The side effect, dubbed “Covid arm”, appears around a week after receiving a dose and affects very few patients, doctors say.
But they stressed that people with the harmless reaction should still get their second dose of the jab for optimal protection against Covid – around 95 per cent for the Moderna jab.
The Moderna vaccine is being dolled out in the US after authorisation in December, but is not expected in the UK until March.
It uses the same mRNA technology as the Pfizer vaccine, which has been given to millions of Britons.
Dr Esther Freeman, director of global health dermatology at Massachusetts General Hospital, told USA Today: “We want to reassure people that [Covid arm] is a known phenomenon.
“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.”
“Angry and painful”
Amelia Brown, who lives in Visalia, California, got her first Moderna vaccine dose on January 11.
Shortly after her jab, Amelia began to feel dizzy and had a minor allergic reaction, she told FoxTV.
Then, a week after her first dose, Amelia said her arm was “just red and angry and had hives”.
The next morning, Amelia says she woke up with a lump.
“It had just grown overnight from like, a dollar size to like, a baseball size,” Amelia said.
“And it was very swollen, hot to the touch, sore, even if I wore my coat that day, the pressure of the coat was painful.”
Worth it for protection
Amelia said the experience led her to consider avoiding her second dose.
But officials have reminded people that some minor side effects, such as a sore arm or light skin rash, are worth the benefit of being protected against a disease.
And it’s often a sign that the body has reacted appropriately to the vaccine.
Dr Praveen Buddiga, an immunologist where Amelia lives, said: “Your immune response is responding and in some cases, some patients have a very hyper or really aggressive immune response.