KEY POINTS

  • Some patients who received the Moderna vaccine suffered from swollen arms
  • A skin biopsy of one of the patients indicated the condition was caused by a drug reaction
  • The condition is harmless, but may be mistaken for an infection

Moderna’s coronavirus vaccine may cause alarming delayed side effects from its first dose among a portion of recipients. 

Some who received their first shot of the Moderna vaccine said their arms became red, sore, itchy, and swollen a week later. The condition appears to be harmless. However, it could easily be mistaken for an infection.

“Delayed cutaneous reactions developed in 12 patients, days after initial injection-related symptoms resolved. The reactions, some as severe as grade 3, resolved in a few days with local therapy,” The New England Journal of Medicine posted on Twitter.

In a letter published Wednesday, a group of doctors shared more details about the side effect of the vaccine in an effort to prevent other health experts from needlessly prescribing antibiotics. 

“We had said it was normal to get redness, itching, and swelling when you get the vaccine. We changed the wording to say it can also start seven to 10 days after you get the vaccine,” Dr. Kimberly G. Bluementhal, an allergist at Massachusetts General Hospital and one of the letter’s authors, said

The letter also noted that 12 people who received a vaccine shot experienced the side effects four to 11 days after inoculation. The delayed reactions only occurred in people who were given the Moderna shot. 

Most patients treated the skin condition with ice or antihistamines. Some were prescribed steroid cream or pills. One of the patients took antibiotics after a doctor mistook the condition for an infection. 

The symptoms lasted up to 11 days. All 12 patients, 10 of whom were women, later received the second dose of Moderna’s vaccine. Three people developed the same symptoms while three experienced milder reactions. 

Doctors at the Massachusetts General Hospital performed a skin biopsy on one patient that indicated the inflammation was caused by a drug reaction. But it was still unclear Wednesday night what the patient’s immune system was reacting to. 

“I don’t understand why,” Blumenthal said. “I’m hoping the companies will figure it out.”

Researchers at the Moderna vaccine clinical trial reported seeing delayed skin reactions in 0.8% of its participants after receiving the first dose. 

Some ten percent of Americans have so far received at least one Covid vaccine dose, with Moderna accounting for just under half the number Moderna logo. Photo: AFP / JOEL SAGET