The "Moderna Arm," or the raised, often itchy red rash that some people develop a week or two after being vaccinated with Moderna's COVID-19 vaccine is harmless, experts reassured. 

There have been reports that people who have experienced the annoying rash may become worse on the second dose, but a report published on Wednesday finds that this is not the case - and many do not have any reactions at all for the second time.

This delayed skin reaction happened in less than 1% of Phase 3 clinical trial participants, according to a letter to the editor published this week in the New England Journal of Medicine.

Such a rash is harmless, causing itchiness or aching at worst cases, and usually goes away within 24 hours a week, according to Esther Freeman, Chief of Global Health Dermatology at Massachusetts General Hospital.

Freeman maintains a global database of COVID-19 skin-related symptoms and vaccine reactions. She told USA Today that only 14 people reported the so-called "COVID arm" as of Jan. 27, but more cases have probably not been reported.

"We want to reassure people that this is a known phenomenon," Freeman told USA Today. "Having a big red splotch on your arm for a couple of days may not be fun, but the reality is there's no need to panic and no reason not to get your second shot."

The reaction happens only in those who receive the Moderna vaccine, not in the case of Pfizer-BioNTech. It is more common in women than in men and is more common in those under the age of 60. The reaction may come as a surprise, as it usually does not occur until seven or eight days after immunization.

The rash is now referred to as the "delayed large local reaction" although many patients have informally started to refer to it as the "Modera arm."

Another positive finding was that it appeared to vanish more quickly after the second shot.

Those that got it after the first shot claimed the rash lasted six to 11 days. Those who had the reaction after the second shot said it had faded within two to three days, Freeman said.

So far, aside from this one reaction, the side effects of both the Moderna and Pfizer-BioNTech vaccines seem to be relatively similar. Experts warn that a small percentage of patients who had a reaction is not a reason to choose one vaccine over another. People should take whatever vaccine they can get, Freeman advised.