against similar reactions. 
A review published in the journal Current Opinion in Allergy and Clinical Immunology found that butterbur works as well as antihistamine pills to relieve itchy eyes caused by allergies.
Butterbur supplements are widely available online and at health food stores.
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