People who have the Moderna coronavirus vaccine should not be alarmed if unusual plaques develop on their arm.
The UK has three jabs in its immunisation arsenal against the pandemic – Pfizer-BioNTech, University of Oxford-AstraZeneca and Moderna – with the latter only recently being rolled out.
Millions in the US have received the Moderna jab, however, with some enduring fleeting swelling.
Writing in the journal JAMA Dermatology, medics from Yale New Haven Hospital in Connecticut reported 16 people who developed so-called Covid-arm – defined as itchy, painful and swollen "pink plaques" – several days after getting vaccinated.
Jabs commonly cause redness, swelling and slight discomfort, however, the Yale medics have stressed the "delayed" plaques are "distinctly different".
Nevertheless, Covid arm is mild and should not put people off getting vaccinated, they added.
The Moderna vaccine was approved in the UK after a study demonstrated it is 94.1% protective against Covid-19, the disease caused by the coronavirus, when administered as two doses with a 28-day interval.
Of the study's more than 30,000 participants, 244 (0.8%) developed Covid arm around eight days after the first jab, which tended to resolve within four to five days.
The Yale medics believe "delayed localised reactions may have been under-reported" in the Moderna study due to such side effects only being "actively solicited until day seven, with unsolicited adverse events collected thereafter".
Overall, the study that led to Moderna being approved "exhibited a favourable tolerability and safety profile".
The vaccine was generally found to be "well tolerated", with most side effects being "mild" or "moderate".
Dermatologist Dr Michele Green, from Lenox Hill Hospital in New York, has "seen several cases of these delayed hypersensitivity reactions after the Moderna Covid-19 vaccine".
"These reactions have presented as localised itchy, red, and sometimes painful plaques near or at the site of injection," she said.
"I have found them responsive to topical steroids and resolve in a short amount of time."
In the Yale study, the 16 patients – aged 25 to 89 – were referred to the hospital with injection-site reactions between 20 January and 12 February, 2021.
Fifteen initially endured the side effect after the first Moderna jab in its two-dose schedule, while the remaining participant only developed Covid arm after their second vaccine.
Eleven of the 15 participants endured the same side effect after their second vaccine dose. Covid arm came about quicker second time round, appearing just two days post-jab compared to a week after the first vaccine.
The plaques were generally hard to touch or swollen with an excessive accumulation of fluid. In rarer cases, the plaques were ring-shaped.
A skin biopsy revealed the participants had infection-fighting white blood cells in fluid-filled spaces in their arm, "consistent with a dermal hypersensitivity reaction". The presence of these cells does not necessarily mean the participants will be better protected against the coronavirus.
Overall, 14 of the 16 patients identified as white. This could reflect Connecticut's population or "erythema [skin inflammation] may be overlooked or not as obvious on darker phototype skin".
Thirteen of the patients were women. At the time of the study, nearly three in five (59.2%) people who had been vaccinated in the US were female.
"Women may also be more prone to developing Covid arm, just as women are more likely to develop anaphylaxis, immediate hypersensitivity and injection-site reactions to other vaccines," wrote the medics.
Females may also be more likely to seek medical attention after developing symptoms, they added.
The 16 patients were generally treated via topical steroids, oral antihistamines and cool compresses.
For unclear reasons, Covid arm has not been observed after the Pfizer-BioNTech jab, which is based on the same technology as Moderna.
The Yale medics concluded other doctors should be aware of Covid arm so they can distinguish it from more serious complications.
Unlike some other side effects, like a severe allergic reaction, Covid arm is "not a contraindication to subsequent vaccination".
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When to seek medical attention
Coronavirus vaccine side effects are mild and fleeting in the vast majority of cases.
While fever can occur post-jab, this may also be a symptom of the coronavirus itself. Anyone who develops a raised temperature, new and continuous cough, or a loss of taste or smell should get tested.
According to the NHS, injection-site tenderness, fatigue, headaches, aching muscles, nausea and vomiting are relatively common after a coronavirus vaccine.
If these symptoms worsen or you are concerned, call the non-emergency NHS number 111.
People should not have a coronavirus vaccine if they have ever had a serious allergic reaction to any of the jab's ingredients or against a previous dose of the two-vaccine schedule.
If a severe allergic reaction does occur, it tends to arise within minutes, with trained staff on hand at vaccination sites.
When it comes to blood clots, which have been linked to the Oxford-AstraZeneca vaccine, call 111 immediately if you endure any of the following from around four days to four weeks post-jab:
Severe headache that is worsening or not relieved with painkillers
Headache that is worse when lying down or bending over
Headache that is "unusual for you" and occurs alongside blurred vision, nausea, vomiting, speech problems, weakness, drowsiness or seizures
Rash that resembles small bruises or bleeding beneath the skin
Shortness of breath
Persistent abdominal pain